Check Atom Conservation - Middle School Physical Science

1. Multiple particle models are proposed. Only ONE correctly conserves atoms. Which reaction has matching atom counts for every element? 1) $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ 2) $2\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$ 3) $\text{N}_2 + \text{H}_2 \rightarrow 2\text{NH}_3$
   Answer: Only reaction 1 This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately for each reaction. For reaction 1: CH₄ + 2O₂ → CO₂ + 2H₂O, counting gives C: 1 before and 1 after ✓, H: 4 before and 4 after ✓, O: 4 before (2×2 from 2O₂) and 4 after (2 from CO₂ + 2 from 2H₂O) ✓—all elements conserved. For reaction 2: 2H₂ + O₂ → H₂O (only ONE water molecule), counting gives H: 4 before (2×2 from 2H₂) and 2 after ✗, O: 2 before and 1 after ✗—atoms NOT conserved. For reaction 3: N₂ + H₂ → 2NH₃, counting gives N: 2 before and 2 after ✓, but H: 2 before (from one H₂) and 6 after (2×3 from 2NH₃) ✗—atoms NOT conserved. Only reaction 1 shows matching counts for all elements. Choice A is correct because only reaction 1 properly conserves atoms for all elements (C, H, and O all match before and after). Reactions 2 and 3 both violate conservation: reaction 2 produces only one H₂O when it needs 2H₂O to balance, and reaction 3 has only one H₂ molecule in reactants when it needs 3H₂ to provide the 6 hydrogen atoms required for 2NH₃. Critical principle: a reaction equation is correct only if EVERY element shows equal atom counts before and after—even one element being unbalanced makes the entire equation incorrect. Common error in checking multiple reactions: assuming equations are correct without verification; always count systematically for each element in each reaction to identify which ones truly conserve atoms.
2. A particle model shows $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$. Verify conservation by counting H and Cl atoms.
   Answer: H: 2 before, 2 after ✓; Cl: 2 before, 2 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for chlorine—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction H₂ + Cl₂ → 2HCl, counting systematically: Hydrogen atoms in reactants: 1 H₂ molecule has 2 H atoms, total is 2 H atoms; Hydrogen atoms in products: 2 HCl molecules, each has 1 H atom, total is 2×1 = 2 H atoms—these counts match (2 = 2) so hydrogen is conserved. Chlorine atoms in reactants: 1 Cl₂ molecule has 2 Cl atoms, total is 2 Cl atoms; Chlorine atoms in products: 2 HCl molecules, each has 1 Cl atom, total is 2×1 = 2 Cl atoms—these counts match (2 = 2) so chlorine is conserved. Since both hydrogen and chlorine show equal before-after counts, this model correctly represents atom conservation. Choice B is correct because it accurately counts atoms showing H: 2 before, 2 after and Cl: 2 before, 2 after, properly verifying that all elements are conserved. Choice A incorrectly claims only 1 H before when H₂ has 2 H atoms; Choice C incorrectly states only 1 H after when 2HCl has 2 H atoms total; Choice D incorrectly claims only 1 Cl before when Cl₂ has 2 Cl atoms. This reaction demonstrates a common pattern where diatomic molecules (H₂, Cl₂) react to form compounds—the coefficient 2 on HCl is essential because it ensures both the 2 H atoms from H₂ and the 2 Cl atoms from Cl₂ are accounted for in the products, maintaining conservation.
3. A student draws a particle model for $2\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$ (only ONE water molecule on the product side). Is this model correct? Use atom counts to decide.
   Answer: No. H: 4 before, 2 after ✗ (hydrogen is not conserved). O: 2 before, 1 after ✗ (oxygen is not conserved). This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for oxygen—only if all elements show equal before-after counts can you conclude atoms are conserved. For this incorrect reaction 2H₂ + O₂ → H₂O (note: only ONE water molecule), counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms; Hydrogen atoms in products: 1 H₂O molecule has 2 H atoms, total is 2 H atoms—these counts don't match (4 ≠ 2) so hydrogen is NOT conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms; Oxygen atoms in products: 1 H₂O molecule has 1 O atom, total is 1 O atom—these counts don't match (2 ≠ 1) so oxygen is NOT conserved. Since neither hydrogen nor oxygen shows equal before-after counts, this model violates conservation and is incorrect. Choice B is correct because it accurately identifies that hydrogen is not conserved (4 before, 2 after) and oxygen is not conserved (2 before, 1 after), properly recognizing that the model is unbalanced. Choice A incorrectly claims the model is correct with matching counts; Choice C incorrectly states hydrogen is conserved when it clearly isn't (4 ≠ 2); Choice D nonsensically claims atoms are conserved despite stating different totals before and after. The key insight is that writing only one H₂O molecule as the product when starting with 2H₂ + O₂ creates an imbalance—you need 2H₂O to balance both hydrogen (4 atoms on each side) and oxygen (2 atoms on each side). Common error: assuming a reaction equation is correct without verifying counts; always check that atoms of each element type are conserved independently before accepting any chemical equation as valid.
4. A particle model shows $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$. Count atoms before and after. Are atoms conserved?
   Answer: N: 2 before, 2 after ✓; H: 6 before, 6 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all nitrogen atoms in all reactant molecules, (2) count all nitrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for hydrogen—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction N₂ + 3H₂ → 2NH₃, counting systematically: Nitrogen atoms in reactants: 1 N₂ molecule has 2 N atoms, total is 2 N atoms; Nitrogen atoms in products: 2 NH₃ molecules, each has 1 N atom, total is 2×1 = 2 N atoms—these counts match (2 = 2) so nitrogen is conserved. Hydrogen atoms in reactants: 3 H₂ molecules, each has 2 H atoms, total is 3×2 = 6 H atoms; Hydrogen atoms in products: 2 NH₃ molecules, each has 3 H atoms, total is 2×3 = 6 H atoms—these counts match (6 = 6) so hydrogen is conserved. Since both nitrogen and hydrogen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing N: 2 before, 2 after and H: 6 before, 6 after, properly verifying that all elements are conserved. Choice B incorrectly claims only 1 nitrogen atom after when there are 2 (2 NH₃ molecules each contribute 1 N); Choice C incorrectly states 3 H before when careful counting shows 6 (3 molecules × 2 H per molecule); Choice D incorrectly states 3 H after when there are 6 (2 molecules × 3 H per molecule). This reaction demonstrates the importance of multiplying by coefficients: 3H₂ means 3 molecules of H₂, giving 3×2 = 6 H atoms total, and 2NH₃ means 2 molecules of NH₃, giving 2×3 = 6 H atoms and 2×1 = 2 N atoms—systematic counting with attention to coefficients ensures accurate verification.
5. Particle model for making ammonia (colors: N = blue, H = white). Reactants (before): 1 molecule N₂ and 3 molecules H₂ - N₂: (N–N) - H₂: (H–H), (H–H), (H–H) Products (after): 2 molecules NH₃ - NH₃: (N with 3 H attached) and (N with 3 H attached) Count atoms of each element. Are atoms conserved?
   Answer: N: 2 before, 2 after ✓; H: 6 before, 6 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction N₂ + 3 H₂ → 2 NH₃, counting systematically: Nitrogen atoms in reactants: 1 N₂ molecule has 2 N atoms, total is 2 N atoms, Nitrogen atoms in products: 2 NH₃ molecules, each has 1 N atom, total is 2×1 = 2 N atoms—these match (2 = 2) so nitrogen is conserved. Hydrogen atoms in reactants: 3 H₂ molecules, each has 2 H atoms, total is 3×2 = 6 H atoms, Hydrogen atoms in products: 2 NH₃ molecules, each has 3 H atoms, total is 2×3 = 6 H atoms—these match (6 = 6) so hydrogen is conserved. Since both nitrogen and hydrogen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 N before and 2 N after, and 6 H before and 6 H after, properly verifying that all elements are conserved by checking each independently. Choice C makes a counting error, stating 3 H before when careful counting shows 6: 3 H₂ molecules × 2 H per molecule = 6 H total. Systematic atom counting procedure: (1) identify all element types present in the reaction (N, H), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (3 H₂ has 3×2 = 6 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (3 H₂ means 3 molecules, each with 2 H atoms = 6 H total, not just 2 H), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding N + H counts instead of checking separately), (d) counting molecules instead of atoms (2 NH₃ is 2 molecules but 8 atoms: 2 N + 6 H), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.
6. A particle model shows $2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2$. Which choice correctly verifies whether each element is conserved?
   Answer: K: 2 before, 2 after ✓; Cl: 2 before, 2 after ✓; O: 6 before, 6 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all potassium atoms in all reactant molecules, (2) count all potassium atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for chlorine and oxygen—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction 2KClO₃ → 2KCl + 3O₂, counting systematically: Potassium atoms in reactants: 2 KClO₃ molecules, each has 1 K atom, total is 2×1 = 2 K atoms; Potassium atoms in products: 2 KCl molecules, each has 1 K atom, total is 2×1 = 2 K atoms—these counts match (2 = 2) so potassium is conserved. Chlorine atoms in reactants: 2 KClO₃ molecules, each has 1 Cl atom, total is 2×1 = 2 Cl atoms; Chlorine atoms in products: 2 KCl molecules, each has 1 Cl atom, total is 2×1 = 2 Cl atoms—these counts match (2 = 2) so chlorine is conserved. Oxygen atoms in reactants: 2 KClO₃ molecules, each has 3 O atoms, total is 2×3 = 6 O atoms; Oxygen atoms in products: 3 O₂ molecules, each has 2 O atoms, total is 3×2 = 6 O atoms—these counts match (6 = 6) so oxygen is conserved. Since all three elements show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing K: 2 before, 2 after; Cl: 2 before, 2 after; O: 6 before, 6 after. Choice B incorrectly claims only 1 K after; Choice C incorrectly states only 3 O before when KClO₃ has 3 O atoms per molecule and there are 2 molecules (2×3 = 6); Choice D makes multiple counting errors. This decomposition reaction demonstrates careful counting with polyatomic groups: each KClO₃ contains 3 oxygen atoms, so 2KClO₃ has 6 total, which perfectly matches the 6 oxygen atoms in 3O₂—systematic element-by-element verification confirms conservation.
7. A particle model shows $\text{H}_2 + \text{Cl}_2 \rightarrow \text{HCl}$. Is this model correct? (It shows only ONE $\text{HCl}$ molecule in the products.)
   Answer: No. H: 2 before, 1 after ✗ and Cl: 2 before, 1 after ✗, so atoms are not conserved. This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for chlorine—only if all elements show equal before-after counts can you conclude atoms are conserved. For this incorrect reaction H₂ + Cl₂ → HCl (note: only ONE HCl molecule), counting systematically: Hydrogen atoms in reactants: 1 H₂ molecule has 2 H atoms, total is 2 H atoms; Hydrogen atoms in products: 1 HCl molecule has 1 H atom, total is 1 H atom—these counts don't match (2 ≠ 1) so hydrogen is NOT conserved. Chlorine atoms in reactants: 1 Cl₂ molecule has 2 Cl atoms, total is 2 Cl atoms; Chlorine atoms in products: 1 HCl molecule has 1 Cl atom, total is 1 Cl atom—these counts don't match (2 ≠ 1) so chlorine is NOT conserved. Since neither hydrogen nor chlorine shows equal before-after counts, this model violates conservation and is incorrect. Choice B is correct because it accurately identifies that hydrogen is not conserved (2 before, 1 after) and chlorine is not conserved (2 before, 1 after), properly recognizing that the model is unbalanced. Choice A incorrectly claims atoms are conserved despite the clear mismatch; Choice C nonsensically claims conservation with different totals; Choice D incorrectly states hydrogen is conserved when it has 2 atoms before and only 1 after. The fundamental problem: starting with diatomic molecules (H₂ and Cl₂) that each contain 2 atoms but producing only one HCl molecule creates an inevitable imbalance—the correct balanced equation requires 2HCl molecules as products to conserve both elements. Key lesson: when reactants are diatomic molecules, products often need coefficients of 2 to maintain atom conservation.
8. A particle model represents the reaction $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$. Which set of atom counts correctly verifies whether atoms are conserved?
   Answer: C: 1 before, 1 after ✓; H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Atoms are conserved. This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all carbon atoms in all reactant molecules, (2) count all carbon atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (hydrogen and oxygen)—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction CH₄ + 2O₂ → CO₂ + 2H₂O, counting systematically: Carbon atoms in reactants: 1 CH₄ molecule has 1 C atom, total is 1 C atom; Carbon atoms in products: 1 CO₂ molecule has 1 C atom, total is 1 C atom—these counts match (1 = 1) so carbon is conserved. Hydrogen atoms in reactants: 1 CH₄ molecule has 4 H atoms, total is 4 H atoms; Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 2 O₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms; Oxygen atoms in products: 1 CO₂ molecule has 2 O atoms plus 2 H₂O molecules each with 1 O atom, total is 2 + 2×1 = 4 O atoms—these match (4 = 4) so oxygen is conserved. Since all three elements show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 1 C before and after, 4 H before and after, and 4 O before and after, properly verifying that all elements are conserved. Choice B incorrectly states 2 H atoms after when careful counting shows 4: 2 H₂O molecules × 2 H per molecule = 4 H total; Choice D incorrectly states 2 O atoms before when there are 2 O₂ molecules × 2 O per molecule = 4 O total. Systematic counting with coefficients is crucial: 2O₂ means count atoms in 2 molecules of O₂, giving 4 oxygen atoms total, and 2H₂O means count atoms in 2 water molecules, giving 4 hydrogen atoms and 2 oxygen atoms total—always multiply the subscript by the coefficient to get total atom count.
9. Particle model for rusting iron (simplified) (colors: Fe = gray, O = red). Reactants (before): 4 atoms Fe and 3 molecules O₂ - Fe: Fe Fe Fe Fe - O₂: (O–O), (O–O), (O–O) Products (after): 2 formula units Fe₂O₃ - Fe₂O₃: (Fe Fe O O O) and (Fe Fe O O O) Which counting is correct, and are atoms conserved?
   Answer: Fe: 4 before, 4 after ✓; O: 6 before, 6 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 4 Fe + 3 O₂ → 2 Fe₂O₃, counting systematically: Iron atoms in reactants: 4 Fe atoms, total is 4 Fe atoms, Iron atoms in products: 2 Fe₂O₃, each has 2 Fe atoms, total is 2×2 = 4 Fe atoms—these match (4 = 4) so iron is conserved. Oxygen atoms in reactants: 3 O₂ molecules, each has 2 O atoms, total is 3×2 = 6 O atoms, Oxygen atoms in products: 2 Fe₂O₃, each has 3 O atoms, total is 2×3 = 6 O atoms—these match (6 = 6) so oxygen is conserved. Since both iron and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 4 Fe before and 4 Fe after, and 6 O before and 6 O after, properly verifying that all elements are conserved by checking each independently. Choice C makes a counting error, stating 2 Fe before when careful counting shows 4: 4 Fe atoms total, and errs on oxygen as 3 after per unit but forgets to multiply by 2 units for 6 O total. Systematic atom counting procedure: (1) identify all element types present in the reaction (Fe, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (3 O₂ has 3×2 = 6 O atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 Fe₂O₃ means 2 units, each with 2 Fe and 3 O = 4 Fe and 6 O total, not just 2 Fe and 3 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding Fe + O counts instead of checking separately), (d) counting molecules instead of atoms (2 Fe₂O₃ is 2 units but 10 atoms: 4 Fe + 6 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.
10. A student draws a particle model for water formation as $2\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$ (only ONE water molecule on the product side). Which statement correctly identifies what is wrong based on atom counts?
    Answer: Both hydrogen and oxygen are not conserved: H is 4 before vs 2 after, and O is 2 before vs 1 after. This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for oxygen—only if all elements show equal before-after counts can you conclude atoms are conserved. For this incorrect reaction model 2H₂ + O₂ → H₂O (with only ONE water molecule), counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms; Hydrogen atoms in products: 1 H₂O molecule has 2 H atoms, total is 2 H atoms—these counts don't match (4 ≠ 2) so hydrogen is NOT conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms; Oxygen atoms in products: 1 H₂O molecule has 1 O atom, total is 1 O atom—these counts don't match (2 ≠ 1) so oxygen is also NOT conserved. Since both hydrogen and oxygen show unequal before-after counts, this model violates atom conservation. Choice D is correct because it accurately identifies that both hydrogen and oxygen are not conserved, with H being 4 before vs 2 after, and O being 2 before vs 1 after, properly checking each element independently. Choice A incorrectly focuses on molecule count (2 molecules before, 1 after) rather than atom counts; Choice B correctly identifies oxygen isn't conserved but misses that hydrogen also isn't conserved; Choice C correctly identifies hydrogen isn't conserved but misses that oxygen also isn't conserved. Systematic atom counting reveals both elements violate conservation: the reactants have 4 H atoms and 2 O atoms, but the single H₂O product has only 2 H atoms and 1 O atom, losing 2 H atoms and 1 O atom in violation of conservation law. Common mistakes include checking only one element or confusing molecule counts with atom counts—proper verification requires checking every element present and finding that ALL must be conserved for the reaction model to be correct.
11. A particle model shows $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$. Which option correctly counts atoms and checks conservation?
    Answer: N: 2 before, 2 after ✓; H: 6 before, 6 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all nitrogen atoms in all reactant molecules, (2) count all nitrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for hydrogen—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction N₂ + 3H₂ → 2NH₃, counting systematically: Nitrogen atoms in reactants: 1 N₂ molecule has 2 N atoms, total is 2 N atoms; Nitrogen atoms in products: 2 NH₃ molecules, each has 1 N atom, total is 2×1 = 2 N atoms—these counts match (2 = 2) so nitrogen is conserved. Hydrogen atoms in reactants: 3 H₂ molecules, each has 2 H atoms, total is 3×2 = 6 H atoms; Hydrogen atoms in products: 2 NH₃ molecules, each has 3 H atoms, total is 2×3 = 6 H atoms—these counts match (6 = 6) so hydrogen is conserved. Since both nitrogen and hydrogen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 N before and 2 N after, and 6 H before and 6 H after, properly verifying that all elements are conserved by checking each independently. Choice B incorrectly states 1 N after when there are 2 (2 NH₃ molecules × 1 N per molecule = 2 N) and 3 H after when there are 6; Choice C reverses the situation claiming 1 N before (when N₂ has 2) and 3 H before (when 3H₂ has 6); Choice D incorrectly states only 2 H before when the coefficient 3 in 3H₂ means 3 molecules of H₂, giving 3×2 = 6 H atoms total. Systematic counting requires careful attention to coefficients: 3H₂ means 3 molecules of H₂, not 3 hydrogen atoms, so the total is 3 molecules × 2 atoms per molecule = 6 hydrogen atoms. Similarly, 2NH₃ means 2 ammonia molecules, each containing 3 hydrogen atoms, for a total of 6 hydrogen atoms in products—these calculations show perfect conservation of both elements.
12. A student draws this particle model for forming carbon dioxide (color key: C = black, O = red). Before: 1 molecule C (single C atom) + 1 molecule O₂ (O–O) After: 2 molecules CO₂ (O–C–O and O–C–O) Is the model correct? If not, what is wrong?
    Answer: Not correct; it creates extra carbon atoms (C: 1 before, 2 after) This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction with before: 1 C + 1 O₂ and after: 2 CO₂, counting systematically: Carbon atoms in reactants: 1 C atom (single), total 1 C; Carbon atoms in products: 2 CO₂ each has 1 C, total 2×1=2 C—these do not match (1 ≠ 2) so carbon is not conserved; Oxygen atoms in reactants: 1 O₂ has 2 O, total 2 O; Oxygen atoms in products: 2 CO₂ each has 2 O, total 2×2=4 O—these do not match (2 ≠ 4) so oxygen is not conserved; Since both carbon and oxygen show unequal before-after counts, this model violates conservation and must be incorrect. Choice B is correct because it correctly identifies the model as not correct due to creating extra carbon atoms (1 before vs 2 after), based on systematic counting. Choice D is wrong because it claims oxygen atoms change into carbon atoms, but actually the model shows creation of extra atoms of both types, not transformation—conservation means no creation or destruction, not changing one element into another. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present, add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 CO₂ means 2 molecules, each with 1 C and 2 O = 2 C and 4 O total), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + O counts instead of checking separately), (d) counting molecules instead of atoms (2 CO₂ is 2 molecules but 6 atoms: 2 C + 4 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.
13. Particle model for forming water (colors: H = white, O = red). Reactants (before): 2 molecules of H₂ and 1 molecule of O₂ - H₂: (H–H) and (H–H) - O₂: (O–O) Products (after): 2 molecules of H₂O - H₂O: (H–O–H) and (H–O–H) Count H and O atoms before and after. Are atoms conserved in this model?
    Answer: H: 4 before, 4 after ✓; O: 2 before, 2 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 2 H₂ + O₂ → 2 H₂O, counting systematically: Hydrogen atoms in reactants: 2 H₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms, Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these counts match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms, Oxygen atoms in products: 2 H₂O molecules, each has 1 O atom, total is 2×1 = 2 O atoms—these also match (2 = 2) so oxygen is conserved. Since both hydrogen and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice B is correct because it accurately counts atoms showing 4 H before and 4 H after, and 2 O before and 2 O after, properly verifying that all elements are conserved by checking each independently. Choice D is wrong because it verifies only total atoms without checking each element separately, missing that you could have 6 total atoms before and after but wrong distribution: for example, 6 H and 0 O before vs 0 H and 6 O after would violate conservation even though totals match. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂ means 2 molecules, each with 2 H atoms = 4 H total, not just 2 H), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 H₂O is 2 molecules but 6 atoms: 4 H + 2 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.
14. A particle model represents: $2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2$. Count C and O atoms before and after. Which statement is correct?
    Answer: Atoms are conserved: C: 2 before, 2 after ✓; O: 4 before, 4 after ✓ This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all carbon atoms in all reactant molecules, (2) count all carbon atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for oxygen—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction 2CO + O₂ → 2CO₂, counting systematically: Carbon atoms in reactants: 2 CO molecules, each has 1 C atom, total is 2×1 = 2 C atoms; Carbon atoms in products: 2 CO₂ molecules, each has 1 C atom, total is 2×1 = 2 C atoms—these counts match (2 = 2) so carbon is conserved. Oxygen atoms in reactants: 2 CO molecules each has 1 O atom (2×1 = 2) plus 1 O₂ molecule has 2 O atoms, total is 2 + 2 = 4 O atoms; Oxygen atoms in products: 2 CO₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms—these also match (4 = 4) so oxygen is conserved. Since both carbon and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice B is correct because it accurately counts atoms showing 2 C before and 2 C after, and 4 O before and 4 O after, properly verifying that all elements are conserved by checking each independently. Choice C makes counting errors, stating 1 carbon atom before when careful counting shows 2: the coefficient 2 in 2CO means 2 molecules × 1 C per molecule = 2 C total, and stating 2 oxygen atoms before when the model shows 4: 2CO has 2×1 = 2 O atoms plus O₂ has 2 O atoms for a total of 4 O atoms. Systematic counting must account for all sources of each element: oxygen appears in both CO and O₂ on the reactant side, so you must add oxygen counts from both molecules (2 from 2CO plus 2 from O₂ = 4 total). Common mistakes to avoid: (a) forgetting coefficients (2CO means 2 molecules, not 1), (b) missing atoms from multiple sources (oxygen comes from both CO and O₂ in reactants), (c) counting molecules instead of atoms—careful systematic counting ensures accurate verification of conservation.
15. Particle model for decomposing hydrogen peroxide (colors: H = white, O = red). Reactants (before): 2 molecules H₂O₂ - H₂O₂: (H–O–O–H) and (H–O–O–H) Products (after): 2 molecules H₂O and 1 molecule O₂ - H₂O: (H–O–H) and (H–O–H) - O₂: (O–O) Complete the conservation check by choosing the correct counts for H and O before and after.
    Answer: H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction 2 H₂O₂ → 2 H₂O + O₂, counting systematically: Hydrogen atoms in reactants: 2 H₂O₂ molecules, each has 2 H atoms, total is 2×2 = 4 H atoms, Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 2 H₂O₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms, Oxygen atoms in products: 2 H₂O molecules, each has 1 O atom (2×1 = 2) plus 1 O₂ with 2 O atoms, total is 2 + 2 = 4 O atoms—these match (4 = 4) so oxygen is conserved. Since both hydrogen and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 4 H before and 4 H after, and 4 O before and 4 O after, properly verifying that all elements are conserved by checking each independently. Choice C makes a counting error, stating 2 H after when careful counting shows 4: 2 H₂O molecules × 2 H per molecule = 4 H total, and similarly errs on oxygen. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂O₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂O means 2 molecules, each with 2 H and 1 O = 4 H and 2 O total, not just 2 H and 1 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 H₂O₂ is 2 molecules but 8 atoms: 4 H + 4 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.
16. Particle model for burning methane (colors: C = black, H = white, O = red). Reactants (before): 1 molecule CH₄ and 2 molecules O₂ - CH₄: C with 4 H attached (C + 4H) - O₂: (O–O) and (O–O) Products (after): 1 molecule CO₂ and 2 molecules H₂O - CO₂: (O–C–O) - H₂O: (H–O–H) and (H–O–H) Which counting table correctly verifies whether atoms are conserved?
    Answer: C: 1 before, 1 after ✓; H: 4 before, 4 after ✓; O: 4 before, 4 after ✓ → Conserved This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For this reaction CH₄ + 2 O₂ → CO₂ + 2 H₂O, counting systematically: Carbon atoms in reactants: 1 CH₄ molecule has 1 C atom, total is 1 C atom, Carbon atoms in products: 1 CO₂ molecule has 1 C atom, total is 1 C atom—these match (1 = 1) so carbon is conserved. Hydrogen atoms in reactants: 1 CH₄ molecule has 4 H atoms, total is 4 H atoms, Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 2 O₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms, Oxygen atoms in products: 1 CO₂ has 2 O atoms plus 2 H₂O each with 1 O atom (2×1 = 2), total is 2 + 2 = 4 O atoms—these match (4 = 4) so oxygen is conserved. Since carbon, hydrogen, and oxygen all show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 1 C before and 1 C after, 4 H before and 4 H after, and 4 O before and 4 O after, properly verifying that all elements are conserved by checking each independently. Choice B makes a counting error, stating 2 C atoms after when careful counting shows only 1: 1 CO₂ molecule has 1 C atom. Systematic atom counting procedure: (1) identify all element types present in the reaction (C, H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 O₂ has 2×2 = 4 O atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂O means 2 molecules, each with 2 H and 1 O = 4 H and 2 O total, not just 2 H and 1 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding C + H + O counts instead of checking separately), (d) counting molecules instead of atoms (CH₄ is 1 molecule but 5 atoms: 1 C + 4 H), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.
17. A particle model represents: $2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$. Which counting table row is correct for verifying conservation?
    Answer: Na: 2 before, 2 after ✓ and Cl: 2 before, 2 after ✓ This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all sodium atoms in all reactant molecules, (2) count all sodium atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for chlorine—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction 2Na + Cl₂ → 2NaCl, counting systematically: Sodium atoms in reactants: 2 Na atoms (coefficient 2 means 2 atoms of elemental sodium), total is 2 Na atoms; Sodium atoms in products: 2 NaCl molecules, each has 1 Na atom, total is 2×1 = 2 Na atoms—these counts match (2 = 2) so sodium is conserved. Chlorine atoms in reactants: 1 Cl₂ molecule has 2 Cl atoms, total is 2 Cl atoms; Chlorine atoms in products: 2 NaCl molecules, each has 1 Cl atom, total is 2×1 = 2 Cl atoms—these also match (2 = 2) so chlorine is conserved. Since both sodium and chlorine show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately shows the counting table with Na: 2 before, 2 after ✓ and Cl: 2 before, 2 after ✓, properly verifying that both elements are conserved. Choice D makes a counting error for chlorine in reactants, stating 1 Cl atom before when Cl₂ clearly has 2 Cl atoms—this is a common mistake of forgetting that Cl₂ is a diatomic molecule with subscript 2 meaning 2 chlorine atoms bonded together, not 1 chlorine atom. The reaction shows how 2 individual sodium atoms react with 1 diatomic chlorine molecule (containing 2 Cl atoms) to form 2 formula units of sodium chloride, with each NaCl containing 1 Na and 1 Cl. Common mistakes to avoid: (a) missing that Cl₂ is diatomic (has 2 atoms, not 1), (b) confusing coefficients with subscripts (2Na means 2 separate Na atoms, while Cl₂ means 1 molecule containing 2 Cl atoms), (c) forgetting to multiply when counting products (2NaCl means 2×1 = 2 Na and 2×1 = 2 Cl total)—systematic counting ensures all atoms are properly accounted for.
18. A student's particle model for burning methane is $\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ (only ONE $\text{O}_2$ molecule is shown on the reactant side). Which statement is correct?
    Answer: Model is incorrect; oxygen is not conserved (O: 2 before, 4 after) This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all carbon atoms in all reactant molecules, (2) count all carbon atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for hydrogen and oxygen—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction CH₄ + O₂ → CO₂ + 2H₂O (with only ONE O₂ molecule shown), counting systematically: Carbon atoms in reactants: 1 CH₄ molecule has 1 C atom, total is 1 C atom; Carbon atoms in products: 1 CO₂ molecule has 1 C atom, total is 1 C atom—these counts match (1 = 1) so carbon is conserved. Hydrogen atoms in reactants: 1 CH₄ molecule has 4 H atoms, total is 4 H atoms; Hydrogen atoms in products: 2 H₂O molecules, each has 2 H atoms, total is 2×2 = 4 H atoms—these counts match (4 = 4) so hydrogen is conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms; Oxygen atoms in products: 1 CO₂ molecule has 2 O atoms plus 2 H₂O molecules each with 1 O atom, total is 2 + 2×1 = 4 O atoms—these counts don't match (2 ≠ 4) so oxygen is NOT conserved. Since oxygen shows unequal before-after counts, this model violates conservation and is incorrect. Choice B is correct because it correctly identifies that oxygen is not conserved, with O: 2 before, 4 after. Choice A incorrectly claims all elements are conserved; Choice C incorrectly identifies hydrogen as not conserved when it actually is (4 = 4); Choice D incorrectly identifies carbon as not conserved when it actually is (1 = 1). The student's error was showing only one O₂ molecule when the balanced equation requires 2O₂ to provide the 4 oxygen atoms needed in products—this demonstrates why checking each element separately is crucial for identifying conservation violations.
19. Two different particle models claim to show the same reaction: $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$ (H = white, O = red). Model 1: Reactants: two H₂ molecules and one O₂ molecule Products: two H₂O molecules Model 2: Reactants: one H₂ molecule and one O₂ molecule Products: one H₂O molecule Which model correctly shows conservation of atoms for the reaction as written?
    Answer: Model 1 only This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all hydrogen atoms in all reactant molecules, (2) count all hydrogen atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for every other element present (oxygen, carbon, nitrogen, etc.)—only if all elements show equal before-after counts can you conclude atoms are conserved. It's not enough to just check one element or count total atoms; each element must be checked independently because the law states that atoms of each type cannot be created or destroyed. For Model 1 (2 H₂ + O₂ → 2 H₂O): Hydrogen atoms in reactants: 2 H₂ (4 H), products: 2 H₂O (4 H)—match; Oxygen: O₂ (2 O), 2 H₂O (2 O)—match, so conserved. For Model 2 (H₂ + O₂ → H₂O): Hydrogen: H₂ (2 H), H₂O (2 H)—match; but Oxygen: O₂ (2 O), H₂O (1 O)—do not match (2 ≠ 1), so oxygen not conserved, model incorrect. Since only Model 1 shows equal counts for all elements, it correctly represents conservation for the reaction as written. Choice A is correct because it properly verifies that Model 1 conserves all elements by checking each independently, while Model 2 violates oxygen conservation. Choice B is wrong because it claims Model 2 is correct when the model actually shows unequal counts for oxygen, missing that all elements must be conserved, not just some. Systematic atom counting procedure: (1) identify all element types present in the reaction (H, O), (2) for each element, count in reactants: look at every reactant molecule, count atoms of that element in each, multiply by coefficient if present (2 H₂ has 2×2 = 4 H atoms), add up all atoms of that element across all reactant molecules, (3) for that same element, count in products using same procedure, (4) compare: before count = after count? (5) repeat steps 2-4 for every element, (6) conclusion: all elements conserved? → model correct; any element not conserved? → model incorrect (unbalanced). Common mistakes to avoid: (a) forgetting coefficients (2 H₂O means 2 molecules, each with 2 H and 1 O = 4 H and 2 O total, not just 2 H and 1 O), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding H + O counts instead of checking separately), (d) counting molecules instead of atoms (2 H₂O is 2 molecules but 6 atoms: 4 H + 2 O), (e) stopping after checking one element (must check all elements present)—careful systematic counting ensures you catch any conservation violations in reaction models and verify that atoms truly are conserved as the law requires.
20. A particle model represents $2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$. Which statement correctly verifies conservation of atoms?
    Answer: Na: 2 before, 2 after ✓; Cl: 2 before, 2 after ✓ → Atoms are conserved. This question tests understanding of how to systematically verify that atoms are conserved in a chemical reaction by counting atoms of each element before and after. To check atom conservation, you must count atoms of each element separately: (1) count all sodium atoms in all reactant molecules, (2) count all sodium atoms in all product molecules, (3) verify these counts are equal, then (4) repeat this process for chlorine—only if all elements show equal before-after counts can you conclude atoms are conserved. For this reaction 2Na + Cl₂ → 2NaCl, counting systematically: Sodium atoms in reactants: 2 Na atoms (the coefficient 2 means 2 individual Na atoms), total is 2 Na atoms; Sodium atoms in products: 2 NaCl molecules, each has 1 Na atom, total is 2×1 = 2 Na atoms—these counts match (2 = 2) so sodium is conserved. Chlorine atoms in reactants: 1 Cl₂ molecule has 2 Cl atoms, total is 2 Cl atoms; Chlorine atoms in products: 2 NaCl molecules, each has 1 Cl atom, total is 2×1 = 2 Cl atoms—these counts match (2 = 2) so chlorine is conserved. Since both sodium and chlorine show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 Na before and after, and 2 Cl before and after, properly verifying that all elements are conserved. Choice B incorrectly states 1 Na before when the equation shows 2Na (2 sodium atoms); Choice C incorrectly states 1 Na after when there are 2 NaCl molecules; Choice D incorrectly states 1 Cl before when Cl₂ contains 2 chlorine atoms. Important distinction: 2Na means 2 individual sodium atoms (Na is monatomic), while Cl₂ means one molecule containing 2 chlorine atoms—coefficients apply differently to monatomic elements versus diatomic molecules. This balanced equation shows how 2 individual Na atoms react with 1 Cl₂ molecule to form 2 NaCl formula units, conserving all atoms.

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