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.

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. For this reaction 2H₂ + O₂ → 2H₂O, 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 A makes a counting error, stating 2 hydrogen atoms before when careful counting shows 4: 2 molecules × 2 H per molecule = 4 H total; Choice C also miscounts hydrogen as 2 after when it's actually 4; Choice D claims conservation based only on total atoms (6 before and after) without checking each element separately, missing that you could have wrong distributions that still total 6. 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 (2H₂ 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. Common mistakes to avoid: forgetting coefficients (2H₂ means 2 molecules, each with 2 H atoms = 4 H total, not just 2 H), counting molecules instead of atoms, or stopping after checking one element—careful systematic counting ensures you catch any conservation violations in reaction models.

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.

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: 2 H₂ + 1 O₂ and after: 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, 2 O before and 2 O after, and properly verifies 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: 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.

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: 2 Mg + 1 O₂ and after: 2 MgO, counting systematically: Magnesium atoms in reactants: 2 Mg atoms, total 2 Mg; Magnesium atoms in products: 2 MgO each has 1 Mg, total 2×1=2 Mg—these match (2=2) so Mg is conserved; Oxygen atoms in reactants: 1 O₂ has 2 O, total 2 O; Oxygen atoms in products: 2 MgO each has 1 O, total 2×1=2 O—these match (2=2) so oxygen is conserved; Since both Mg and oxygen show equal before-after counts, this model correctly represents atom conservation. Choice A is correct because it accurately counts atoms showing 2 Mg before and 2 Mg after, 2 O before and 2 O after, and properly verifies that all elements are conserved by checking each independently with ✓. Choice B makes a counting error, stating 1 Mg before when careful counting shows 2 (from 2 Mg atoms), confusing the lack of coefficient with count. Systematic atom counting procedure: (1) identify all element types present in the reaction (Mg, 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 Mg means 2 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 MgO means 2 units, each with 1 Mg and 1 O = 2 Mg and 2 O total), (b) counting only one side (must count both before and after to compare), (c) mixing elements (adding Mg + O counts instead of checking separately), (d) counting molecules instead of atoms (2 MgO is 2 units but 4 atoms: 2 Mg + 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.

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 C₂H₄ + 3 O₂ and after: 2 CO₂ + 2 H₂O, counting systematically: Carbon atoms in reactants: 1 C₂H₄ has 2 C atoms, total 2 C; Carbon atoms in products: 2 CO₂ each has 1 C, total 2×1=2 C—match (2=2); Hydrogen atoms in reactants: 1 C₂H₄ has 4 H, total 4 H; Hydrogen atoms in products: 2 H₂O each has 2 H, total 2×2=4 H—match (4=4); Oxygen atoms in reactants: 3 O₂ each has 2 O, total 3×2=6 O; Oxygen atoms in products: 2 CO₂ each has 2 O (2×2=4) plus 2 H₂O each has 1 O (2×1=2), total 4+2=6 O—match (6=6); 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 2 C before and 2 C after, 4 H before and 4 H after, 6 O before and 6 O after, and properly verifies that all elements are conserved by checking each independently. Choice D makes a counting error, claiming 3 O before when careful counting shows 6 (3 O₂ × 2 = 6), missing the coefficient and counting molecules instead of atoms. 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 (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 (3 O₂ means 3 molecules, each with 2 O = 6 O total, not just 3 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 (2 CO₂ + 2 H₂O is 4 molecules but 12 atoms: 2 C + 4 H + 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.

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: 2 H₂ + 1 O₂ and after: 1 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: 1 H₂O molecule has 2 H atoms, total is 2 H atoms—these counts do not 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 do not match (2 ≠ 1) so oxygen is not conserved; Since both hydrogen and oxygen show unequal before-after counts, this model violates conservation and must be incorrect. Choice D is correct because it correctly identifies the model as unbalanced based on systematic counting, properly verifying that both elements are not conserved with specific mismatched counts. Choice A is wrong because it claims conservation when the model actually shows unequal counts for both elements, 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 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 (1 H₂O is 1 molecule but 3 atoms: 2 H + 1 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.

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 CO + 1 O₂ and after: 1 CO₂, counting systematically: Oxygen atoms in reactants: 1 CO molecule has 1 O atom plus 1 O₂ molecule has 2 O atoms, total is 1+2=3 O atoms; Oxygen atoms in products: 1 CO₂ molecule has 2 O atoms, total is 2 O atoms—these counts do not match (3 ≠ 2) so oxygen is not conserved; However, for completeness, carbon: 1 before (from CO), 1 after (from CO₂)—matches, but since oxygen mismatches, the model is incorrect. Choice B is correct because it accurately counts oxygen atoms as 3 before and 2 after, properly calculates counts accounting for all molecules, and correctly identifies the mismatch with ✗. Choice A makes a counting error, stating 2 O before when careful counting shows 3 (1 from CO + 2 from O₂), missing the O in CO. 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 to count atoms from all molecules (missing the O in CO, counting only O₂ as 2 O), (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 reactants but 4 atoms: C, O, O, 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.

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 CH₄ + 2 O₂ and after: 1 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 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); 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 has 1 O atom (2×1=2), total is 2 + 2 = 4 O atoms—these match (4 = 4); 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, 4 O before and 4 O after, and properly verifies that all elements are conserved by checking each independently. Choice B makes a counting error, stating 2 C after when careful counting shows only 1 (from 1 CO₂), and claiming 2 O before when it's 4 (from 2 O₂), while also incorrectly showing O as 4 after but marking it as mismatched. 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 (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.

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 → 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: 1 H₂O molecule has 2 H atoms, total is 2 H atoms—these do not 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 do not match (2 ≠ 1) so oxygen is not conserved. Since both hydrogen and oxygen show unequal before-after counts, this model violates conservation and must be incorrect. Choice D is correct because it correctly identifies the model as unbalanced based on systematic counting, properly calculating counts accounting for coefficients and showing both H: 4→2 and O: 2→1 are not conserved. Choice A is wrong because it claims conservation when the model actually shows unequal counts for both elements, 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 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 (1 H₂O is 1 molecule but 3 atoms: 2 H + 1 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.