Unbalanced (incorrect) particle model for forming carbon dioxide (colors: C = black, O = red).
Reactants (before): 1 molecule C and 1 molecule O₂
- C: (C)
- O₂: (O–O)
Products (after): 2 molecules CO₂
- CO₂: (O–C–O) and (O–C–O)
Which option correctly identifies what is wrong by counting atoms?
- The model is correct: C and O counts match on both sides.
- Carbon is not conserved: 1 C before, 2 C after.
- Oxygen is not conserved: 2 O before, 4 O after.
- Both carbon and oxygen are not conserved (C: 1→2, O: 2→4). (correct answer)
Explanation: 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 C + O₂ → 2 CO₂, counting systematically: Carbon atoms in reactants: 1 C atom, total is 1 C atom, Carbon atoms in products: 2 CO₂ molecules, each has 1 C atom, total is 2×1 = 2 C atoms—these do not match (1 ≠ 2) so carbon is not conserved. Oxygen atoms in reactants: 1 O₂ molecule has 2 O atoms, total is 2 O atoms, Oxygen atoms in products: 2 CO₂ molecules, each has 2 O atoms, total is 2×2 = 4 O atoms—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 D is correct because it correctly identifies the model as unbalanced based on systematic counting, showing both C: 1→2 and O: 2→4 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 (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 (1 O₂ has 1×2 = 2 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 CO₂ means 2 molecules, each with 1 C and 2 O = 2 C and 4 O total, not just 1 C and 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 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.