The answer, as given, given correctly stated that the atomic number depended on the number of protons in the nucleus and not the number of neutrons (which is always approximately the same, but often slightly different). But it should, in my opinion, have allowed "the number of electrons in the uncharged (or uncompounded) atom" – or similar. The point is that when an atom exists as an atom, as opposed to being part of a compound molecule, it has the same the number of protons and electrons (hence its neutral charge). When two or more atoms combine to form a molecule, some of them lose one or more electrons and some of them gain. It's the propensity of an atom to gain or lose electrons that determines the element's chemical properties, and so the number of electrons is arguably more important than the number of protons when we're talking about atomic numbers and the periodic table.
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