The chemical behaviour of an atom is mostly determined by its:

Correct answer: (b) Valence electrons

Explanation

The chemical behaviour of an atom depends on its valence electrons, the electrons in its outermost shell. Atoms react by losing, gaining or sharing these electrons, so the number of valence electrons decides an element's valency and the bonds it forms.

That is why elements in the same group of the periodic table, which have the same number of valence electrons, behave alike. Isotopes have the same electrons but different numbers of neutrons, so their mass numbers and atomic weights differ while their chemical properties stay the same.