The chemical behavior of an atoms is determine by the formation or destruction of chemical bonds. The chemical bonds are the result of the interaction of the electrons of the atoms. Chemical properties of the atoms are given by how attached are the shell electrons attached to the nucleus and how they interact with other atoms. Chemical changes are the result of exchange valence electrons of the atoms. So, the answer is the atomic particle that determines the chemical behavior of an atom is the electron, because it is the particle that is active in chemical bonding.
