What determines the chemical behavior of an atom?

Question

title

Answer 1

the number of electrons on its outer shell

Answer 2

The number of Valence Electrons in the outer shell.

Answer 3

Number of electrons or number of protons or size of an atom are reasonable answers. All the three are inter-related and may mean the same.

Number of electrons decide the number of protons or vice-versa. Total number of electrons also decide the number of valance electrons and number of shells of electrons in an atom. It decides valancy.

Number of shells, number of valance electrons and number of electrons decide the size of atom. It decides electron affinity and ionization energy of an atom.