compare chlorine atom with chlorine ion  

In chlorine atom, the number of protons (17) is equal to the number of electrons (17) Chlorine atom is reactive. It reacts with sodium vigorously forming sodium chloride. The properties of chlorine atom are due to the presence of electrons in its valence shell. In chloride ion, there are 17 protons but 18 electrons.Chloride ion is unreactive. It does not react with sodium. The properties of chloride ion are due to its charge.