Chemical reactivity is a measure of how likely atoms are to form bonds with other atoms.
Because bonding occurs when atoms gain, lose, or share their electrons, the reactivity of an atom is strongly dependent on the arrangement and number of electrons in the atom. These bonds are not generally as strong as ionic bonds and elements which form these type of bonds are generally not as reactive as those which form ionic bonds. Finally there are elements which are unlikely to gain, lose or share electrons, so they don't form bonds easily.