The fact that some elements are more atomically stable than others accounts for their tendency to form compounds. This atomic stability is basically the amount of electrons an atom holds in its outer shell. When the outer shell is completely full, an atom is more stable than when the outer shell is only half full.
The Octet Rule says that atoms form compounds to reach eight electrons in their outer shell. This rule sums up the chemical activity that occurs between the elements on the periodic table. The reason for this tendency is electron affinity, which varies between metal to non-metal atoms. Non-metallic atoms tend to have a high electron affinity where metals electron affinity is generally low. Although there are other factors, electron affinity is the reason behind chemical bonding between atoms.
In direct correlation to an atoms outer shell and electron affinity, the differences in nuclear shielding helps atoms in their tendency to form compounds. Nuclear shielding is when the electrons within the inner shell of an atom keep the protons in the nucleus and electrons in the outer shell apart from each other. Because there is nuclear shielding in every element, the electrons in the outer shell are free to form bonds with other atoms.