When an atom has a full outer shell, it is considered to be stable and less likely to react with other atoms. The outermost shell of an atom is known as the valence shell, and it determines the atom's chemical behavior and reactivity. The stability associated with a full outer shell is based on the octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a stable configuration with eight electrons in their valence shell (except for hydrogen and helium, which require only two electrons to complete their outer shell).
There are two main ways an atom can achieve a full outer shell:
Electron transfer: Atoms can transfer electrons to or from other atoms to achieve a stable configuration. For example, if an atom has one or a few electrons in its valence shell, it can lose those electrons to achieve a full shell. On the other hand, if an atom has almost a full outer shell, it can gain electrons from another atom to complete its valence shell. This transfer of electrons forms ionic bonds between atoms.
Electron sharing: Atoms can also achieve a full outer shell by sharing electrons with other atoms. In covalent bonding, atoms share pairs of electrons, allowing each atom to complete its outer shell. This type of bonding commonly occurs between nonmetal atoms.
When an atom has a full outer shell, it becomes more stable and less likely to undergo chemical reactions. The stable configuration of a full outer shell minimizes the atom's tendency to gain, lose, or share electrons, as it already has the desired electron configuration.