Ionization is the process during which an atom gains or loses electron(s) and becomes charged ions. Thus, ionization is the process that occurs when a sodium atom (Na) turns into a sodium ion (Na+).
Ions are atoms that have lost or gained electrons. Electrons are negatively charged subatomic particles that move around the central nucleus of an atom. Because electrons are negatively charged, the charge of an atom changes when electrons are gained or lost.
Atoms gain or lose electrons in order to obtain the octet rule. The octet rule states that, in order to be stable, most main-group elements need eight valence electrons. Valence electrons are the electrons found in the outermost orbital of an atom.
Cations are positively charged ions that have lost electrons. Element that form cations lose their valance electrons in order to fulfill the octet rule. In this way, they drop to the next lower orbital that is full.
Anions are negatively charged ions. Anions are negatively charged because they have gained additional electrons in order to fulfill the octet rule.
Neutrally charged sodium atoms have one valence electron. In order to fulfill the octet rule, this single valence electron is lost. In this way, sodium drops to the next lower orbital that is full.
Because a sodium atom loses an energy level (orbital) when becoming an ion, sodium ions are smaller than a neutrally-charged sodium atom.