What is ionization?

Ionization is the process of turning uncharged atoms or groups of atoms into ions. Ions carry an electric charge. This occurs when atoms either lose or gain electrons. With fewer electrons, the charge is positive. More electrons create a negative charge.
It is rather striking that after ionization most atoms have the same number of electrons as the inert gases (those that do not readily combine with others). The resistance to change (stability) of these ions and the chemical inactivity of the inert gases is attributed to a special electronic arrangement. These atoms, except for helium, have eight electrons in the outermost energy level. metals, such as sodium, lose elec­trons readily, ionize, since they have one or two more electrons than the inert gases.
Non metals such as chlorine, gain electrons easily in ionization since they have one or two fewer electrons than the corresponding inert gas.