IONIC BONDING
Atoms are most stable when their outermost energy level has two electrons in each orbital. The second energy level has a 2s orbital and the three p orbitals (x, y, z). Similarly, the third energy level has a 3s orbital and three p orbitals (x, y, z). Both of these energy levels are full when they contain eight electrons.

Note that a sodium atom (Na) has only one electron in its outermost (third) energy level. If the sodium atom is able to "lose" this electron, it becomes a sodium ion (Na). The sodium ion has a full complement of eight electrons in its outermost (now second) energy level, so the sodium ion is very stable.

Note that the adjacent chlorine atom (Cl) has seven electrons in its outermost (third) energy level. If a chlorine atom can gain an extra electron, it can complete its third energy level and become a very stable chloride ion (Cl).

When sodium and chlorine atoms are mixed together, the sodium atom gives its electron to the chlorine atom. The sodium and chlorine atoms become the sodium and chloride ions (Na and Cl, respectively). The sodium and chloride ions have opposite electrical charges and so the two ions are electrostatically attracted to each other. The electrostatic attraction between two ions, each with a full (not partial) electric charge, is called an ionic bond.