Magnesium is an alkali earth metal and chlorine is a nonmetal. Due to this, and their differences in electronegativity, the elements form an ionic bond. Since magnesium has two valence electrons, it will give them up to two chlorine atoms (which has 7 valence electrons), which needs them to complete its octet. From this, magnesium forms a cation, and will have the ion formula Mg(2+). The two chlorine atoms form anions, and will have the ion formula Cl(1-).
Terms:
Cation: a positively charged ion
Anion: a negatively charge ion.
Electronegativity: the tendency of an atom in a molecule to attract the shared pair of electrons towards itself.
