Let us consider the formation of magnesium ion from magnesium atom. We see that it has 2 electrons in its outermost shell. If it loses these two electrons, then it can achieve the stable configuration of 2, 8 (that of noble gas neon). This can be represented in Fig. 5.4 as follows:

Formation of magnesium ion

You can see that the resulting magnesium ion has only 10 electrons and hence it has 2+ charge. It is a dipositive ion and can be represented as Mg2+ion. The two electrons lost by the magnesium are gained one each by two chlorine atoms to give two chloride ions.
The formation of chloride ion has already been explained above.
Thus, one magnesium ion and two chloride ion joins together to give magnesium chloride, MgCl₂. Hence, we can write

Mg	+	2Cl	->	Mg2+	2(Cl)
(2,8,2)		2(2,8,7) or MgCl2		(2,8)	2(2,8,8)