Magnesium chloride is a salt formed from ionic bonding between one magnesium atom and two chlorine atoms. A magnesium atom has two electrons in its outer (third) shell; chlorine atoms have seven electrons in their outer shell. How many electrons would be in the outer shell of the magnesium once it becomes an ion

Outermost shell of magnesium ion has 8 electrons.

Explanation:

Mg + Cl → MgCl2

Mg electronic configuration  =

Cl  electronic configuration =

In the third shell there are two electrons while in Chlorine there are 7 electrons.

The ions formed will be  Mg^+2 and Cl-

Since Magnesium has donated 2 electrons to form Mg^+2 ion now the its ionic electronic configuration becomes  

      Mg^+2 = ( 2 , 8)

Therefore the outermost shell of magnesium ion has 8 electrons.

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