BeF4^2- is called the fluoberyllate ion. The formal charge on the beryllium atom in this ion is would be positive two. We calculate the formal charge of an atom in a molecule or ion by adding the charges of all atoms and equating this to the charge of the molecule or ion. For this case, the total charge of the ion is -2 and the charge of a fluoride ion is always -1. We let x be the formal charge of the beryllium atom. We calculate as follows:
-2 = x + (-1) (4)
-2 = x + -4
x = -2 + 4
x = 2
So, the formal charge of beryllium atom is 2.
