Answer: Ionic formula will be
.
Explanation:
and
ions will form a ionic compound. Ionic compounds have both metals and non-metals.
Here
is a metal and
is a non-metal.
The net charge on any compound must be 0.
So we need 2 phosphate ions to balance the charge on
ions. Similarly we need 3 Magnesium ions to balance the charge on
ions.
Criss-crossing the charges, we will get the formula as 
Criss-crossing is shown in the image below.
Fluorine(F)
Explanation:-
- Fluorine has Z=9
- +2 charge means 2 electrons donated (7left only)
Lets look at Electronic configuration

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