Concerning the formula for sodium phosphate for an example...?

Question

I don't understand how to use the subscripts in the formula...

for me, sodium phospate is NaPO

but, since the subscripts are required I need to put Na3PO4

I know that Na needs one more electron to have a full set and mg needs 3 so 3 Na will combine with P ,but why is there 4 electrons.help i'm really confused.

Answer 1

The bonds are between O and Na.

There are 3 oxigen and we have 3 bonds O-Na+

we have a double bond between P and O

..........O- Na+
....O.= P - O - Na+
...........O- Na+

Answer 2

When 1 x P combines with 4 x O, there are three spare electrons, so we give the particle a charge, and call it (PO4)3-.
Similarly, Na gets rid of just one electron and so we also give it a charge: Na+. Now the charges must balance, and so you need 3 x Na - hence Na3PO4.