[yodalr]

Why is it exactly -3? Can it be more or less. What caused it to have -3 charge. The book only tells me that it has -3, but I can't find explanation anywhere.

[chenbeier]

Phosphate is PO₄^3-.

You can calculate: Phosphorus has 5 Electrons to spend and you have 4 Oxygen which can accept 2 electrons each, what means 8 Electrons. You have -8 +5 = -3

[yodalr]

Phosphate is PO₄^3-.

You can calculate: Phosphorus has 5 Electrons to spend and you have 4 Oxygen which can accept 2 electrons each, what means 8 Electrons. You have -8 +5 = -3

I see, thanks.

So the overall charge is still 0 yes? Only oxygens have -3 charge and Phosphorus has +5, yes?

[chenbeier]

Yes, if you have the partner ions included, like  Na₃PO₄. The charge is zero.

[mjc123]

So the overall charge is still 0 yes? Only oxygens have -3 charge and Phosphorus has +5, yes?

That doesn't give an overall charge of 0, does it? Formally it can be regarded as P⁵⁺ surrounded by 4 x O^2- (though it is not really ionic like that), with an overall charge of +5 -8 = -3.
As chenbeier says, a phosphate compound, like Na₃PO₄, is electrically neutral.