Its been awhile since I did this, so please forgive me of any inconsistances. A very nice way to grasp covalant bonding is to make a lewis dot diagrams of the molecule ( :shock: !). Heh, enjoy:
1- You have 4 oxygen atoms, each with a valence of 6eˉ so total 4x6=24eˉ
2- You have one phosphorus with 5eˉ
3- Since it is an ion, with charge of 3eˉ you have grand total of 24+5+3=32eˉ
4- Now write the letter P on a peice of paper and an O on top, right, bottom, and left sides of it. Start adding dots first (rememb, total of 32) to where the oxygens bond with phosphorus; between their symbols. Next start dotting the phosphorus around their outsides (2 each side) one by one until you have used up 32 dots.
You should find that all 32 dots are used up and make all the atoms' valences 'full', and between each oxygen and the phosphorus is two dots denoting a single bond. Now you can convert to a structural formula consisting of single bonds.
Repeat the above with something like CO (carbon monoxide), you'll find C is 4 dots short from a full valence, therefore requires a triple bond.
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