What is the Charge on PO4 (Phosphate ion)? And Why?

The Charge of PO4 (Phosphate ion) is 3-.

But the question is how can you find the charge on PO4 (phosphate ion)?

Well there are 2 methods by which you can find the charge of PO4.

Lets dive right into these methods one by one.

If you are a visual learner like me, then here is a short one minute video for you.

Method 1: By looking at what it is bonded to

The charge of PO4 (Phosphate ion) can be found out by looking at what it is bonded to.

So let’s take some examples of compounds that contain PO₄; like H₃PO₄, K₃PO₄, etc.

Example 1: H₃PO₄
In H₃PO₄, the PO₄ is bonded to Hydrogen (H).
You know that the ionic charge of H is 1+.
So you can easily say that the charge of PO₄ should be 3-, then only it will get canceled out.
Hence the charge of PO₄ in H₃PO₄ is 3-.

Example 2: K₃PO₄
In K₃PO₄, the PO₄ is bonded to Potassium (K).
And again, you know that the ionic charge of K is 1+.
So here also you can easily say that the charge of PO₄ should be 3-, then only it will get canceled out.
Hence the charge of PO₄ in K₃PO₄ is 3-.

As seen from the above examples,
The charge of PO4 is 3-.

In this way, you can easily find the charge of PO4 by looking at what it is bonded to.

Method 2: By calculating the formal charge using lewis structure

In order to calculate the formal charge on PO4 (Phosphate ion), you should know the Lewis dot structure of PO4 (Phosphate ion).

Here is the lewis structure of PO4.

Now using the above lewis structure of PO4, you have to find the formal charge on each atom that is present in the PO4 molecule.

For calculating the formal charge, you need to remember this formula;

Formal charge = Valence electrons – Nonbonding electrons – (Bonding electrons)/2

You can see the bonding and nonbonding electrons of PO4 from the image given below.

So now let’s calculate the formal charge on each individual atom present in PO4.

Formal charge on Phosphorus atom:
Valence electrons = 5 (as it is in group 15 on periodic table) ^[1]
Nonbonding electrons = 0
Bonding electrons = 10

So according to the formula of formal charge, you will get;

Formal charge on Phosphorus = Valence electrons – Nonbonding electrons – (Bonding electrons)/2 = 5 – 0 – (10/2) = 0

So the formal charge on phosphorus atom is 0.

Formal charge on double bonded Oxygen:
Valence electron = 6 (as it is in group 16 on periodic table) ^[2]
Nonbonding electrons = 4
Bonding electrons = 4

So according to the formula of formal charge, you will get;

Formal charge on double bonded Oxygen = Valence electrons – Nonbonding electrons – (Bonding electrons)/2 = 6 – 4 – (4/2) = 0

So the formal charge on double bonded oxygen atom is 0.

Formal charge on single bonded Oxygen:
Valence electron = 6 (as it is in group 16 on periodic table)
Nonbonding electrons = 6
Bonding electrons = 2

So according to the formula of formal charge, you will get;

Formal charge on single bonded Oxygen = Valence electrons – Nonbonding electrons – (Bonding electrons)/2 = 6 – 6 – (2/2) = 1-

So the formal charge on single bonded oxygen atom is 1-.

Now let’s put all these charges on the lewis dot structure of PO4.

So there is overall 3- charge left on the entire molecule.

This indicates that the PO4 (Phosphate ion) has 3- charge.

I hope you have understood the above calculations of PO4 (Phosphate ion). But for your tests, you don’t need to remember the entire calculations. You should just try to remember that PO4 has 3- charge.

Check out some other related topics for your practice.

Related topics:
Charge of Silver (Ag)
Charge of Zinc (Zn)
Charge of Aluminum (Al)
Charge of Sodium (Na)
Charge of Chlorine (Cl)