PO43- Lewis structure

PO₄^3- (phosphate) has one phosphorus atom and four oxygen atoms.

In the PO₄^3- Lewis structure, there is one double bond and three single bonds around the phosphorus atom, with four oxygen atoms attached to it. One oxygen atom with a double bond has two lone pairs, and three oxygen atoms with single bonds have three lone pairs.

Also, there is a negative (-1) charge on the three oxygen atoms with single bonds.

Alternative method: Lewis structure of PO₄^3-

Contents

Rough sketch

First, determine the total number of valence electrons

In the periodic table, phosphorus lies in group 15, and oxygen lies in group 16.

Hence, phosphorus has five valence electrons and oxygen has six valence electrons.

Since PO₄^3- has one phosphorus atom and four oxygen atoms, so…

Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of four oxygen atoms = 6 × 4 = 24

Now the PO₄^3- has a negative (-3) charge, so we have to add three more electrons.

So the total valence electrons = 5 + 24 + 3 = 32

Learn how to find: Phosphorus valence electrons and Oxygen valence electrons

Second, find the total electron pairs

We have a total of 32 valence electrons. And when we divide this value by two, we get the value of total electron pairs.

Total electron pairs = total valence electrons ÷ 2

So the total electron pairs = 32 ÷ 2 = 16

Third, determine the central atom

We have to place the least electronegative atom at the center.

Since phosphorus is less electronegative than oxygen, assume that the central atom is phosphorus.

Therefore, place phosphorus in the center and oxygens on either side.

And finally, draw the rough sketch

PO43- Lewis Structure (Step 1) — Rough sketch of PO₄^3- Lewis structure | Image: Learnool

Lone pair

Here, we have a total of 16 electron pairs. And four P — O bonds are already marked. So we have to only mark the remaining twelve electron pairs as lone pairs on the sketch.

Also remember that phosphorus is a period 3 element, so it can keep more than 8 electrons in its last shell. And oxygen is a period 2 element, so it can not keep more than 8 electrons in its last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are oxygens.

So for each oxygen, there are three lone pairs, and for phosphorus, there is zero lone pair because all twelve electron pairs are over.

Mark the lone pairs on the sketch as follows:

PO43- Lewis Structure (Step 2) — Lone pairs marked on PO₄^3- Lewis structure | Image: Learnool

Formal charge

Use the following formula to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

For phosphorus atom, formal charge = 5 – 0 – ½ (8) = +1

For each oxygen atom, formal charge = 6 – 6 – ½ (2) = -1

Here, both phosphorus and oxygen atoms have charges, so mark them on the sketch as follows:

PO43- Lewis Structure (Step 3) — Formal charges marked on PO₄^3- Lewis structure | Image: Learnool

The above structure is not a stable Lewis structure because both phosphorus and oxygen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.

Convert a lone pair of the oxygen atom to make a new P — O bond with the phosphorus atom as follows:

PO43- Lewis Structure (Step 4) — Lone pair of left oxygen is converted, and got the most stable Lewis structure of PO₄^3- | Image: Learnool

Final structure

PO43- Lewis Structure (Final) — PO₄^3- Lewis structure showing a negative (-3) charge | Image: Learnool

The final structure of PO₄^3- includes a central phosphorus atom connected to four oxygen atoms. In this configuration, the phosphorus atom forms a double bond with one oxygen and single bonds with the three remaining oxygens. The double-bonded oxygen retains two lone pairs, while each single-bonded oxygen maintains three lone pairs. This arrangement is the most stable because it minimizes formal charges; the phosphorus and the double-bonded oxygen both have a formal charge of zero, while each single-bonded oxygen carries a formal charge of -1. Consequently, this specific electronic distribution serves as the definitive and most accurate Lewis representation of the phosphate ion.

To properly represent this as a polyatomic ion, the entire Lewis structure is enclosed within square brackets. The overall charge of 3- is then written as a superscript outside the brackets at the top right, indicating that the structure has gained three extra electrons beyond the valence electrons of the neutral atoms.

Next: I₃^– Lewis structure

External video

PO4 3- Lewis Structure: How to Draw the Lewis Structure for PO4 3- – YouTube • Wayne Breslyn

External links

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