H3PO4 Lewis structure

H₃PO₄ (phosphoric acid) has three hydrogen atoms, one phosphorus atom, and four oxygen atoms.

In the H₃PO₄ Lewis structure, there is one double bond and three single bonds around the phosphorus atom, with four oxygen atoms attached to it. The oxygen atom with a double bond has two lone pairs, and the left oxygen, right oxygen, and the bottom oxygen atom (with which the hydrogen atom is attached) also has two lone pairs.

Alternative method: Lewis structure of H₃PO₄

Rough sketch

• First, determine the total number of valence electrons

In the periodic table, hydrogen lies in group 1, phosphorus lies in group 15, and oxygen lies in group 16.

Hence, hydrogen has one valence electron, phosphorus has five valence electrons, and oxygen has six valence electrons.

Since H₃PO₄ has three hydrogen atoms, one phosphorus atom, and four oxygen atoms, so…

Valence electrons of three hydrogen atoms = 1 × 3 = 3
Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of four oxygen atoms = 6 × 4 = 24

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

Learn how to find: Hydrogen valence electrons, 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

Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.

Now we have to choose the central atom from phosphorus and oxygen. 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 hydrogen and oxygen on either side.

• And finally, draw the rough sketch

Lone pair

Here, we have a total of 16 electron pairs. And seven bonds are already marked. So we have to only mark the remaining nine electron pairs as lone pairs on the sketch.

Also remember that hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell. 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 hydrogens and oxygens. But no need to mark on hydrogen, because each hydrogen has already two electrons.

So for top oxygen, there are three lone pairs. For left oxygen, right oxygen, and bottom oxygen, there are two lone pairs, and for phosphorus, there is zero lone pair because all nine electron pairs are over.

Mark the lone pairs on the sketch as follows:

Formal charge

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

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

For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0

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

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

For left oxygen, right oxygen, and bottom oxygen atom, formal charge = 6 – 4 – ½ (4) = 0

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

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 top oxygen atom to make a new P — O bond with the phosphorus atom as follows:

Final structure

The final structure of H₃PO₄ consists of a central phosphorus atom linked to four oxygen atoms, three of which are further bonded to hydrogen atoms. In this arrangement, the phosphorus atom forms a double bond with one lone oxygen atom and single covalent bonds with the three hydroxyl (OH) groups, utilizing an expanded valence shell to accommodate ten electrons. Within this configuration, the double-bonded oxygen maintains two lone pairs, while each of the three single-bonded oxygen atoms also retains two lone pairs alongside their bonds to hydrogen. This arrangement represents the most stable state for the molecule because it minimizes formal charges, resulting in a formal charge of zero for every atom involved. Accordingly, this specific electronic distribution serves as the definitive and most accurate Lewis representation of phosphoric acid.

Next: IF₃ Lewis structure

External video

• H3PO4 Lewis Structure: How to Draw the Lewis Structure for H3PO4 – YouTube • Wayne Breslyn

External links

• https://www.chemistryscl.com/general/lewis-structure-of-H3PO4/
• https://www.thegeoexchange.org/chemistry/bonding/Lewis-Structures/H3PO4-lewis-structure.html
• https://techiescientist.com/h3po4-lewis-structure/