How to calculate formal charges of water (H2O) with lewis structure?

In covalently bonded molecules, formal charge is the charge assigned to an atom based on the assumption that the bonded electrons are equally shared between concerning atoms, regardless of their electronegativity.

The overall formal charge present on a molecule is a measure of its stability.

The fewer the formal charges present on the bonded atoms in a molecule (close to zero), the greater the stability of its Lewis structure.

In this article, we will calculate the formal charges present on the bonded atoms in H₂O and also the overall charge present on the molecule.

So, continue reading!

How to calculate the formal charges on H₂O atoms?

The formal charges can be calculated using the formula given below:

The formal charge of an atom = [valence electrons of an atom – non-bonding electrons – ½ (bonding electrons)]

• The valence electrons (V.E) of an atom are the total number of electrons present in its valence shell. Valence electrons can be calculated by locating the position of the elemental atom in the Periodic Table.
• Non-bonding electrons (N.E) are the number of lone pairs present on the atom. (1 lone pair means 2 nonbonding electrons).
• Bonding electrons (B.E) are the total electrons shared with the atom via covalent chemical bonds. (1 single bond means 2 bonding electrons).

Now let us use this formula to calculate the formal charges in the most preferred Lewis structure of H₂O.

The most preferred Lewis representation of H₂O is as shown below.

The water molecule consists of a total of 8 valence electrons. An oxygen (O) atom is present at the center, which is bonded to two atoms of hydrogen (H), one on each side, via a single covalent bond.

Two lone pairs of electrons are also present at the central O-atom, while no lone pair of electrons is present on any of the two H-atoms.

It is the best possible Lewis structure of H₂O because the formal charges are minimized in it, and thus, it is the most stable.

Let’s find out how we can determine the formal charges present on each atom in the H₂O Lewis structure.

For the central oxygen atom

• Valence electrons of oxygen = It is present in Group VI A = 6 valence electrons
• Bonding electrons = 2 single bonds = 2 (2) = 4 electrons
• Non-bonding electrons = 2 lone pairs = 2(2) = 4 electrons
• Formal charge on the Oxygen atom = 6 – 4 – 4/2 = 6 – 4 – 2 = 6 – 6 = 0

∴ The formal charge on the O-atom in H₂O is 0.

 For each hydrogen atom

• Valence electrons of hydrogen = It is present in Group I A = 1 valence electron
• Bonding electrons = 1 single bond = 2 electrons
• Non-bonding electrons = no lone pair = 0 electrons
• Formal charge on the Hydrogen atom = 1 – 0 – 2/2 = 1 – 0 – 1 = 1 – 1 = 0

∴ The formal charge on each H-atom in H₂O is 0.

This calculation shows that zero formal charges are present on the central O-atom and also on both H-atoms in the H₂O Lewis structure.

The overall formal charge present on the molecule is thus zero, as shown below. 

Also, check –

• How to draw H₂O lewis structure?
• Formal charge calculator
• SO₃ formal charge
• CO₂ formal charge
• HCN formal charge
• SO₄^2- formal charge
• PO₄^3- formal charge
• SO₃^2- formal charge
• CN^– formal charge
• SO₂ formal charge
• O₃ formal charge
• SCN^– formal charge
• POCl₃ formal charge
• NH₃ formal charge
• CO formal charge
• ClO₃^– formal charge
• NH₄⁺ formal charge
• H₃O⁺ formal charge
• OH^– formal charge
• HSO₄^– formal charge
• ClO^– formal charge
• BH₄^– formal charge
• N₃^– formal charge
• H₂SO₄ formal charge
• NCO^– formal charge
• NO₃^– formal charge
• NO₂^– formal charge
• CH₃ formal charge

FAQ

Summary

• The best possible Lewis structure of a molecule is the one in which the bonded atoms carry formal charges as close to zero as possible.
• The formal charge formula is [ V.E – N.E – B.E/2].
• In H₂O, zero formal charges are present on the central oxygen atom.
• Both single-bonded hydrogen atoms also have zero formal charges in H₂
• The overall formal charge on H₂O is 0. It is thus a neutral molecule.