How to calculate formal charges of POCl3 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 POCl₃ and also the overall charge present on the molecule.

So, continue reading!

How to calculate the formal charges on POCl₃ 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 determined 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 POCl_3.

The most preferred Lewis representation of POCl₃ is as shown below.

It consists of a total of 32 valence electrons. A Phosphorus (P) atom is present at the center, which is bonded to four atoms, including one oxygen (O) atom via a double covalent bond and three Chlorine (Cl) atoms, each via single covalent bonds.

No lone pair of electrons is present at the central P-atom. Each of the outer chlorine atoms contains 3 lone pairs of electrons, while the double-bonded O-atom contains 2 lone pairs of electrons.

It is the best possible Lewis structure of POCl₃ 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 POCl₃ Lewis structure.

For the central phosphorus atom

• Valence electrons of phosphorus = It is present in Group VA = 5 valence electrons
• Bonding electrons = 3 single bonds + 1 double bond = 3(2) + 2 (2) = 10 electrons
• Non-bonding electrons = no lone pair = 0 electrons
• Formal charge on the Phosphorous atom = 5 – 0 – 10/2 = 5 – 0 – 5 = 5 – 5 = 0

∴ The formal charge on the P-atom in POCl₃ is 0.

For oxygen atom

• Valence electrons of oxygen = It is present in Group VI A = 6 valence electrons
• Bonding electrons = 1 double bond = 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 POCl₃ is 0.

For each chlorine atom

• Valence electrons of chlorine = It is present in Group VII A = 7 valence electrons
• Bonding electrons = 1 single bond = 2 electrons
• Non-bonding electrons = 3 lone pairs = 3(2) = 6 electrons
• Formal charge on the Chlorine atom = 7 – 6 – 2/2 = 7 – 6 – 1 = 7 – 7 = 0

∴ The formal charge on each Cl-atom in POCl₃ is 0.

This calculation shows that zero formal charges are present on the central P-atom, one double-bonded O-atom, and three single-bonded Cl-atoms in the POCl₃ Lewis structure, as shown below. 

In conclusion, no overall charge is present on the phosphorus oxychloride (POCl₃) Lewis structure; hence it is a neutral molecule.

Also, check –

• How to draw POCl₃ 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
• ClO₃^– formal charge
• NH₃ formal charge
• CO formal charge
• H₂O 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 POCl_3, zero formal charges is present on the central P-atom.
• The O-atom also has a zero formal charge in POCl₃.
• The Cl-atoms also carry zero formal charges in POCl₃.
• The overall formal charge on POCl₃ is thus 0, and it is a neutral molecule.