How to calculate formal charges of POCl3 with lewis structure?

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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!

Name of the molecule Phosphorus oxychloride
Chemical formulaPOCl3
The formal charge on the P-atom0
The formal charge on O-atom0
The formal charge on Cl-atoms0
The overall formal charge on POCl30

How to calculate the formal charges on POCl3 atoms?

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

formal charge formula

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 POCl3.

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

best stable lewis structure of POCl3

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

calculating formal charge on P atom in POCl3

  • 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

calculating formal charge on double bonded oxygen atom in POCl3

  • 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

calculating formal charge on chlorine atom in POCl3

  • 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. 

POCl3 lewis structure with formal charge

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

pocl3 formal charge

Also, check –

FAQ

How can you calculate POCl3 formal charges?

The formal charges present on the bonded atoms in POCl3 can be calculated using the formula given below:

V.E – N.E – B.E/2

Where –

⇒ V.E = valence electrons of an atom

⇒ N.E = non-bonding electrons, i.e., lone pairs

⇒ B.E = bonding electrons

What is the formal charge on P-atom in POCl3?

The central phosphorus (P) atom carries zero formal charges in POCl3.

What is the formal charge on O-atom in POCl3?

Zero or no formal charge is present on the oxygen (O) atom in POCl3.

What is the formal charge on Cl-atoms in POCl3?

Zero or no formal charge is present on either of the three chlorine (Cl) atoms in POCl3.

What is the overall formal charge on POCl3?

The overall formal charge on POCl3 is 0.

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 POCl3, 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.
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