Understanding Hybridisation of PCl₃: Phosphorus Trichloride

Phosphorus trichloride (PCl₃) is a molecule with a trigonal pyramidal structure. It’s an excellent example of sp³ hybridisation in inorganic chemistry.

Let us understand how hybridisation happens in PCl₃. Read on to learn how it leads to its bonding and molecular shape.

What is the Hybridisation of PCl₃?

Phosphorus trichloride consists of one phosphorus atom and three chlorine atoms. Each chlorine atom is bonded to phosphorus. In order to form these bonds, phosphorus undergoes sp³ hybridisation.

Using the Hybridisation Formula

We can determine the hybridisation of phosphorus trichloride using the simple formula:

formula

Step-by-step calculation:

Valence electrons of central atom (P): 5
Monovalent atoms (Cl): 3
Negative charge: 0
Positive charge: 0

formula

Interpretation:

3 σ bonds + 1 lone pair → 4 regions of electron density → sp³ hybridisation

Breakdown of PCl₃ Hybridisation

Phosphorus trichloride consists of one pair of lone electrons and has a trigonal pyramidal molecular shape. It has 3 covalent single P–Cl bonds.

Here is a complete understanding of its hybridisation.

Electronic Configuration of Phosphorus

The atomic number of phosphorus is 15.

The ground state of phosphorus :

1s² 2s² 2p⁶ 3s² 3p³

Phosphorus has three unpaired electrons in 3p orbitals, enough to form 3 σ bonds; the lone pair remains in the 3s orbital.

Excited state configuration: Since PCl₃ can already form 3 bonds using the 3 unpaired electrons of p orbital, and the remaining lone pair will be in the s orbitals, it does not need any excited state.

Formation of Hybrid Orbitals

sp³ hybridisation occurs when 1 s orbital and 3 p orbitals mix.

The result:
→ 4 sp³ hybrid orbitals on phosphorus atom

→ 3 sp³ hybrid orbitals used for σ bonding with chlorine atoms
→ 1 sp³ hybrid orbital has the lone pair of electrons

Bond Formation in PCl₃

Each phosphorus atom uses:

3 sp³ orbital to form a σ bond with chlorine
1 sp³ orbital to have the lone pair

Result:

3 P–Cl σ bonds
1 Lone Pair (on the phosphorus atom)
Hybridisation type: sp³
Bond angle: ~107°
Geometry: Trigonal pyramidal

Geometry and bonding of PCl₃

Details At A Glance

Property	Details
Molecule	Phosphorus trichloride (PCl₃)
Hybridisation	sp³
Geometry	Trigonal pyramidal
Bond angle	~107°
Bonding	3 σ bonds (P–Cl), 1 LP (lone pair)
Unhybridised Orbitals	0 (hybridisation happens for all the valence orbitals)
Phosphorus valency satisfied?	Yes, by forming 3 bonds and having 1 lone pair of electron

Formal Charge in PCl₃

To determine if the Lewis structure of PCl₃ is stable, we calculate the formal charge on each atom using the formula:

Formal charge = Valence electrons – (Lone pair electrons + ½ × Bonding electrons)

Step-by-step for each atom:

Phosphorus (P) – central atom

Valence electrons: 5
Lone pairs: 1
Bonding electrons: 6 (from three single bonds with Cl)

Formal charge = 5 – (2 + ½×6) = 5 – (2 + 3) = 0

Chlorine (Cl) – each

Valence electrons: 7
Lone pairs: 3
Bonding electrons: 2 (1 single bond with Phosphorus)

Formal charge = 7 – (6 + ½×2) = 7 – (6 + 1) = 0

Thus, all atoms in PCl₃ carry zero formal charge, confirming that the Lewis structure is stable and correct.

Summing Up

The phosphorus in PCl₃ forms 3 single covalent bonds with chlorine and has one lone pair of electron. sp³ hybridisation leads to a trigonal pyramidal shape with ~107° bond angles. The decrease in bond angle is because of the presence of a lone pair.

Frequently Asked Questions

Q1. Why does Phosphorus undergo hybridisation in PCl₃?

To form 3 covalent bonds and arrange electrons such that minimal repulsion is present, phosphorus mixes orbitals to form hybrid orbitals.

Q2. How many σ and π bonds are present in PCl₃?

There are 3 σ bonds and 0 π bonds in total.

Q3. What is the shape of PCl₃?

Trigonal pyramidal, due to sp³ hybridisation and presence of lone pair.

Q4. Is PCl₃ polar or non-polar?

PCl₃ is polar, due to the asymmetrical electron distribution, as there is presence of lone pair of electron.

Q5. What are some uses of PCl₃ in our daily lives?

PCl₃ is commercially used as the chlorinating agent in chemical synthesis. It is also used in manufacturing organophosphorus compounds, pesticides, etc.