Steric Number

What is a Steric Number?

The Steric number is the total number of atoms directly bonded to a central metal atom and the number of lone pairs attached. Steric number plays an essential role in VSEPR (Valence Shell Electron Pair Repulsion) theory in assessing the molecular geometry.

• It plays a vital role in determining the shape of the molecule.
• It also helps in evaluating the final product of organic reactions.

Table of Content

• What is a VSEPR Theory
• Postulates of VSEPR Theory
• How to find Steric Number
• Examples of Steric Number
• Frequently Asked Questions – FAQs

What is a VSEPR Theory?

The VSEPR theory is used to depict the shape of molecules. It is based on the principle that there is repulsion in between valence electrons, due to which atoms tend to arrange themselves in a way that experiences the most negligible repulsion.

The arrangement of molecules in the three dimensions determines the molecule’s geometry.

Postulates of VSEPR Theory

• There is a central atom in a polyatomic molecule to which all other atoms are joined.
• The total number of valence shell electron pairs determines the molecule’s shape.
• The electron pairs tend to arrange themselves to exert minimum repulsion.
• Repulsion is maximum between two lone pairs and minimum between two bond pairs.
• If both lone pairs and bond pairs surround the central atom, the molecule would have a distorted shape.

How to find a Steric Number?

We can calculate steric numbers using the formula:

Steric Number=½ X [(number of valence electron) + (number of monovalent atoms attached to the central atom) – (cationic charge) + (anionic charge)]

Once the steric number is calculated, we can quickly assess the molecule’s shape.

Examples of Steric Number

Steric Number 2

Carbon dioxide is an example of a molecule having steric number 2.

½ X [4+0-0+0] = 2

.

Steric Number 3

Boron trifluoride is an example of a molecule having steric number 3.

½ X [3+3-0+0] = 3

Steric Number 4

H₂O, NH₃ and CH₄ are examples of molecules having steric number 4.

For H₂O: ½ X [6+2-0+0] = 4

For NH₃: ½ X [5+3-0+0] = 4

For CH₄: ½ X [4+4-0+0] = 4

Steric Number 5

PCl₅ is an example of a molecule having the steric number 5.