Predict the shape of the molecule using Steric Number

Steric number is the number of atoms bonded to a central atom of a molecule plus the number of lone pairs attached to the central atom.

The steric number of a molecule is used in VSEPR (valence shell electron pair repulsion) theory to determine the molecular geometry of a molecule.

Steric Number helps to predict the geometry and shape of the molecule easily.

HOW TO FIND THE STERIC NUMBER

Use the Lewis structure to determine the steric number. The steric number gives the electron-pair arrangement for the geometry that maximizes distance between valence electron pairs.

To minimize the repulsion between Valence shell electrons , the distance between them should be maximum.

When the distance between valence electrons is maximized, the energy of the molecule is at its lowest state.

When the Energy (potential energy) of the molecule is lowest , molecule is in its most stable arrangement.

The steric number is calculated using the following formula:

Steric Number = (number of lone electron pairs on the central atom) + (number of atoms bonded to the central atom)

or 

Steric Number = number of lone pairs of electrons + number of σ (sigma) bonds formed by central atom

Steric Number Hybridization* Geometry Examples 
2 sp linear BeCl2
3 sp2 trigonal planar BF3
4 sp3 Tetrahedral CH4
5 sp3d Trigonal bipyramidal PCl5
6 sp3d2 Octahedral

or square bipyramidal

SF6
7 sp3d3 pentagonal bipyramidal IF7
1 pure s spherical H atom

* Steric number can be used to predict the hybridization. 

Examples for Calculation of Steric Number 

(i) Methane (CH4) – Methane consists of carbon bonded to 4 hydrogen atoms and 0 lone pairs. Steric numer is 4.

Water (H2O) – Water has two hydrogen atoms bonded to oxygen and also 2 lone pairs, so its steric number is 4.

Ammonia (NH3) – Ammonia also has a steric number of 4 because it has 3 hydrogen atoms bonded to nitrogen and 1 lone electron pair.

 

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