How do you determine steric numbers?

The concept of steric number is primarily used in VSEPR theory (Valence Shell Electron Pair Repulsion theory) to predict the geometry of molecules based on the arrangement of electron pairs around a central atom. The steric number determines the number of regions of electron density around the central atom.

To determine the steric number, follow these steps:

Draw the Lewis structure of the molecule, which shows the connectivity of atoms and the location of lone pairs.

Count the number of electron pairs (both bonding pairs and lone pairs) around the central atom. A bonding pair is a pair of electrons involved in a covalent bond, and a lone pair is a pair of electrons that are not involved in bonding and reside on the central atom.

Add up the number of bonding pairs and lone pairs to calculate the steric number.

The steric number corresponds to a specific molecular geometry based on VSEPR theory. The most common geometries are:

Steric number 2: Linear geometry (180° bond angle).
Steric number 3: Trigonal planar geometry (120° bond angles).
Steric number 4: Tetrahedral geometry (109.5° bond angles).
Steric number 5: Trigonal bipyramidal geometry (90° and 120° bond angles).
Steric number 6: Octahedral geometry (90° bond angles).
Note that the steric number is determined by the total number of electron pairs, regardless of whether they are bonding or lone pairs. It provides a general framework for predicting molecular geometries based on the repulsion between electron pairs.