Molecule Geometry

What is the Steric Number?

The Steric Number is the sum of the number of atoms bonded to the central atom and the number of lone pairs on the central atom. It determines the electron geometry according to VSEPR theory.

How to determine molecular geometry using VSEPR Theory:

1. Count Electron Domains (Steric Number): Determine the total number of electron regions around the central atom. This includes every bond (single, double, or triple counts as one domain) and every lone pair.
2. Identify Electron Geometry: Based on the Steric Number, determine the arrangement of all electron domains. This is the 'base' geometry for the molecule.
3. Determine Molecular Shape: Account for lone pairs. Lone pairs occupy space but are not visible in the molecular shape. Their stronger repulsion influences the spatial arrangement of the bonding pairs, leading to the final molecular geometry (which may differ from the electron geometry).

Steric Number = 2

• Electron Geometry: Linear. This refers to the arrangement of all electron domains (bonds and lone pairs) around the central atom.
• Molecular Shape (0 Lone Pairs): Linear. When there are no lone pairs, the molecular shape is identical to the electron geometry. The bond angle is 180^\circ.

Steric Number = 3, 0 Lone Pairs

• Electron Geometry: Trigonal Planar. All electron domains are arranged in a flat, triangular shape.
• Molecular Shape: Trigonal Planar. The molecular shape matches the electron geometry due to no lone pair repulsion. Bond angles are generally 120^\circ.

Steric Number = 3, 1 Lone Pair

• Electron Geometry: Trigonal Planar (Still the overall electron domain arrangement).
• Molecular Shape: Bent. The lone pair exerts more repulsion, pushing the bonding pairs closer together, resulting in bond angles of <120^\circ (e.g., in SO_2).

Steric Number = 4, 0 Lone Pairs

• Electron Geometry: Tetrahedral. All electron domains point towards the corners of a tetrahedron.
• Molecular Shape: Tetrahedral. The molecular shape is tetrahedral with ideal bond angles of 109.5^\circ (e.g., in CH_4).