Topic 14.1: Shapes of molecules and ions

14.1.1 Predict the shape and bond angles for species with five and six negative charge centres using the VSEPR theory.

Expanded Octet is when compounds have more than eight electrons around the central atom is possible because the d orbitals avaliable in the valence shell of these atoms have energy values relatively close to those of the p orbitals. So promtions of electrons from 3p to 3d orbitals will allow additional electron pairs to form. This is how elements such as phosphorus and sulfur expand their octets forming species with five or six charge centers.

This table shows all the shapes you need to know. Remember the names for all of the different shapes.

A is central atom
X are other atoms
E are lone pairs

Five negative charge centres.

Triangular bipyramidal

Consists of 90 degrees, 120 degrees and 180 degree bonds.

Seesaw

With one pair of lone pair, it reduces the degree is reduced from 120 to 117 between the angles

T-shaped

With two pairs of lone pair, the bond angles are 90 degrees and 180 degrees.

Linear

It is just 180 degrees

Six negative charged centres

Octahedral

All 90 degrees or 180 degrees bond.

Square Pyramidal

All degrees are <90 degrees or <180 degrees.

Square Planar

All angles are 90 degrees or 180 degrees

Summary

2 charge centers are linear

3 charge centers are planar triangular
With lone pair V-shaped

4 charge centers are tetrahedral
With lone pair Pyramidal
With 2 lone pair V-shaped

5 charge centers are triangular bipyramidal
With lone pair See-saw
With 2 lone pair T-shaped
With 3 lone pairs Linear

6 charge centers are octahedral
With lone pair Square pyramidal
With 2 lone pair Square planar