Molecular Geometry

Question 0

No. of sets electron pairs / geometry of sets of electron pairs

Answer 0

2 / linear
3 / trigonal planar
4 / tetrahedral
5 / trigonal bi pyramidal 
6 / octahedral

Question 1

Steps to determining the shape of a molecule using the VSEPR theory:

Answer 1

1. Draw the Lewis structure of the molecule
2. Count the number of sets of bonding pairs and lone pairs around any inner atom, then use that number (between 2-8 pairs) to determine the optimum geometry of these sets.
3. Modify the geometry if necessary, to take account of the fact that magnitudes of repulsions between electron pairs.

Question 2

Repulsion strength relationship between bonding pairs and lone pairs

Answer 2

LP-LP > BP-LP > BP-BP

Lone pairs occupy a larger volume than bonding pairs.

Question 3

Two sets of EP: linear geometry

Answer 3

BeH2 for example: H-Be-H
We have two sets of electron pairs around the inner atom that we need to situate as far away from each other as possible. This is satisfied by a linear arrangement. No geometrical adjustment is necessary, as there are only two BPs, and no LPs.

Question 4

Three sets of EP: trigonal planar geometry

Answer 4

BF3 for example:

We have three sets of electron pairs around the inner atom. Optimum arrangement is 120 degrees apart. This leads to a trigonal planar and are coplanar. As all the electron pairs are equivalent, we do not adjust their optimal geometry.

Question 5

Four sets of EPs: tetrahedral

Answer 5

For example methane:
It has four single C-H single covalent bonds, therefore the four sets of electron pairs are equivalent.

Ammonia, NH3:
Four electron pairs in total: 3 BPs and 1 LP. We have BP-BP and BP-LP bonds to consider. The single lone pair on the N atom takes up more space than each of the three BPs and this distorts the shape to 107 degrees between each atom.