Everything Science Grade 11

1.10 CHAPTER 1. ATOMICCOMBINATIONS

Step 2 : Count the number of electron pairs around thecentral atom

There are three electronpairs.

Step 3 : Determine the basic geometry of the molecule

Since there are three electron pairs, the molecule must be trigonal planar.

Extension: More about molecular shapes

Determining the shapeof a molecule can be abit more complicated. In the examples we

have used above, we looked only at the number ofbondingelectronpairs when we were trying

to decide on the molecules’ shape. But there are also other electron pairs in the molecules.

These electrons, whichare not involved in bonding but which are alsoaround the central

atom, are called lone pairs. The worked examplebelow will give you anidea of how these

lone pairs can affect theshape of the molecule.

Example 15: Molecular shape III

QUESTION

Determine the shape ofa molecule of NH 3

SOLUTION

Step 1 : Draw the molecule using Lewis notation

lone pair of electrons

H N H

H

ו

ו××

ו

Step 2 : Count the number of electron pairs around thecentral atom

There are four electron pairs.

Step 3 : Determine the basic geometry of the molecule

Since there are four electron pairs, the moleculemust be tetrahedral.

Step 4 : Determine how many lone pairs are around the central atom

There is one lone pair of electrons and this willaffect the shape of the molecule.

Step 5 : Determine the final shape of the molecule

The lone pair needs more space than the bonding pairs, and thereforepushes

the three hydrogen atoms together a little more.The bond angles between the

hydrogen and nitrogenatoms in the molecule become 106 degrees, rather than

the usual 109 degrees of a tetrahedral molecule. The shape of the molecule is