Everything Science Grade 11

1.5 CHAPTER 1. ATOMICCOMBINATIONS

Example 6: Lewis notation: Atomswith variable valencies

QUESTION

Represent the moleculeH 2 S using Lewis notation

SOLUTION

Step 1 : Determine the numberof valence electrons foreach atom.

Hydrogen has an electron configuration of 1s^1 and sulphur has an electron con-

figuration of [Ne] 3s^2 3p^4. Each hydrogen atom has one valence electronwhich

is unpaired, and sulphur has six valence electrons. Since there are 2 hydrogen

atoms to 1 sulphur atom, sulphur must have a valency of 2.

2 H• S

×

××

××

×

Step 2 : Arrange the atoms in the molecule so that theoutermost energy levelin each

atom is full.

The H 2 S molecule is represented below.

H S

ו

××

××

ו

H

Another way of representing molecules is using Couper notation. In this case, only the electrons that
are involved in the bondbetween the atoms are shown. A line is used foreach covalent bond. Using
Couper notation, a molecule of water and a molecule of HCN wouldbe represented as shown in
Figures 1.3 and 1.4 below.

H

O

H

Figure 1.3: A water molecule represented usingCouper notation

H C N

Figure 1.4: A moleculeof HCN represented using Couper notation

Extension: Dative covalent bonds