1.1 Covalent bonding and lewis notation  (Page 2/3)

The above examples all show single covalent bonds , where only one pair of electrons is shared between the same two atoms . If two pairs of electrons are shared between the same two atoms, this is called a double bond . A triple bond is formed if three pairs of electrons are shared.

You will have noticed in the above examples that the number of electrons that are involved in bonding varies between atoms. We say that the valency of the atoms is different.

Valency

The number of electrons in the outer shell of an atom which are able to be used to form bonds with other atoms.

In the first example, the valency of both hydrogen and chlorine is one, therefore there is a single covalent bond between these two atoms. In the second example, nitrogen has a valency of three and hydrogen has a valency of one. This means that three hydrogen atoms will need to bond with a single nitrogen atom. There are three single covalent bonds in a molecule of ammonia. In the third example, the valency of oxygen is two. This means that each oxygen atom will form two bonds with another atom. Since there is only one other atom in a molecule of O ${}_2$ , a double covalent bond is formed between these two atoms.

Covalent bonding and valency

1. Explain the difference between the valence electrons and the valency of an element.
2. Complete the table below by filling in the number of valence electrons and the valency for each of the elements shown:

   Element	No. of valence electrons	No. of electrons needed to fill outer shell	Valency
   F
   Ar
   C
   N
   O

3. Draw simple diagrams to show how electrons are arranged in the following covalent molecules:
   1. Water (H ${}_2$ O)
   2. Chlorine (Cl ${}_2$ )