Valency

Valency is the combining power of an element. Elements in the same group of the periodic table have the same valency. The valency of an element is related to how many electrons are in the outer shell.

Table showing how the valency of an element is related to how many electrons are in the outer shell.

Group number	1	2	3	4	5	6	7	8
Valency	1	2	3	4	3	2	1	0

Group number
1
2
3
4
5
6
7
8

Valency
1
2
3
4
3
2
1
0

The noble gases have the valency 0 as they do not usually combine with other elements.

To write the chemical formula for a compound it is best to use the S.V.S.D.F system.

S - write down the symbols of both the elements involved.
V - beneath each symbol, write it’s valency. Memorising the above table is useful.
S - swap the valencies over.
D - if the valencies can be simplified, divide them both by the smaller of the two numbers. If one of the numbers is already one, then they cannot be divided and simplified any further.
F - write the formula.