Valency can be thought of as a measure of the combining capacity of atoms. Different atoms can combine with each other by donating or accepting electrons or by sharing electrons. The valency of combining atoms gives us an idea about the possibility and type of combinations. The valency of an element can be determined by writing its electronic configuration, for which the atomic number is needed, and then using the octet rule to find out the number of electrons that can be donated or accepted to achieve a fully filled orbital.
For example, sodium (Na) has an atomic number of 11. Its electronic configuration can be written as 1s2, 2s2, 2p6, 3s1. In this case, sodium can either gain or donate 1 electron to have either fully filled 3s orbital or a completely vacant 3s orbital, respectively. A sodium atom chooses to donate this 1 electron, and hence its valency is 1. Chlorine (atomic number 17) will, on the other hand, prefer to gain 1 electron and hence has a valency of 1.
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