Valency

It is the combining capacity of an element. It is the number of electrons in an atom that actually take part in bond formation. For example, carbon atom has 4 valence electrons.

K LC₍₆₎ 2  4

Carbon atom is capable of forming 4 bonds. Hence, its valency is four.

Calculation of valency

• The number of valence electrons is the valency of the element.

• The valency of an element can also be calculated by finding the number of electrons required to complete octet (8).

i.e., 8 number of valence electrons = valency

Example

Cl - 17 : 2, 8, 7

Valency = 8 - Number of valence electrons

= 8 - 7

= 1

Example

0 - 8 : 2, 6

Valency = 8 - Number of valence electrons

= 8 - 6

= 2

Let us test our understanding of valency and valence electrons before we proceed ahead.

Example 1

What is the number of valence electrons, valency and valence shell of the atom whose atomic number is 17?

K, L, M

Electron configuration : 2, 8, 7

Number of valence electrons = 7

Valency = 8 - 7 = 1

Valence shell = M

Example 2

Which of the two elements would be more reactive, Element A of atomic number 36 or element B of atomic number 19?

A (36) Electron configuration : 2, 8, 18, 8

B (19) Electron configuration : 2, 8, 8, 1

Since the outermost shell of element A is completely filled it is stable and unreactive.

Element B has one electron in its valence shell thus it is more reactive.