Valency – Definition, Examples and How to Determine It

Valency is an important concept in Chemistry that helps us understand how atoms form bonds and write chemical formulae correctly.

What is Valency?

Valency is the number of electrons an atom uses to form bonds with other atoms.

Atoms gain, lose or share valence electrons to achieve stable noble gas electronic configuration. Valence electrons are the electrons present in the outermost shell of an atom.

Examples of Valency

Hydrogen (H)

Hydrogen has 1 electron in its first shell. It needs 1 more electron to achieve a stable duplet configuration. (Remember, first shell can only hold 2 electrons.) Hydrogen uses 1 electron to form a bond, hence its valency is 1.

Magnesium (Mg)

Magnesium has 12 electrons (2,8,2). It is easier to lose 2 electrons than to take in 6 electrons to achieve stable octet configuration. It loses its 2 valence electrons to form ionic bond, hence its valency of Mg is 2.

Carbon (C)

Carbon uses all its 4 valence electrons to form bonds, hence it has a valency of 4.

Oxygen (O)

Oxygen has 8 electrons (2,6). It has 6 valence electrons, however it only uses 2 of its outermost electrons in bonding to achieve stable octet electronic configuration. Hence, valency of oxygen is 2.

Valency of Common Elements

Here are the valencies of some common elements:

Element	Valency	Type of Bond Usually Formed
Hydrogen (H)	1	Covalent
Oxygen (O)	2	Covalent
Nitrogen (N)	3	Covalent
Carbon (C)	4	Covalent
Chlorine (Cl)	1	Covalent / Ionic
Sodium (Na)	1	Ionic
Magnesium (Mg)	2	Ionic
Aluminium (Al)	3	Ionic

Valency of Noble Gases

All elements in group 0 (Modern: 18) of the Periodic Table have a valency of 0. They already have a full outer shell and have achieved stable noble gas electronic configuration. Therefore, they do not gain, lose, or share electrons and are chemically unreactive (inert).

Click here to learn how to draw dot and cross diagrams of covalent molecules.