Short Answer

Valency is the combining capacity of an element. It tells us how many electrons an atom can gain, lose, or share to form chemical bonds. Elements with similar valency often show similar chemical behaviour. For example, hydrogen has valency 1, oxygen has valency 2, and nitrogen has valency 3.

Valency helps explain how elements form compounds like H₂O, CO₂, and NaCl. It is based on the number of electrons in the outermost shell (valence electrons). Valency is one of the most important concepts in understanding chemical bonding and reactions.

Detailed Explanation :

Valency

Valency is a key concept in chemistry that helps us understand how and why atoms combine with each other. Since atoms are not stable on their own if their outermost shell is not completely filled, they form chemical bonds to achieve stability. Valency describes the ability of an atom to combine with other atoms by gaining, losing, or sharing electrons.

Different elements have different valencies depending on their electronic configuration. The valency of an element determines the type of compounds it can form and how it reacts with other elements. Understanding valency helps in writing chemical formulas, predicting reactions, and studying chemical structures.

Meaning of Valency

Valency is defined as:

“The combining capacity of an element, determined by the number of electrons an atom can gain, lose, or share during a chemical reaction.”

It is decided mainly by the number of valence electrons, which are the electrons present in the outermost shell of an atom.

Why Valency is Important

Valency helps us:

Understand how atoms combine
Predict the formulas of compounds
Write chemical equations correctly
Identify bonding patterns
Explain the behaviour of elements in reactions

Without valency, chemistry would not have a clear system for naming or understanding compounds.

How Valency is Determined

Valency depends on the number of electrons in the outermost shell:

If the atom has 1, 2, or 3 electrons in its outer shell

It will lose electrons, and its valency equals the number of electrons lost.

Examples:

Sodium (Na) → 1 electron in outer shell → valency = 1
Magnesium (Mg) → 2 electrons → valency = 2
Aluminium (Al) → 3 electrons → valency = 3

These elements are metals.

If the atom has 5, 6, or 7 electrons in its outer shell

It will gain electrons to complete 8 electrons.
Valency = 8 − number of valence electrons.

Examples:

Nitrogen (5 electrons) → valency = 3
Oxygen (6 electrons) → valency = 2
Fluorine (7 electrons) → valency = 1

These elements are non-metals.

If the outer shell has 4 electrons

The atom can gain or lose electrons.
Valency = 4
Example: Carbon → valency = 4

Examples of Valency in Elements

Here are simple examples to understand valency:

Hydrogen (H) → valency = 1
Oxygen (O) → valency = 2
Nitrogen (N) → valency = 3
Carbon (C) → valency = 4
Sodium (Na) → valency = 1
Calcium (Ca) → valency = 2

These numbers decide how these elements combine in compounds.

Valency in Chemical Compounds

The valency of elements helps determine the formula of compounds:

Water (H₂O)
Hydrogen valency = 1
Oxygen valency = 2
So, 2 hydrogen atoms combine with 1 oxygen atom.
Carbon dioxide (CO₂)
Carbon valency = 4
Oxygen valency = 2
Carbon shares electrons with 2 oxygen atoms.
Sodium chloride (NaCl)
Sodium valency = 1
Chlorine valency = 1
So, Na combines with Cl in a 1:1 ratio.

Without valency, chemical formulas would not make sense.

Variable Valency

Some elements, especially metals like iron, copper, and manganese, show variable valency. This happens because they can lose different numbers of electrons in different reactions.

Examples:

Iron shows valency 2 and 3
Copper shows valency 1 and 2

This explains why compounds like FeO (iron(II) oxide) and Fe₂O₃ (iron(III) oxide) exist.

Relation Between Valency and Periodic Table

The periodic table is arranged in such a way that elements in the same group have the same valency.

For example:

Group 1 → valency 1 (H, Li, Na, K)
Group 2 → valency 2 (Be, Mg, Ca)
Group 13 → valency 3
Group 14 → valency 4
Group 15 → valency 3
Group 16 → valency 2
Group 17 → valency 1

Group 18 elements have a valency of 0 because they have complete outer shells and do not react.

Valency and Chemical Bonding

Valency explains why atoms form:

Ionic bonds (by gaining or losing electrons)
Covalent bonds (by sharing electrons)

For example:

Sodium loses 1 electron → Na⁺
Chlorine gains 1 electron → Cl⁻
They combine to form NaCl.

Carbon shares 4 electrons to form covalent bonds.

Thus, valency is central to understanding bonding.

Conclusion

Valency is the combining capacity of an element and depends on the number of electrons in its outermost shell. It tells us how atoms bond by gaining, losing, or sharing electrons. Valency helps explain chemical formulas, bonding behaviour, and periodic table trends. Understanding valency is essential for learning how elements form compounds and participate in chemical reactions.