Variable Valency: Learn Why Some Elements Show More than One Valency

Did you ever wonder why some elements do not seem to stick to just one bonding style? One minute, they're bonding with an element using one valency and the next, they are presenting a completely different valency! This is known as variable valency.This Variable valency guide will outline what valency is, why some elements have variable valency, how it varies, and how you can detect it by using simple formulas. 

Table of Contents

• What is Valency
• What is Variable Valency
• What is Electrovalency
• Common Examples of Variable Valency
• How to Find Valency from Compound Formulas
• Quick Tips to Learn Variable Valency

What is Valency?

Valency of elements refers to the combining capacity of an element - simply put, it’s the number of electrons an atom needs to gain, lose, or share to achieve stability. Most atoms aim to fill their outermost shell (usually 8 electrons), and they bond accordingly.

For example:

• Oxygen has a valency of 2 because it needs 2 electrons to complete its outer shell.
• Sodium has a valency of 1 because it loses 1 electron to become stable.

What is Variable Valency

While many elements have a fixed valency, some can exhibit more than one valency. This means they can bond in multiple ways, forming different compounds with different chemical formulas. This property is known as variable valency.

This usually happens in transition metals and elements that have more than one stable oxidation state. The electrons in their inner orbitals can also take part in bonding, allowing multiple valency states.

What is Electrovalency?

Electrovalency refers to the valency shown by an element when it forms an ionic bond, where it completely donates or accepts electrons.Some elements show variable electrovalency because they can lose different numbers of electrons depending on the chemical environment. This is commonly seen in:

• Transition metals (like iron, copper, chromium)
• Elements that form both lower and higher oxidation states

That is: Two major factors explain why certain elements exhibit variable valency:

• In transition elements, the small energy gap between the ns and (n–1)d orbitals, and in inner transition elements between ns and (n–2) orbitals, allows different numbers of electrons to be involved in bonding.
• In p-block elements, the inert pair effectmakes the outer s-electrons less likely to participate in bonding.

For example:

• Iron (Fe) can show +2 and +3 oxidation states (Fe²⁺ and Fe³⁺).
• Copper (Cu) shows +1 and +2 valency (Cu⁺ and Cu²⁺).

Another one of the inert pair effects explains why heavier p-block elements sometimes show lower oxidation states than expected. 

Read More: Atomic number and Mass-number

But what exactly is the Inner Pair Effect?

The inert pair effect is when the two s-electrons (ns²) in the valence shell of heavy p-block elements prefer to stay “inactive” or inert, instead of joining in chemical bonding. 

As atoms get heavier, their outermost s-electrons are held more tightly by the nucleus due to poor shielding by inner electrons. This makes it harder for them to participate in reactions, leaving only the p-electrons to form bonds.

In Groups III A and IV A, lighter elements usually form +3 and +4 states, but as we move down the groups, +1 and +2 states become more stable. 

This happens because the two outer s-electrons (ns²) stay “inert,” held tightly by the nucleus and less willing to bond. 

As a result, elements like thallium (+1) and lead (+2) prefer lower oxidation states, a key idea behind variable valency.

Common Examples of Variable Valency

Let’s look at a few common elements that exhibit variable valency:

• Copper (Cu):

• Cuprous compounds: Cu⁺ → Valency = 1
• Cupric compounds: Cu²⁺ → Valency = 2

• Iron (Fe):

• Ferrous sulfate (FeSO₄): Fe²⁺ → Valency = 2
• Ferric chloride (FeCl₃): Fe³⁺ → Valency = 3

• Tin (Sn):

• Stannous chloride (SnCl₂): Sn²⁺ → Valency = 2
• Stannic oxide (SnO₂): Sn⁴⁺ → Valency = 4

• Lead (Pb):

• Lead(II) nitrate (Pb(NO₃)₂): Pb²⁺ → Valency = 2
• Lead(IV) oxide (PbO₂): Pb⁴⁺ → Valency = 4

These different forms lead to different chemical and physical properties in the compounds they form.

How to Find Valency from Compound Formulas

Even if you're not given the valency of an element directly, you can easily determine it from the chemical formula of a compound.

Here’s how:

• Identify the valency of the other known element in the compound.
• Use the formula to balance the total valency.

Example:

In FeCl₃, we know that chlorine (Cl) has a valency of 1. 

There are three Cl atoms, so the total negative charge = 3. 

Therefore, Fe must have a valency of 3 to balance it → Fe³⁺.

Another example:

In Cu₂O, oxygen (O) has a valency of 2. 

Two Cu atoms = 2 positive charges, 

which means each Cu must be +1 → Cu⁺.

Quick Tips to Learn Variable Valency

• Understand that some elements can lose different numbers of electrons, which leads to variable valency.
• Learn common examples like iron showing valency of 2 and 3, and copper showing 1 and 2.
• Remember that variable valency often comes from the use of inner orbitals such as d-orbitals.
• Know that variable valency results in different oxidation states in different compounds.

Also Read: Atoms and Molecules

Variable valency helps us better understand how elements behave differently in a variety of chemical environments. By learning how to read and interpret compound formulas, you'll be able to identify valencies confidently-a must-have skill for any budding chemist.