How is valency used to write chemical formulas?

Short Answer

Valency is used to write chemical formulas by determining how many atoms of each element must combine to form a stable compound. Each element has a specific valency, which tells how many electrons it can gain, lose, or share. By balancing the valencies of two elements, we can write the correct chemical formula.

For example, magnesium has valency 2 and chlorine has valency 1. To balance their valencies, one magnesium atom combines with two chlorine atoms, giving the formula MgCl₂. Thus, valency helps us identify the simple whole-number ratio in which elements combine.

Detailed Explanation :

Use of Valency to Write Chemical Formulas

Valency is one of the most important tools in chemistry because it helps determine how elements combine to form compounds. Chemical formulas represent the composition of compounds, showing the exact number of atoms of each element. To write these formulas correctly, we need to know the valency of each element involved. Valency allows us to balance the combining capacities of atoms so that the resulting compound is stable and electrically neutral.

When atoms combine, the total positive charge must equal the total negative charge. Valency tells us how many electrons an atom can gain, lose, or share during bonding. By using this information, we can find the correct ratio of atoms in a compound and write the chemical formula accurately.

Basic Idea Behind Using Valency

The core concept is:

Elements combine in such a way that their valencies balance, forming a neutral compound.

For example:

  • Sodium (valency 1) combines with chlorine (valency 1) → NaCl
  • Aluminium (valency 3) combines with oxygen (valency 2) → Al₂O₃

The valency method ensures that the final compound has no overall charge.

Steps to Write Chemical Formulas Using Valency

To write a chemical formula correctly, we follow these steps:

Step 1: Write the symbols of the combining elements

For example:
Magnesium and oxygen → Mg and O

Step 2: Write the valency of each element

Magnesium = 2
Oxygen = 2

Step 3: Cross-multiply the valencies

Place the valency of one element as the subscript of the other.

For Mg and O:

  • Mg gets 2 (from oxygen)
  • O gets 2 (from magnesium)

Result: Mg₂O₂

Step 4: Reduce to the simplest ratio

Mg₂O₂ reduces to MgO.

This is the final formula.

This method is often called the criss-cross method.

Examples of Writing Chemical Formulas Using Valency

Let us look at several examples to understand this process clearly.

Example 1: Sodium chloride

Sodium (Na): valency 1
Chlorine (Cl): valency 1

Cross-multiplying:
Na₁Cl₁ → NaCl

Final formula: NaCl

Sodium and chlorine combine in a 1:1 ratio because their valencies are equal.

Example 2: Magnesium chloride

Magnesium (Mg): valency 2
Chlorine (Cl): valency 1

Cross-multiplying:
Mg₁Cl₂ → MgCl₂

Final formula: MgCl₂

One magnesium atom combines with two chlorine atoms.

Example 3: Aluminium oxide

Aluminium (Al): valency 3
Oxygen (O): valency 2

Cross-multiplying:
Al₂O₃

Final formula: Al₂O₃

Two aluminium atoms combine with three oxygen atoms.

Example 4: Calcium fluoride

Calcium (Ca): valency 2
Fluorine (F): valency 1

Cross-multiplying:
Ca₁F₂ → CaF₂

Final formula: CaF₂

Example 5: Carbon tetrachloride

Carbon (C): valency 4
Chlorine (Cl): valency 1

Cross-multiplying:
C₁Cl₄ → CCl₄

Final formula: CCl₄

Carbon needs four chlorine atoms to complete its valency.

Role of Valency in Covalent Compounds

In covalent compounds, atoms share electrons but valency still decides how many bonds will form.

For example:

  • Hydrogen has valency 1
  • Oxygen has valency 2

So, oxygen forms two bonds with two hydrogen atoms → H₂O.

Water formula: H₂O

The same method applies even when bonding is covalent.

Why Valency Is Important in Writing Formulas

Valency helps us:

  • Predict how elements combine
  • Determine the correct number of atoms in a compound
  • Write formulas that follow chemical laws
  • Ensure compounds are electrically neutral
  • Avoid errors in chemical equations

Without valency, chemical formulas would be confusing and incorrect.

Valency and Polyatomic Ions

Sometimes, compounds contain polyatomic ions (groups of atoms with a charge).

For example:

  • Sulphate (SO₄²⁻)
  • Nitrate (NO₃⁻)
  • Ammonium (NH₄⁺)

Valency is applied in the same way.

Example:

Calcium sulphate:

  • Calcium (Ca²⁺): valency 2
  • Sulphate (SO₄²⁻): valency 2

Cross-multiplying gives Ca₂(SO₄)₂
Simplify → CaSO₄

Conclusion

Valency is used to write chemical formulas by ensuring that the combining capacities of atoms balance each other. By knowing how many electrons an element can gain, lose, or share, we can determine the correct ratio in which different atoms join to form a compound. The criss-cross method, along with knowledge of valency, allows us to write formulas like NaCl, MgCl₂, Al₂O₃, and many more accurately. Valency forms the foundation for understanding chemical bonding, composition, and reactions.