Chemical Formula

Last Updated : 23 Apr, 2026

A chemical formula is a symbolic representation of a chemical substance. It shows the types of elements present, the number of atoms of each element in one molecule or formula unit and the ratio in which atoms are combined.

Chemical formulas use:

Symbols of elements (H, O, Na, Cl, etc.)
Subscripts (small numbers written below) show the number of atoms.
Sometimes brackets ( ), +, − signs for ions or complex compounds.

Example: Water → H₂O, Carbon dioxide → CO₂

Features of Chemical Formula

1. It represents the name of the compound in symbolic form.

2. It shows the elements present in the compound.

3. It tells the exact number of atoms of each element.

4. It represents one molecule or one formula unit of a substance.

5. It helps in calculating:

Molecular mass
Molar mass
Chemical reactions

6. It shows the definite mass ratio in which elements combine.

Types of Chemical Formulae

A chemical formula is a symbolic representation of a substance that shows the elements present and the number of atoms of each element in a molecule or compound. Different types of chemical formulas are used to represent substances in various ways, depending on the information required.

The main types of chemical formulas are discussed below:

1. Molecular Formula

Shows the actual number of atoms of each element in one molecule.
Written using element symbols and subscripts.

Compound	Molecular Formula
Urea	CH₄N₂O
Acetic Acid	CH₃COOH
Glucose	C₆H₁₂O₆
Methane	CH₄

2. Empirical Formula

Shows the simplest whole number ratio of atoms.
Does not show actual number, only ratio.

Compound	Molecular Formula	Empirical Formula
Urea	CH₄N₂O	CH₄N₂O
Acetic Acid	C₂H₄O₂	CH₂O
Glucose	C₆H₁₂O	CH₂O

3. Structural Formula

Shows the arrangement of atoms in a molecule.
Displays how atoms are bonded.

Example:

👁 structural_formula_of_acetic_acide

Valency

Valency is the combining capacity of an atom, i.e., the number of electrons an atom can lose, gain, or share to form a stable compound.

Atoms try to achieve a stable outer shell (like noble gases).
Metals usually lose electrons, so they have positive valency.
Non-metals usually gain electrons, so they have negative valency.

Examples:
Aluminium (Al) → valency = 3 (loses 3 electrons)
Oxygen (O) → valency = 2 (gains 2 electrons)

Ions

An ion is an atom or group of atoms that carries a charge due to loss or gain of electrons.

Cation → positively charged ion (loss of electrons)

Example: Al ³⁺

Anion → negatively charged ion (gain of electrons)

Example: O ^2-

Rules for Writing Chemical Formula

The chemical formula is written by balancing the valencies of the combining elements so that the overall compound becomes electrically neutral.

👁 al

Step 1: Write the symbols of the elements

Aluminium → Al
Oxygen → O

Step 2: Write their valencies

Aluminium (Al) → 3
Oxygen (O) → 2

Step 3: Find the LCM of valencies

LCM of 3 and 2 = 6

Step 4: Cross the valencies

Al gets 2 (from O)
O gets 3 (from Al)
Formula becomes → Al₂O₃

Step 5: Write metal first, then non-metal

Aluminium (metal) comes first, then Oxygen → Al₂O₃

Step 6: Simplify if possible

2 and 3 cannot be simplified further → final formula remains Al₂O₃

Examples:

1. Chemical Formula of Rust:

Elements present in Rust are Iron and Oxygen.
Symbol
Iron, Fe (26)
Oxygen, O (8)
Valency
3
2
Lowest common multiple of valency is 6. Therefore,
Fe = 2 × 3 = 6
O = 3 × 2 = 6
This implies that the chemical formula is Fe₂O_3.

Symbol	Iron, Fe (26)	Oxygen, O (8)
Valency	3	2

2. Chemical Formula of Aluminium Oxide:

The elements present in Aluminium Oxide are Aluminium and Oxygen.
Symbol
Aluminium, Al (13)
Oxygen, O (8)
Valency
3
2
The lowest common multiple of valency is 6. Therefore,
Al = 2 × 3 = 6
O = 3 × 2 = 6
This implies that the chemical formula is Al₂O₃.

Symbol	Aluminium, Al (13)	Oxygen, O (8)
Valency	3	2

3. Chemical Formula of Methane:

Elements present in Methane are Carbon and Hydrogen.
Symbol Carbon, C (6) Hydrogen, H (1)
Valency
4
1
Lowest common multiple of valency is 4. Therefore,
C = 1 × 4
H = 4 × 1
This implies that the chemical formula is CH₄.

Symbol	Carbon, C (6)	Hydrogen, H (1)
Valency	4	1

4. Chemical Formula of Sodium Chloride:

Elements present in Sodium Chloride are Sodium and Chloride.
Symbol Sodium, Na (11) Chlorine, Cl (17)
Valency
1
1
Lowest common multiple of valency is 1. Therefore,
Na = 1
Cl = 1
This implies that the chemical formula is NaCl

Symbol	Sodium, Na (11)	Chlorine, Cl (17)
Valency	1	1

5. Chemical Formula of Calcium Oxide:

Elements present in Calcium Oxide are Calcium and Oxygen.
Symbol Calcium, Ca (20) Oxygen, O (8)
Valency
2
2
Lowest common multiple of valency is 2. Therefore.
Ca = 1 × 2
O = 1 × 2
This implies that the chemical formula is CaO.

Symbol	Calcium, Ca (20)	Oxygen, O (8)
Valency	2	2