A molecule is the smallest particle of an element or compound that can exist independently and shows its properties.

What is Avogadro number?

Avogadro number is 6.022 x 10^23, the number of particles present in one mole of a substance.

Class 10

Tamil Nadu Class 10 Science Chapter 7 - Atoms and Molecules

Samacheer Kalvi Class 10 Science Atoms and Molecules guide with atomicity, molecular mass, mole concept, Avogadro number, formula mass, important questions, and revision checklist.

Updated 6 June 20264 min read

Quick Summary

Atoms and molecules explain how matter is built. This chapter is important for atomic mass, molecular mass, valency, chemical formulae, mole concept, and numerical problems.

Key Definitions

Atom: Smallest particle of an element that takes part in chemical reactions.
Molecule: Smallest particle that can exist independently and retain the properties of a substance.
Atomicity: Number of atoms present in one molecule of an element.
Valency: Combining capacity of an atom.
Mole: Amount of substance containing 6.022 x 10^23 particles.
Molar mass: Mass of one mole of a substance.

Important Laws

Law of Conservation of Mass

Mass can neither be created nor destroyed in a chemical reaction.

Law of Constant Proportions

A pure chemical compound always contains the same elements combined in the same fixed proportion by mass.

Atomicity Examples

Type	Example	Atomicity
Monoatomic	Helium	1
Diatomic	Hydrogen, Oxygen	2
Triatomic	Ozone	3
Polyatomic	Phosphorus	4 or more

Formulae and Calculations

Concept	Method
Molecular mass	Sum of atomic masses of all atoms in the molecule
Formula mass	Sum of atomic masses of ions in an ionic compound
Number of moles	`given mass / molar mass`
Number of particles	`moles x 6.022 x 10^23`

Mole Concept

One mole of any substance contains Avogadro number of particles.

1 mole = 6.022 x 10^23 particles
Number of moles = mass / molar mass

For example, one mole of oxygen molecules contains 6.022 x 10^23 oxygen molecules.

Worked Examples

Example 1 — Molecular mass

Find the molecular mass of water (H₂O). (H = 1, O = 16)

= (2 × 1) + (1 × 16) = 2 + 16 = 18 u

Example 2 — Molecular mass of a compound

Molecular mass of CaCO₃. (Ca = 40, C = 12, O = 16)

= 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100 u

Example 3 — Mass to moles

How many moles are in 36 g of water?

moles = mass / molar mass = 36 / 18 = 2 moles

Example 4 — Moles to particles

Number of molecules in 2 moles of water.

= moles × 6.022 × 10²³ = 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules

Example 5 — Mass of given particles

Mass of 3.011 × 10²³ atoms of carbon (atomic mass 12).

moles = (3.011 × 10²³)/(6.022 × 10²³) = 0.5 mol
mass = moles × molar mass = 0.5 × 12 = 6 g

Textbook Solution Strategy

1. Identify whether the question asks for atoms, molecules, ions, moles, or mass. 2. Write the given mass and molar mass. 3. Use moles = mass / molar mass. 4. Convert moles to particles if needed using Avogadro number. 5. Keep units clear in the final answer.

Important Questions

One Mark

Define atom.
What is atomicity?
Write the value of Avogadro number.

Two Marks

Differentiate between atom and molecule.
Define valency with an example.
Calculate the molecular mass of water.

Five Marks

Explain the mole concept with formulae.
State and explain the law of conservation of mass.
Solve a numerical problem involving mass, molar mass, and number of particles.

Common Mistakes

Writing atomic mass and molecular mass as the same thing.
Forgetting to multiply by the number of atoms in a formula.
Missing 10^23 while writing Avogadro number.
Not converting given mass into moles before finding particles.

Revision Checklist

Definitions of atom, molecule, atomicity, valency revised
Molecular mass calculations practised
Mole formula memorised
Avogadro number written correctly
At least five numerical problems solved

Frequently Asked Questions

An atom is the smallest particle of an element that takes part in a chemical reaction.