Class 9 Science – Atoms and Molecules

Class 9 Science – Atoms and Molecules

Introduction

All matter is made up of tiny particles called atoms and molecules. Atoms combine to form molecules. This chapter explains the laws of chemical combination, atomic and molecular masses, and the mole concept.

Laws of Chemical Combination

1. Law of Conservation of Mass

  • Mass is neither created nor destroyed in a chemical reaction.
  • Total mass of reactants = Total mass of products.

Example:
Hydrogen + Oxygen → Water
Mass before reaction = Mass after reaction

2. Law of Constant Proportions

  • A pure compound always contains the same elements in fixed proportion by mass.
  • Example: Water always has hydrogen and oxygen in a fixed ratio.

Atoms

  • Smallest particle of an element that takes part in a chemical reaction.
  • Atoms are very small and cannot exist freely (except noble gases).

Atomic Mass

  • Mass of an atom compared to 1/12th mass of carbon-12 atom.
  • Unit: atomic mass unit (amu or u).

Molecules

  • Group of two or more atoms chemically bonded.
  • Can exist independently.

Types of Molecules

  1. Molecules of Elements
    • Made of same atoms.
    • Example: O₂, N₂, H₂
  2. Molecules of Compounds
    • Made of different atoms.
    • Example: H₂O, CO₂, NH₃

Molecular Mass

  • Sum of atomic masses of all atoms in a molecule.

Example:
Molecular mass of H₂O
= 2 × 1 (H) + 16 (O)
= 18 u

Mole Concept

  • A mole is a unit used to measure the amount of substance.
  • 1 mole = 6.022 × 10²³ particles (Avogadro’s number).

Relation:

  • Number of moles = Given massMolar mass\frac{\text{Given mass}}{\text{Molar mass}}Molar massGiven mass​

Molar Mass

  • Mass of 1 mole of a substance.
  • Unit: grams per mole (g/mol).

Example:
Molar mass of CO₂ = 44 g/mol

Importance of Atoms and Molecules

  • Helps in understanding chemical reactions.
  • Used in calculating reactants and products.
  • Important for stoichiometry and numericals.

Quick Short Q&A (Most Possible)

QuestionShort Answer
What are atoms?Smallest particles of elements.
What are molecules?Group of bonded atoms.
State law of conservation of mass.Mass is neither created nor destroyed.
Law of constant proportions?Fixed mass ratio in compounds.
Atomic mass unit?1/12th mass of carbon-12 atom.
Molecular mass?Sum of atomic masses.
What is a mole?Amount containing 6.022×10²³ particles.
Avogadro’s number?6.022 × 10²³
Molar mass unit?g/mol
Example of element molecule?O₂
Example of compound molecule?H₂O