Class 9th Chemistry - Atoms and Molecules

Atoms and Molecules - Class 9th Chemistry

Understanding the basic building blocks of matter is fundamental to chemistry.

Atoms

Atoms are the smallest particles of an element that retain the properties of that element.

• Atomic Number: Number of protons in the nucleus
• Mass Number: Sum of protons and neutrons in the nucleus
• Isotopes: Atoms of the same element with different mass numbers

Molecules

Molecules are groups of two or more atoms held together by chemical bonds.

• Molecular Formula: Shows the actual number of atoms in a molecule
• Empirical Formula: Shows the simplest ratio of atoms in a compound

Atomic Mass and Molecular Mass

• Atomic Mass Unit (u): 1/12th the mass of a carbon-12 atom
• Atomic Mass: Average mass of all isotopes of an element
• Molecular Mass: Sum of atomic masses of all atoms in a molecule

Mole Concept

One mole contains 6.022 × 10²³ particles (Avogadro's number).

Practice Problems

1. Calculate the molecular mass of H₂SO₄
2. How many moles are present in 18 g of water?
3. Find the number of atoms in 2 moles of oxygen gas.