Chapter 9 — Atomic Foundations of Matter

Class 9 Science Chapter 9 'Atomic Foundations of Matter' covers the Law of Conservation of Mass, the Law of Constant Proportions, Dalton's Atomic Theory, covalent and ionic bonding, chemical formulae, and molecular and formula unit mass calculations.

Chapter 9 of NCERT Class 9 Science (Exploration, 2026-27 edition) establishes the atomic basis of matter through two foundational laws: the Law of Conservation of Mass (Lavoisier, 1789), which states matter cannot be created or destroyed in a chemical reaction, and the Law of Constant Proportions (Proust), which states elements in a compound always combine in a fixed mass ratio. These laws underpin Dalton's Atomic Theory. The chapter then explains how atoms form molecules via covalent bonds (electron sharing) and ionic bonds (electron transfer), covers systematic naming and formula writing for both compound types, and introduces molecular mass and formula unit mass calculations.

1. 01Law of Conservation of Mass: total mass of reactants equals total mass of products in any chemical reaction, proposed by Antoine Lavoisier in 1789.
2. 02Law of Constant Proportions (Proust's Law): elements in a compound always combine in a fixed ratio by mass regardless of source — water always contains hydrogen and oxygen in the ratio 1:8.
3. 03Dalton's Atomic Theory postulates that atoms are indivisible, identical within an element, differ between elements, and combine in simple whole-number ratios to form compounds.
4. 04Covalent bonds form by sharing of electrons between atoms (single bond: one shared pair; double bond: two shared pairs); examples include H2, Cl2, O2, HCl, and H2O.
5. 05Ionic bonds form by transfer of electrons, producing cations (e.g., Na+) and anions (e.g., Cl-) held together by electrostatic attraction; ionic compounds generally have high melting points and conduct electricity only when dissolved in water.
6. 06Molecular mass is the sum of atomic masses of all atoms in a molecule (e.g., H2O = 18 u); formula unit mass applies to ionic compounds and represents the mass of the simplest whole-number ion ratio (e.g., Na2O = 62 u).