Chapter 9 — Atomic Foundations of Matter

Chapter 9 of the Class 9 Science NCERT textbook, "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.

• Two Foundational Laws — The Law of Conservation of Mass (Lavoisier, 1789) says matter is neither created nor destroyed in a reaction, and the Law of Constant Proportions (Proust) says a compound's elements combine in a fixed mass ratio.
• Dalton's Atomic Theory — Underpinned by these laws, Dalton's theory holds that atoms are indivisible, identical within an element but different between elements, and combine in simple whole-number ratios to form compounds.
• Covalent and Ionic Bonds — Atoms join into molecules by sharing electrons in covalent bonds (single or double pairs, as in H2 or H2O) or by transferring electrons in ionic bonds, forming cations and anions held by electrostatic attraction.
• Formulae and Mass — Compounds are written with systematic formulae, and their mass is found by adding atomic masses: molecular mass for molecules such as H2O = 18 u, and formula unit mass for ionic compounds such as Na2O = 62 u.

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).