class9 ch 3 class notes

It explains how all matter is made up of tiny particles called atoms, which combine in fixed ratios to form molecules. The chapter introduces the laws of chemical combination: Law of Conservation of Mass – mass is neither created nor destroyed in a chemical reaction. Law of Definite Proportions – a compound always contains elements in a fixed ratio by mass. Law of Multiple Proportions – if two elements form more than one compound, they combine in small whole number ratios. It covers Dalton’s Atomic Theory (1803), which states that matter is made of indivisible atoms, atoms of the same element are identical, and compounds form when atoms combine in fixed ratios. The document defines atoms as the smallest unit of an element (represented by chemical symbols like H, O, Na, Cl) and molecules as stable combinations of atoms (e.g., H₂, O₂, H₂O, CO₂). It also introduces atomic mass (relative mass compared to carbon-12) and molecular mass (sum of atomic masses of atoms in a molecule). The mole concept is explained as 1 mole = 6.022 × 10²³ particles (Avogadro number), with formulas like: Molecular mass (M) = sum of atomic masses Number of moles (n) = Mass of substance / Molar mass Mass of substance (m) = Number of moles × Molar mass In short, the document gives a foundation of modern chemistry, focusing on atoms, molecules, chemical laws, atomic masses, molecular masses, and mole calculations for numerical problems.