Atoms and Molecules
Gujarat Board · Class 9 · Science
Quick revision notes for Atoms and Molecules — Gujarat Board Class 9 Science. Key concepts, formulas, and definitions for last-minute revision.
Key Topics to Revise
Laws of Chemical Combination
- Antoine Lavoisier established the foundation of chemical sciences with two important laws
- Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction
- Law of Constant Proportions (Definite Proportions): Elements in a compound are always present in definite proportions by mass
Dalton's Atomic Theory
- John Dalton (1766-1844) provided the first scientific atomic theory in 1808
- All matter is made of tiny indivisible particles called atoms
- Atoms cannot be created or destroyed in chemical reactions
Atoms - Size, Symbols, and Mass
- Atoms are extremely small (radius ~10⁻¹⁰ m)
- Millions of atoms stacked would be barely as thick as paper
- Modern symbols use 1-2 letters: first letter uppercase, second lowercase
Molecules and Ions
- Molecule: smallest particle showing all properties of substance
- Molecules formed by chemical bonding of atoms
- Atomicity: number of atoms in a molecule
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