Atoms and Molecules Notes: Class 9 Science Chapter 3

Short Note:

• Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction.

• Law of Constant Proportions: In a chemical substance, elements are always present in definite proportions by mass (e.g., in H₂O, hydrogen and oxygen ratio is always 1:8 by mass).

• Dalton's Atomic Theory: All matter is made up of tiny indivisible particles called atoms, which take part in chemical reactions.

Short Note:

• Atom: The smallest particle of an element that takes part in a chemical reaction and may or may not exist independently.

• Atomic Radius: Measured in nanometers (1 nm = 10^-9 m).

• Atomic Mass Unit (u): Defined as a mass unit equal to exactly 1/12th the mass of one carbon-12 atom.

Short Note:

• Molecule: A group of two or more atoms chemically bonded together (e.g., O₂, H₂O).

• Atomicity: The number of atoms constituent in a molecule (e.g., Argon = Monoatomic, Oxygen = Diatomic, Ozone = Triatomic).

• Ion: A charged species formed by loss or gain of electrons. Positively charged ion is a cation (Na⁺), negatively charged ion is an anion (Cl^-).

Short Note:

• Valency: Combining capacity of an element.

• Writing Chemical Formulae: Write symbols side-by-side with valencies below them, then cross-over the valencies.

• Molecular Mass: Sum of the atomic masses of all the atoms present in a molecule of the substance.

• Formula Unit Mass: Sum of atomic masses of all atoms in a formula unit of an ionic compound (e.g., NaCl).