NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules
NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules covers laws of chemical combination, Dalton's atomic theory, atoms, atomic mass, molecules, chemical formulae writing, molecular mass, and formula unit mass.
Overview of Topics & Subtopics (Click to Jump)
1. Laws of Chemical Combination & Dalton's Atomic Theory
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.
2. What is an Atom? & Atomic Mass Unit
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.
3. What is a Molecule & Ions?
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-).
4. Writing Chemical Formulae & Molecular Mass
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).
Intext Questions - Page Number: 32
Total Mass of Reactants: Mass of sodium carbonate + Mass of ethanoic acid = 5.3 g + 6.0 g = 11.3 g
Total Mass of Products: Mass of sodium ethanoate + Mass of carbon dioxide + Mass of water = 8.2 g + 2.2 g + 0.9 g = 11.3 g
Since Total Mass of Reactants = Total Mass of Products (11.3 g), this reaction follows the Law of Conservation of Mass.
Intext Questions - Page Number: 35
Intext Questions - Page Number: 39
(i) Sodium oxide: Na2O
(ii) Aluminium chloride: AlCl3
(iii) Sodium sulphide: Na2S
(iv) Magnesium hydroxide: Mg(OH)2
Textbook Chapter End Exercise Questions
(a) Magnesium chloride: MgCl2
(b) Calcium oxide: CaO
(c) Copper nitrate: Cu(NO3)2
(d) Aluminium chloride: AlCl3
(e) Calcium carbonate: CaCO3
• H2 = 2 × 1 = 2 u
• O2 = 2 × 16 = 32 u
• Cl2 = 2 × 35.5 = 71 u
• CO2 = 12 + (2 × 16) = 12 + 32 = 44 u
• CH4 = 12 + (4 × 1) = 16 u
• NH3 = 14 + (3 × 1) = 17 u
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