CBSE Class 9 Science Notes: Atoms and Molecules

Concept of Atoms and Molecules

This section explores the fundamental building blocks of matter: atoms and molecules. It covers how matter is composed of tiny, indivisible particles called atoms, and how these atoms combine to form molecules.

Definitions

• Atom: The smallest particle of an element that can take part in a chemical reaction. Atoms possess all the properties of that element.
• Molecule: A group of two or more atoms that are chemically bonded together and act as a unit. Molecules can be of the same element (e.g., O₂) or different elements (e.g., H₂O).
• Element: A pure substance made of only one kind of atom.
• Compound: A substance formed when two or more elements are chemically combined in a fixed ratio.

Laws of Chemical Combination

These laws govern how elements combine to form compounds. Understanding these laws is crucial to grasping chemical reactions.

Core Principles

• Law of Conservation of Mass: In a chemical reaction, mass is neither created nor destroyed. The total mass of the reactants equals the total mass of the products.
• Law of Constant Proportions (or Definite Proportions): A chemical compound always contains the same elements combined together in the same proportion by mass.

Molecular and Chemical Formulas of Common Compounds

This section introduces how to represent compounds using chemical formulas and provides examples of common compounds.

Definitions & Examples

• Chemical Formula: A shorthand representation of a compound using the symbols of the elements present and subscripts to indicate the number of atoms of each element.
• Examples:
  • Water: H₂O
  • Carbon Dioxide: CO₂
  • Sodium Chloride (Common Salt): NaCl
  • Hydrochloric acid: HCl

Atomic Mass and Molecular Mass

This section defines and explains how to calculate atomic and molecular masses, which are essential for stoichiometry.

Definitions

• Atomic Mass: The mass of an atom, usually expressed in atomic mass units (amu or u). 1 amu is equal to $1/12$ of the mass of a carbon-12 atom.
• Molecular Mass: The sum of the atomic masses of all the atoms in a molecule.

Formulas & Calculations

To calculate molecular mass:

1. Identify the chemical formula of the molecule.
2. Find the atomic masses of each element from the periodic table.
3. Multiply the atomic mass of each element by the number of atoms of that element in the molecule.
4. Add the results from step 3 to find the molecular mass.

Example

Calculate the molecular mass of water (H₂O). Atomic masses: H = 1 u, O = 16 u.

Molecular mass of H₂O = (2 × Atomic mass of H) + (1 × Atomic mass of O) = (2 × 1 u) + (1 × 16 u) = 18 u