CBSE Class 9 Science Chapter 3 Notes: Atoms And Molecules

Important Chapter Terms & Glossary

Law of Conservation of Mass

Mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

Law of Constant Proportions

In a chemical substance, the elements are always present in definite proportions by mass, irrespective of its source.

Atom

The smallest constituent unit of ordinary matter that retains the chemical properties of an element.

Molecule

The smallest particle of an element or a compound that is capable of independent existence and shows all properties of that substance.

Valency

The combining capacity of an atom of an element, which determines how it links with other atoms.

Polyatomic Ion

A cluster of atoms carrying a net positive or negative charge that behaves as a single unit.

Molar Mass

The mass of one mole of a substance (atoms, molecules, or formula units) expressed in grams.

Avogadro Constant

The defined constant value of 6.022 × 10²³ particles (atoms, molecules, or ions) present in one mole of any substance.

Must-Remember Key Revision Points

• Antoine L. Lavoisier established the experimental foundations of chemistry through the Laws of Chemical Combination.
• John Dalton proposed the Dalton's Atomic Theory, stating that all matter consists of indivisible atoms that combine in simple whole-number ratios.
• Atoms are extremely tiny; their size is measured in nanometers (1 nm = 10⁻⁹ m).
• The atomic mass of an element is calculated relative to 1/12th of the mass of a Carbon-12 atom.
• Molecules can be monoatomic (He), diatomic (O₂), triatomic (O₃), or polyatomic (P₄, S₈) depending on the number of atoms forming them.
• Chemical formulas are written using symbols of constituents crossed with their respective valencies.
• The formula mass of ionic compounds is calculated similarly to molecular mass but is called 'formula unit mass' since they contain ions instead of discrete molecules.
• One mole of any chemical substance represents exactly 6.022 × 10²³ representative particles of that substance.

Core Concept: Laws of Chemical Combination & Dalton's Theory

The foundation of modern chemical science rests upon the Laws of Chemical Combination. The Law of Conservation of Mass states that in any closed system, mass is conserved. Hence, the sum of reactants' masses equals the sum of products' masses. Secondly, the Law of Constant Proportions (or Definite Proportions) shows that chemical compounds contain constituents in a fixed mass ratio. For instance, pure water (H₂O) always contains Hydrogen and Oxygen in a mass ratio of 1:8.

Dalton's Atomic Theory successfully explained these laws by postulating that all matter is made of tiny indivisible particles called atoms. According to Dalton, atoms of a given element are identical in mass and properties, whereas atoms of different elements differ. He proposed that chemical reactions represent a rearrangement of atoms, which combine in simple, whole-number ratios to form compounds. This provided a physical model for chemical combinations.

Differences: Atoms vs. Molecules

Steps to Write the Chemical Formula of a Compound

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Worked Revision Examples

• {"title":"Writing a Chemical Formula","problem":"Write the chemical formula for Calcium Chloride.","solution":"1. Write elements side-by-side: Ca and Cl.\n2. Write valencies beneath them: Ca has a valency of 2+; Cl has a valency of 1-.\n3. Criss-cross the valencies: Ca₁ Cl₂.\n4. Simplify and write: CaCl₂."}
• {"title":"Calculating Molecular Mass","problem":"Calculate the molecular mass of Nitric Acid (HNO₃). Given: H = 1 u, N = 14 u, O = 16 u.","solution":"Molecular Mass of HNO₃ = (1 × Atomic mass of H) + (1 × Atomic mass of N) + (3 × Atomic mass of O)\n= (1 × 1) + (1 × 14) + (3 × 16)\n= 1 + 14 + 48\n= 63 u."}
• {"title":"Mole Concept Calculation","problem":"Calculate the number of moles in 52 grams of Helium (He). Given: Atomic mass of He = 4 u.","solution":"Molar mass of Helium (M) = 4 g/mol.\nGiven Mass (m) = 52 g.\nFormula: Number of moles (n) = Given Mass (m) / Molar Mass (M)\nn = 52 / 4 = 13 moles."}

Board Exam Alert & Common Mistakes

Beware of Molar Mass vs. Atomic Mass Units (u)!
Many students lose marks by swapping units. u represents relative atomic mass (mass of one single atom or molecule), while g (grams) represents molar mass (mass of 1 mole of atoms or molecules).

• If the question asks for molecular mass of H₂O, the answer is 18 u.
• If the question asks for molar mass of H₂O, the answer is 18 g (or 18 g/mol).

Marking Scheme Tip: Always write down the core formula (e.g., n = m / M) and specify your steps to secure partial step marks in numeric questions.

Quick Chapter Check

• What is the ratio by mass of nitrogen and hydrogen in Ammonia (NH₃)? Nitrogen has an atomic mass of 14 u, and three Hydrogen atoms have a combined mass of 3 u. Therefore, the ratio by mass of Nitrogen to Hydrogen in ammonia is always 14:3.
• Define atomicity and state the atomicity of ozone (O₃) and phosphorus (P₄). Atomicity is the number of atoms present in one molecule of an element. The atomicity of ozone (O₃) is 3 (triatomic), and for phosphorus (P₄), it is 4 (tetraatomic).
• What is the formula unit mass of Sodium Chloride (NaCl)? (Na = 23 u, Cl = 35.5 u) Formula Unit Mass of NaCl = Atomic mass of Na + Atomic mass of Cl = 23 + 35.5 = 58.5 u.
• How many atoms are present in 1 mole of any monatomic element? 1 mole of any monatomic element contains exactly 6.022 × 10²³ atoms (Avogadro's number).

Frequently Asked Questions