Class IX SEBA Science – Chapter 3: Atoms and Molecules
Class IX SEBA Science – Chapter 3: Atoms and Molecules
⚛️ 1. Laws of Chemical Combination
Law of Conservation of Mass
- Mass can neither be created nor destroyed in a chemical reaction.
- Example: Mass of reactants = Mass of products.
Law of Constant Proportions
- In a pure compound, the elements are always present in a definite proportion by mass.
- Example: Water (H₂O) always contains hydrogen and oxygen in 1:8 ratio by mass.
🧪 2. Dalton’s Atomic Theory
- Matter is made up of indivisible particles called atoms.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements have different masses and properties.
- Atoms combine in small whole numbers to form compounds.
- Atoms cannot be created or destroyed in a chemical reaction.
🧬 3. Atoms
- Atom is the smallest particle of an element that can take part in a chemical reaction.
- Represented by symbols (e.g., H for hydrogen, O for oxygen).
- Some elements exist as atoms (e.g., He, Ne), others as molecules (e.g., O₂, N₂).
🔗 4. Molecules
- A molecule is a group of two or more atoms chemically bonded together.
- Molecule of element → O₂, N₂
- Molecule of compound → H₂O, CO₂
🧩 5. Ions
- Charged particles formed by loss or gain of electrons.
- Cation: Positively charged (e.g., Na⁺, Ca²⁺)
- Anion: Negatively charged (e.g., Cl⁻, SO₄²⁻)
🔢 6. Chemical Formula
- Represents elements in a compound and the ratio of atoms.
- Example: H₂O → 2 atoms of hydrogen + 1 atom of oxygen
- Uses valency to balance elements.
🧮 7. Molecular Mass and Formula Unit Mass
- Molecular mass = Sum of atomic masses of all atoms in a molecule.
- Formula unit mass = Sum of atomic masses in the formula of an ionic compound.
- Example: Molecular mass of H₂O = 2×1 + 16 = 18 u
⚖️ 8. Mole Concept
- 1 mole = 6.022 × 10²³ particles (Avogadro number).
- 1 mole of any substance has mass equal to its molar mass in grams.
- Moles = Given mass / Molar mass
📘 Exercise Questions and Answers
1. A 2 g of hydrogen reacts with 16 g of oxygen to form water. What mass of water will be formed?
Total mass = 2 g + 16 g = 18 g (Law of conservation of mass)
2. Which postulate of Dalton’s theory explains the law of conservation of mass?
- “Atoms can neither be created nor destroyed in a chemical reaction.”
3. Which postulate of Dalton’s theory explains the law of constant proportion?
- “Atoms combine in simple whole number ratios to form compounds.”
4. Define atomic mass unit.
- 1 amu = 1/12 the mass of one atom of carbon-12 isotope.
5. Why is it not possible to see an atom with the naked eye?
- Atoms are extremely small (size ~10⁻¹⁰ m) and cannot be seen even with powerful microscopes.
6. Write the chemical formula of:
a) Hydrogen chloride → HCl
b) Calcium oxide → CaO
c) Aluminium chloride → AlCl₃
d) Sodium sulphide → Na₂S
e) Magnesium hydroxide → Mg(OH)₂
7. What is the molecular mass of:
a) H₂O → 2×1 + 16 = 18 u
b) CO₂ → 12 + 2×16 = 44 u
c) CH₄ → 12 + 4×1 = 16 u
✍️ Extra Important Questions with Answers
1. What is an atomic mass unit (amu)?
It is the standard unit of measuring atomic mass. 1 amu = 1/12 mass of carbon-12 atom.
2. Define valency.
Valency is the combining capacity of an element to form chemical bonds.
3. What is the difference between an atom and a molecule?
An atom is the smallest unit of an element, whereas a molecule is made of two or more atoms chemically bonded.
4. What is a chemical formula?
A symbolic representation of a compound showing types and number of atoms.
5. Define mole.
One mole is the amount of substance containing 6.022 × 10²³ particles (atoms/molecules/ions).
proceed to Chapter 4: Structure of the Atom?
