3. Atoms and Molecules Class 9 science Solutions English Medium-Additional -Questions 3

NCERT Solutions Class 9 science English Medium

3. Atoms and Molecules

Topic: Additional -Questions 3

Additional -Questions:

Q1. What is atomicity?

Ans: The number of atoms contained in a molecule of a substance is called its atomicity. 

Q2. How molecules are categrised on the base of atomicity?

Ans:

1. Monoatomic Molecules (Molecule having 1 atom)

2. Diatomic Molecules (molecule having 2 atoms)

3. Triatomic Molecules (molecules having 3 atoms)

4. Tetraatomic Molecules (molecules having 4 atoms)

5. Ployatomic Molecules (molecules having more than 4 molecules) 

1. Atom

• The smallest particle of an element that cannot be divided further by chemical means.

• It retains all the properties of that element.

• Example: H (Hydrogen atom), O (Oxygen atom).

2. Molecule

• A group of two or more atoms (same or different) chemically bonded together.

• Example: H₂ (hydrogen molecule), H₂O (water molecule).

3. Ion

• A charged particle formed when an atom or molecule loses or gains electrons.

• Positive ions → Cations (formed by loss of electrons).

• Negative ions → Anions (formed by gain of electrons).

4. Atomicity

• The number of atoms present in one molecule of an element.

• Example:

  • O₂ → Atomicity = 2

  • P₄ → Atomicity = 4

5. Anion

• A negatively charged ion.

• Formed by gain of electrons.

• Example: Cl⁻, O²⁻.

6. Cation

• A positively charged ion.

• Formed by loss of electrons.

• Example: Na⁺, Ca²⁺.

7. One mole atom

• The amount of a substance containing 6.022 × 10²³ atoms of that element.

• Example: 1 mole of carbon = 6.022 × 10²³ atoms of carbon.

8. One mole molecule

• The amount of a substance containing 6.022 × 10²³ molecules.

• Example: 1 mole of water (H₂O) = 6.022 × 10²³ molecules of H₂O.

9. Monoatomic

• A molecule that contains only one atom.

• Example: Noble gases (He, Ne, Ar).

10. Diatomic

• A molecule that contains two atoms.

• Example: H₂, O₂, Cl₂.

11. Triatomic

• A molecule that contains three atoms.

• Example: O₃ (ozone), H₂O (water).

12. Polyatomic

• A molecule that contains more than three atoms.

• Example: P₄ (phosphorus), CH₄ (methane), C₆H₁₂O₆ (glucose).

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