📘 Chapter 3 – Atoms and Molecules
🧠 Detailed Notes
🔹 Atom:
- The smallest particle of an element.
- Takes part in chemical reactions.
- Cannot be created, destroyed or divided further (as per Dalton).
- Example: H, O, N
🔹 Molecule:
- Group of atoms bonded together.
- Can be:
- Molecule of element: O₂, N₂
- Molecule of compound: H₂O, CO₂
🔹 Atomicity:
| Type | Example | Atomicity |
|---|---|---|
| Monoatomic | He, Ne | 1 |
| Diatomic | H₂, O₂ | 2 |
| Triatomic | O₃ | 3 |
| Tetratomic | P₄ | 4 |
| Octatomic | S₈ | 8 |
🔹 Laws of Chemical Combination
1. Law of Conservation of Mass:
➡️ Mass is neither created nor destroyed during a chemical reaction.
2. Law of Constant Proportions:
➡️ In a compound, elements are always combined in a fixed ratio by mass.
🔹 Dalton’s Atomic Theory:
- Matter is made of atoms.
- Atoms are indivisible and indestructible.
- Atoms of the same element are identical.
- Different elements have different atoms.
- Compounds form by simple ratios.
- Reactions = rearrangement of atoms.
🔹 Symbols of Elements
| Element | Symbol |
|---|---|
| Hydrogen | H |
| Oxygen | O |
| Carbon | C |
| Sodium | Na |
| Chlorine | Cl |
| Calcium | Ca |
🔹 Ions:
- Cations (Positive): Na⁺, Ca²⁺
- Anions (Negative): Cl⁻, SO₄²⁻
- Formed by gain/loss of electrons.
🔹 Chemical Formula:
- Tells us type and number of atoms.
- Based on valency of elements.
Examples: - NaCl (1:1), H₂O (2 H, 1 O), CO₂ (1 C, 2 O)
🔹 Valency Table:
| Element | Valency |
|---|---|
| H | 1 |
| O | 2 |
| N | 3 |
| C | 4 |
| Na | 1 |
| Cl | 1 |
🔹 Molecular Mass:
- Add atomic masses of all atoms.
Examples: - CO₂ = 12 + (16 × 2) = 44 u
- H₂O = (1×2) + 16 = 18 u
🔹 Mole Concept:
- 1 mole = 6.022 × 10²³ particles
- 1 mole of H₂O = 18g
- 1 mole of CO₂ = 44g
✅ MCQs
Q1. Which of these is a molecule of compound?
a) O₂
b) H₂
c) CO₂
d) N₂
Q2. Water’s chemical formula is:
a) HO
b) OH₂
c) H₂O
d) O₂H
Q3. Atomicity of phosphorus (P₄) is:
a) 1
b) 2
c) 4
d) 8
Q4. Valency of oxygen is:
a) 1
b) 2
c) 3
d) 4
Q5. 1 mole contains:
a) 6.022 × 10⁶
b) 6.022 × 10²
c) 6.022 × 10²³
d) 6.022
Q6. Which is a cation?
a) OH⁻
b) Cl⁻
c) Na⁺
d) SO₄²⁻
Q7. Dalton proposed that atoms are:
a) Visible
b) Neutral
c) Divisible
d) Indivisible
Q8. Which law explains fixed ratio of elements in water?
a) Law of motion
b) Law of constant proportion
c) Law of mass
d) Law of volume
Q9. Which of the following is a monoatomic gas?
a) O₂
b) H₂
c) He
d) CO₂
Q10. Symbol of calcium is:
a) C
b) Cl
c) Ca
d) Cu
Q11. What is the molecular mass of CO₂?
a) 28 u
b) 44 u
c) 12 u
d) 32 u
Q12. Which of the following is a triatomic molecule?
a) O₂
b) CO₂
c) H₂
d) O₃
Q13. Ions are formed by:
a) Changing neutrons
b) Changing protons
c) Gain or loss of electrons
d) None
Q14. What is the formula of Sodium Chloride?
a) NaCl
b) Na₂Cl
c) NaCl₂
d) Na₂Cl₂
Q15. Which element has valency 1?
a) O
b) N
c) H
d) C
✅ Assertion & Reason
Q16.
A: Atoms cannot be divided.
R: Dalton’s theory said atoms are indivisible.
Q17.
A: Water is a compound.
R: It contains hydrogen and oxygen combined in fixed ratio.
Q18.
A: H₂ is a compound.
R: It has only one kind of atom.
Q19.
A: CO₂ follows Law of Constant Proportion.
R: It has C and O in same ratio in all samples.
Q20.
A: An ion is a neutral atom.
R: Ions do not have charges.
✅ Case-Based Study
Passage:
Reena mixed 12g carbon and 32g oxygen to make carbon dioxide. The total mass after reaction was 44g. She also tested the same compound from another source and found same C:O ratio.
Q21. What is the total mass after reaction?
a) 12g
b) 32g
c) 44g
d) 24g
Q22. Which law is followed by constant mass?
a) Conservation of energy
b) Constant proportion
c) Conservation of mass
d) Avogadro law
Q23. Why is the ratio always 12:32 in CO₂?
a) Due to Dalton’s theory
b) Because of pressure
c) Because of Law of Constant Proportion
d) Law of demand
✅ Reason-Based Questions
Q24. Why is H₂O a compound?
➡️ Because it contains different elements (H and O) chemically bonded.
Q25. Why is O₂ a molecule but not a compound?
➡️ It contains same atoms only (oxygen + oxygen).
Q26. Why do ions carry charge?
➡️ Because of gain or loss of electrons.
Q27. Why is CO₂ mass = 44 u?
➡️ Carbon = 12, Oxygen = 16 × 2 = 32 → 12 + 32 = 44 u.
Q28. Why is valency important?
➡️ It tells how many atoms will combine to form chemical formulas.
Q29. Why are atoms neutral?
➡️ Equal protons and electrons cancel each other’s charges.
Q30. Why is mole concept helpful?
➡️ It helps count huge number of atoms or molecules.
Q31. Why is atomic mass expressed in u?
➡️ Actual mass is too small to write in grams.
Q32. Why does Na become Na⁺?
➡️ It loses one electron and becomes positive.
Q33. Why are chemical formulas used?
➡️ To show types and numbers of atoms in a compound.
Q34. Why does Dalton’s theory say atoms are indivisible?
➡️ Because at that time sub-atomic particles weren’t discovered.
Q35. Why is CO₂ always made from same elements in same ratio?
➡️ Due to Law of Constant Proportion.
Q36. Why is Cl⁻ an anion?
➡️ It gains one electron and becomes negatively charged.
Q37. Why can’t atoms be seen?
➡️ They are extremely small (in nanometers).
Q38. Why is H₂ not a compound?
➡️ It is made of two same atoms (hydrogen).
📌 Answer Key
✅ MCQs:
1–c, 2–c, 3–c, 4–b, 5–c, 6–c, 7–d, 8–b, 9–c, 10–c, 11–b, 12–d, 13–c, 14–a, 15–c
✅ Assertion & Reason:
16 – A: True, R: True ✅
17 – A: True, R: True ✅
18 – A: False, R: True
19 – A: True, R: True ✅
20 – A: False, R: False
✅ Case-Based:
21 – c) 44g
22 – c) Law of conservation of mass
23 – c) Law of constant proportion