📘 Chapter 4: Structure of the Atom
🔹 1. Discovery of Subatomic Particles
- Electron discovered by J.J. Thomson (1897)
- Proton discovered by Goldstein
- Neutron discovered by James Chadwick
🔹 2. Subatomic Particles Summary
| Particle | Charge | Mass (approx.) | Symbol |
|---|---|---|---|
| Electron | -1 | 1/1836 u | e⁻ |
| Proton | +1 | 1 u | p⁺ |
| Neutron | 0 (Neutral) | 1 u | n⁰ |
🔹 3. Atomic Models
a) Thomson’s Model (“Plum Pudding”)
- Atom = Positive sphere with electrons inside.
- Like raisins in pudding.
- ⚠ Not accepted: couldn’t explain nucleus.
b) Rutherford’s Model (Gold Foil Experiment)
- Atom has a dense nucleus (positive).
- Electrons revolve around the nucleus.
- Most of the atom is empty space.
- ⚠ Failed: could not explain stability.
c) Bohr’s Model
- Electrons revolve in fixed circular orbits (shells).
- Electrons don’t lose energy in orbits.
- Energy is absorbed/released only when electrons jump between orbits.
🔹 4. Atomic Number & Mass Number
- Atomic Number (Z): Number of protons (also = electrons in neutral atom)
🔸 Z = p⁺ - Mass Number (A): Number of protons + neutrons
🔸 A = p⁺ + n⁰
Example (Carbon):
Z = 6, A = 12 → 6 protons, 6 neutrons, 6 electrons
🔹 5. Isotopes
- Atoms of same element with same atomic number but different mass number.
- Example:
- Hydrogen: ¹H, ²H (Deuterium), ³H (Tritium)
🔸 Used in:
- Cancer treatment (Cobalt-60)
- Radioactive dating
- Nuclear reactors
🔹 6. Isobars
- Atoms with same mass number but different atomic numbers.
- Example:
- Argon (Ar) and Calcium (Ca) → Both A = 40
🔹 7. Electronic Configuration
- Distribution of electrons in shells:
K (2), L (8), M (18), N (32) - Formula: 2n², where n = shell number
Examples:
| Element | Atomic No. | Configuration |
|---|---|---|
| H | 1 | 1 |
| He | 2 | 2 |
| O | 8 | 2, 6 |
| Na | 11 | 2, 8, 1 |
🔹 8. Valency
- Valency = combining capacity = electrons lost/gained/shared to complete outer shell.
Examples:
| Element | Valency |
|---|---|
| H | 1 |
| O | 2 |
| N | 3 |
| C | 4 |
| Na | 1 |
| Cl | 1 |
✅ MCQs
Q1. Who discovered the electron?
a) Bohr
b) Rutherford
c) Thomson
d) Chadwick
Q2. Charge on proton is:
a) Negative
b) Zero
c) Positive
d) Neutral
Q3. Mass of neutron is:
a) 0
b) 1 u
c) 2 u
d) 1/1836 u
Q4. Atomic number is equal to:
a) Protons
b) Electrons
c) Both (a) and (b)
d) Neutrons
Q5. Who proposed nuclear model of atom?
a) Bohr
b) Dalton
c) Rutherford
d) Thomson
Q6. Maximum electrons in L shell:
a) 2
b) 8
c) 18
d) 32
Q7. What is mass number of carbon (Z = 6, neutrons = 6)?
a) 6
b) 12
c) 3
d) 10
Q8. Which is an isotope of hydrogen?
a) O₂
b) Tritium
c) Argon
d) Sodium
Q9. Bohr’s model explains:
a) Neutron discovery
b) Nucleus structure
c) Stability of atom
d) All
Q10. Electron revolves around nucleus in:
a) Spiral path
b) Straight line
c) Random path
d) Fixed orbits
Q11. Which is isobar pair?
a) ¹H and ²H
b) ⁴⁰Ar and ⁴⁰Ca
c) ¹⁴C and ¹²C
d) ²³Na and ³⁵Cl
Q12. Element with 2, 8, 1 electronic configuration:
a) Oxygen
b) Neon
c) Sodium
d) Carbon
Q13. What is valency of Cl (2, 8, 7)?
a) 1
b) 7
c) 2
d) 8
Q14. Symbol of isotope of carbon with mass 14:
a) C₁₄
b) ¹⁴C
c) C⁺
d) C₆
Q15. Electron charge is:
a) 1 u
b) +1
c) –1
d) 0
✅ Assertion & Reason
Q16.
A: Electrons are negatively charged.
R: They are present inside the nucleus.
Q17.
A: Atomic number of Na is 11.
R: It has 11 protons.
Q18.
A: Neutrons have no charge.
R: They are lighter than protons.
Q19.
A: Electrons revolve in fixed shells.
R: Bohr proposed this model.
Q20.
A: Isotopes have different chemical properties.
R: They have different number of protons.
✅ Case-Based Study
Passage:
Rutherford performed an experiment by bombarding alpha particles on thin gold foil. Most particles passed straight, few deflected, and some bounced back.
Q21. What did this experiment prove?
a) Atom is solid
b) Atom has no space
c) Atom has empty space
d) Atom has no electrons
Q22. Why did some particles deflect?
a) Due to outer electrons
b) Due to nucleus
c) Due to gravity
d) Due to neutrons
Q23. What was concluded about nucleus?
a) It’s heavy and neutral
b) It’s light and positive
c) It’s dense and positively charged
d) It’s made of electrons
✅ Reason-Based Questions
Q24. Why is atom electrically neutral?
➡️ Because it has equal number of protons and electrons.
Q25. Why is nucleus positively charged?
➡️ Due to presence of protons.
Q26. Why neutron is called neutral particle?
➡️ It has no charge.
Q27. Why electron remains outside nucleus?
➡️ Due to electrostatic attraction, it revolves in orbits.
Q28. Why did Rutherford say atom is hollow?
➡️ Most alpha particles passed without deflection.
Q29. Why valency of carbon is 4?
➡️ It needs 4 electrons to complete octet.
Q30. Why do isotopes have same chemical properties?
➡️ They have same number of electrons.
Q31. Why is 2n² used in Bohr’s model?
➡️ To calculate maximum electrons in a shell.
Q32. Why is neutron’s discovery important?
➡️ It explained atomic mass and isotopes.
Q33. Why is Na’s configuration 2, 8, 1?
➡️ It has 11 electrons and they fill shells accordingly.
Q34. Why electrons don’t fall into nucleus?
➡️ Because they move in stable orbits as per Bohr.
Q35. Why isotopes have different masses?
➡️ Due to different number of neutrons.
Q36. Why mass number = p + n?
➡️ Because electrons have negligible mass.
Q37. Why is electronic configuration important?
➡️ It helps to understand chemical behavior.
Q38. Why valency of noble gases is 0?
➡️ Because they have complete outer shell.
📌 Answer Key
✅ MCQs:
1–c, 2–c, 3–b, 4–c, 5–c, 6–b, 7–b, 8–b, 9–c, 10–d, 11–b, 12–c, 13–a, 14–b, 15–c
✅ Assertion & Reason:
16 – A: True, R: False
17 – A: True, R: True ✅
18 – A: True, R: False
19 – A: True, R: True ✅
20 – A: False, R: False
✅ Case-Based:
21 – c) Atom has empty space
22 – b) Due to nucleus
23 – c) It’s dense and positively charged