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📚 NCERT & ASSAM BOARD · CLASS 9 · SCIENCE

Chapter 4 – Structure of the Atom
Complete Study Notes

Notes · Summary · Definitions · Models · MCQs · Board Questions · Memory Tricks

🔬 Subatomic Particles ⚛️ Atomic Models 🔢 Electronic Configuration 🧲 Valency 🧪 Isotopes 🎯 40+ MCQs

🌱

Welcome to Jnaanangkur – The Learning Hub!
Dear students, in this chapter we will explore the fascinating world inside an atom. You will learn how scientists discovered electrons, protons, and neutrons, understand atomic models, electronic configuration, valency, isotopes, and much more through easy explanations, diagrams, memory tricks, and exam-oriented questions. Let's begin this exciting journey into the world of atoms! 🚀

📋 Table of Contents

1. Chapter Overview & Learning Outcomes
2. Quick Chapter Summary
3. Important Definitions
4. Discovery of Subatomic Particles
5. Atomic Models (Thomson, Rutherford, Bohr)
6. Electronic Configuration
7. Valency Made Easy
8. Atomic Number & Mass Number
9. Isotopes
10. Memory Tricks 🧠
11. Exam Booster Notes
12. NCERT Questions & Answers
13. Extra Exam-Oriented Questions
14. MCQ Practice Zone (40 Questions)
15. Previous Year Board Questions
16. One-Page Revision Sheet
17. Frequently Asked Questions
18. Last-Minute Exam Tips

1

Chapter Overview

Chapter

4 – Structure of the Atom

Subject

Science (Chemistry)

Class

Class 9

Board

NCERT & Assam Board

Exam Weightage

15–20 Marks

Difficulty

⭐⭐⭐ Moderate

🎯 Learning Outcomes

• Describe the discovery of electrons, protons & neutrons
• Explain Thomson's, Rutherford's & Bohr's atomic models
• Write electronic configurations of elements up to Z=20
• Calculate valency from electronic configuration
• Define atomic number, mass number & isotopes
• Solve numerical problems on mass number & atomic number
• List applications of isotopes in medicine & industry
• Compare different atomic models with their limitations

2

Quick Chapter Summary

An atom is the smallest unit of matter that retains the chemical properties of an element. Atoms consist of three fundamental subatomic particles:

⚡

Electron (e⁻)

Negatively charged particle
Discovered by J.J. Thomson (1897)
Revolves in shells around nucleus

➕

Proton (p⁺)

Positively charged particle
Discovered by E. Goldstein (1886)
Present inside the nucleus

⚪

Neutron (n)

Neutral (no charge) particle
Discovered by J. Chadwick (1932)
Present inside the nucleus

💡

Scientists gradually developed atomic models: Thomson's Plum Pudding Model → Rutherford's Nuclear Model → Bohr's Planetary Model. Each model improved upon the previous, leading to our modern understanding of atomic structure.

3

Important Definitions

Term	Definition
Atom	The smallest particle of an element that can take part in a chemical reaction and retains its chemical identity.
Electron	A negatively charged subatomic particle with negligible mass (9.11 × 10⁻³¹ kg), found revolving around the nucleus in fixed energy shells.
Proton	A positively charged subatomic particle (charge = +1) found in the nucleus of an atom, with mass approximately equal to 1 atomic mass unit (amu).
Neutron	A neutral subatomic particle (charge = 0) found in the nucleus, with mass approximately 1 amu.
Valency	The combining capacity of an element; the number of electrons an atom can donate, accept, or share to achieve a stable (fully filled) outermost shell.
Isotopes	Atoms of the same element that have the same atomic number (same protons) but different mass numbers (different neutrons).
Atomic Number (Z)	The number of protons present in the nucleus of an atom of an element. It is unique for each element.
Mass Number (A)	The total number of protons and neutrons (nucleons) present in the nucleus of an atom.

4

Discovery of Subatomic Particles

1897 J.J. Thomson ⚡ Electron

Experiment: Cathode Ray Tube (CRT) Experiment. Thomson passed electricity at high voltage through a gas at very low pressure in a sealed glass tube (cathode ray tube). He observed a stream of particles moving from the cathode (negative plate) to the anode (positive plate).

Key Observations:

• The cathode rays were deflected toward the positively charged plate, confirming the particles are negatively charged.
• The same particles were obtained regardless of the gas used or the material of the cathode.
• These particles were named electrons.

Conclusion: Electrons are fundamental negatively charged subatomic particles present in all atoms. Charge on electron = −1.6 × 10⁻¹⁹ C. Mass of electron ≈ 1/1840 of hydrogen atom.

🖥️

Shows cathode rays bending toward the positive plate

⭐ Exam Tip: Thomson's cathode ray experiment is frequently asked. Remember: rays move from cathode (−) to anode (+) and bend toward positive plate, proving electrons are negatively charged.

1886 E. Goldstein ➕ Proton

Experiment: Canal Ray (Anode Ray) Experiment. Goldstein used a specially modified discharge tube with a perforated (holed) cathode. He observed rays passing through the holes in the cathode moving in the opposite direction to cathode rays.

Key Observations:

• Canal rays were deflected toward the negatively charged plate, proving they carry positive charge.
• The charge-to-mass ratio of canal rays depended on the gas used in the tube.
• With hydrogen gas, the lightest positively charged particles were obtained — named protons.

Conclusion: Protons are positively charged subatomic particles. Charge = +1.6 × 10⁻¹⁹ C. Mass of proton ≈ 1836 times the mass of electron.

🔋

Shows positive rays passing through perforated cathode

⭐ Exam Tip: Canal rays are also called anode rays or positive rays. The mass of the particle depends on the gas used. Lightest → Hydrogen → Proton.

1932 James Chadwick ⚪ Neutron

Experiment: Chadwick bombarded Beryllium (Be) with alpha particles. He detected a new type of radiation that could not be deflected by electric or magnetic fields, proving the particles had no charge.

Key Finding:

• The new particles had mass approximately equal to that of a proton.
• These particles were neutral — called neutrons.
• This discovery explained the difference between atomic number and mass number (mass defect).

⭐ Exam Tip: Neutron was the last subatomic particle to be discovered (1932). Chadwick received the Nobel Prize in Physics in 1935 for this discovery.

📊 Subatomic Particles at a Glance

Property	Electron	Proton	Neutron
Symbol	e⁻	p⁺	n
Charge	−1 (negative)	+1 (positive)	0 (neutral)
Relative Mass	1/1840	1 amu	1 amu
Location	Shells/Orbitals	Nucleus	Nucleus
Discoverer	J.J. Thomson	E. Goldstein	J. Chadwick
Year	1897	1886	1932

5

Atomic Models

1904

A. Thomson's Plum Pudding Model

J.J. Thomson proposed that an atom is a positively charged sphere with electrons embedded in it like plums (raisins) in a pudding. The total positive charge equals the total negative charge, making the atom electrically neutral.

🍮

Sphere of positive charge with electrons distributed throughout

✅ Advantages

• First model to include electrons
• Explained electrical neutrality of atom
• Explained why atoms don't fly apart

❌ Limitations / Failures

• Could not explain Rutherford's scattering experiment results
• Did not account for the nucleus
• Could not explain atomic spectra

⭐ Exam Tip: Thomson's model is also called "Plum Pudding Model," "Watermelon Model," or "Raisin Pudding Model." Electrons = plums/raisins; positive sphere = pudding.

1911

B. Rutherford's Nuclear Model

Rutherford conducted the famous Gold Foil (Alpha Scattering) Experiment to test Thomson's model. He bombarded a thin gold foil (0.00004 cm thick) with a stream of fast-moving alpha (α) particles from a radioactive source.

🔬

Alpha Scattering Experiment Setup: A radioactive source emits α-particles → narrow beam directed at thin gold foil → ZnS (zinc sulfide) screen surrounding the foil detects the scattered particles as flashes of light.

🎯

Gold foil, ZnS screen, and three types of alpha particle paths

📋 Observations & Conclusions

Observation	Conclusion
Most α-particles passed straight through the gold foil	Most of the atom is empty space
A few α-particles were deflected at small angles	A small positive charge is concentrated inside the atom
Very few α-particles (1 in 12,000) bounced back 180°	There is a tiny, dense, positively charged core — the nucleus

⚛️

Rutherford's Postulates: (1) The atom has a tiny, dense, positively charged nucleus at its centre. (2) Electrons revolve around the nucleus in circular orbits. (3) The size of the nucleus is very small compared to the size of the atom (nucleus ≈ 10⁻¹⁵ m; atom ≈ 10⁻¹⁰ m).

✅ Achievements

• Discovered the nucleus
• Established nuclear model of atom
• Showed atoms are mostly empty space

❌ Limitations

• Could not explain atomic stability (electrons should spiral into nucleus)
• Could not explain atomic spectra (line spectra)
• No explanation for energy of electrons

⭐ Exam Tip (Most Important!): Rutherford's experiment is one of the most asked topics. Remember the three types of α-particle paths. The 180° deflection proves the existence of nucleus.

1913

C. Bohr's Planetary Model

Niels Bohr modified Rutherford's model to overcome its limitations, drawing on quantum theory proposed by Max Planck.

🪐

Bohr's Postulates:
1. Electrons revolve around the nucleus in fixed, discrete circular paths called orbits / shells / energy levels.
2. While revolving in permitted orbits, electrons do NOT radiate energy (they are in stationary states).
3. Electrons can jump to a higher shell by absorbing energy and return to a lower shell by emitting energy.
4. Each shell has a fixed energy: K < L < M < N.

🪐

Nucleus at centre with K, L, M, N shells containing electrons

🔤 Energy Shells

Shell	Symbol	n value	Max electrons (2n²)
1st Shell	K	1	2 × 1² = 2
2nd Shell	L	2	2 × 2² = 8
3rd Shell	M	3	2 × 3² = 18
4th Shell	N	4	2 × 4² = 32

✅ Advantages

• Explained stability of atoms
• Explained hydrogen's line spectrum
• Introduced energy shells

❌ Limitations

• Could not explain spectra of multi-electron atoms
• Did not account for wave nature of electrons
• Could not explain Zeeman & Stark effects

📊 Comparison of Atomic Models

Aspect	Thomson (1904)	Rutherford (1911)	Bohr (1913)
Nucleus	Not present	Present, +ve, small	Present, small
Electrons	Embedded in +ve sphere	Revolve around nucleus	Revolve in fixed shells
Atomic Stability	Explained (neutral)	Not explained	Explained
Spectra	Not explained	Not explained	Explained (H only)
Energy Levels	No	No	Yes (K, L, M, N)
Base Experiment	CRT Experiment	Gold Foil Experiment	Hydrogen Spectrum

6

Electronic Configuration

📏

Bohr-Bury Rules for Filling Electrons:
Rule 1: Maximum electrons in any shell = 2n² (n = shell number)
Rule 2: The outermost shell cannot have more than 8 electrons
Rule 3: The second-last shell cannot have more than 18 electrons
Rule 4: Electrons are not put in a new shell unless the preceding shells are filled

MAXIMUM ELECTRONS PER SHELL

Maximum Electrons = 2n²

K(n=1): 2 | L(n=2): 8 | M(n=3): 18 | N(n=4): 32

⚛️ Electronic Configurations of Common Elements

H

Hydrogen

1

Z=1 | K:1

He

Helium

2

Z=2 | K:2

Li

Lithium

2,1

Z=3 | K:2, L:1

C

Carbon

2,4

Z=6 | K:2, L:4

O

Oxygen

2,6

Z=8 | K:2, L:6

Na

Sodium

2,8,1

Z=11 | K:2, L:8, M:1

Mg

Magnesium

2,8,2

Z=12 | K:2, L:8, M:2

Al

Aluminium

2,8,3

Z=13 | K:2, L:8, M:3

Cl

Chlorine

2,8,7

Z=17 | K:2, L:8, M:7

Ca

Calcium

2,8,8,2

Z=20 | K:2, L:8, M:8, N:2

📊

Shell-wise diagram showing electrons in K, L, M, N shells

7

Valency Made Easy

Valency is the combining capacity of an element. It tells us how many bonds an atom can form. Valency depends on the number of valence electrons (electrons in the outermost shell).

FOR METALS

Valency = Valence Electrons

Na has 1 valence e⁻ → Valency = 1

FOR NON-METALS

Valency = 8 − Valence e⁻

O has 6 valence e⁻ → Valency = 8−6 = 2

🧪 Valency of Common Elements

H

Hydrogen

Config: 1 | Valence e⁻: 1

Valency = 1

O

Oxygen

Config: 2,6 | Valence e⁻: 6

Valency = 8−6 = 2

N

Nitrogen

Config: 2,5 | Valence e⁻: 5

Valency = 8−5 = 3

Na

Sodium

Config: 2,8,1 | Valence e⁻: 1

Valency = 1

Mg

Magnesium

Config: 2,8,2 | Valence e⁻: 2

Valency = 2

Cl

Chlorine

Config: 2,8,7 | Valence e⁻: 7

Valency = 8−7 = 1

💡

Special Case: Noble gases (He, Ne, Ar) have fully filled outermost shells (He: 2e⁻; others: 8e⁻) and hence have valency = 0. They are inert/unreactive.

📊

Metal → Valency = valence electrons | Non-metal → Valency = 8 − valence electrons

8

Atomic Number & Mass Number

ATOMIC NUMBER (Z)

Z = No. of Protons

In neutral atom: Z = No. of Electrons

MASS NUMBER (A)

A = P + N

Protons + Neutrons (Nucleons)

NUMBER OF NEUTRONS

Neutrons (N) = A − Z

Mass Number − Atomic Number

📋 Worked Examples

Element	Symbol	Atomic No. (Z)	Mass No. (A)	Protons	Neutrons	Electrons
Hydrogen	¹₁H	1	1	1	0	1
Carbon	¹²₆C	6	12	6	6	6
Nitrogen	¹⁴₇N	7	14	7	7	7
Oxygen	¹⁶₈O	8	16	8	8	8
Sodium	²³₁₁Na	11	23	11	12	11
Chlorine	³⁵₁₇Cl	17	35	17	18	17

9

Isotopes

⚛️

Definition: Atoms of the same element that have the same atomic number (same protons) but different mass numbers (different neutrons) are called isotopes. They have identical chemical properties but different physical properties.

🔬 Isotopes of Hydrogen

¹H – Protium

Z=1 | A=1 | Neutrons=0

Most abundant isotope

²H – Deuterium (D)

Z=1 | A=2 | Neutrons=1

Used in heavy water (D₂O)

³H – Tritium (T)

Z=1 | A=3 | Neutrons=2

Radioactive; nuclear weapons research

⚗️ Isotopes of Carbon

¹²C – Carbon-12

Z=6 | A=12 | Neutrons=6

Standard atomic mass reference

¹⁴C – Carbon-14

Z=6 | A=14 | Neutrons=8

Carbon dating of ancient fossils

🏥 Applications of Isotopes

Isotope	Field	Application
Uranium-235	Energy / Industry	Nuclear fuel in atomic reactors to generate electricity
Cobalt-60	Medicine	Treatment of cancer (radiation therapy)
Iodine-131	Medicine	Treatment of goitre (thyroid disorders)
Carbon-14	Research	Radiocarbon dating of fossils and archaeological artifacts
Deuterium (²H)	Industry	Used as a moderator in nuclear reactors (heavy water)

🧠 Memory Tricks

TRICK 1

EPN

E = Electron (−)
P = Proton (+)
N = Neutron (0)
Three main subatomic particles!

TRICK 2

TGC

Thomson → e⁻ (Electron)
Goldstein → p⁺ (Proton)
Chadwick → n (Neutron)
Scientists & their discoveries!

TRICK 3

KLMN

"Kids Love Mangoes Naturally"
K → 1st Shell (2e⁻)
L → 2nd Shell (8e⁻)
M → 3rd Shell (18e⁻)
N → 4th Shell (32e⁻)

TRICK 4

A=P+N

"Mass Means Protons + Neutrons"
A = Mass Number
P = Protons
N = Neutrons
Always remember this formula!

TRICK 5

2n²

Max electrons in shell n = 2n²
Shell K(1): 2×1²=2
Shell L(2): 2×2²=8
Shell M(3): 2×3²=18

TRICK 6

V=8−e

Valency of Non-metals
V = 8 − valence electrons
O has 6 → 8−6=2
Cl has 7 → 8−7=1

🎯 Exam Booster — Very Important Topics

• ⭐⭐⭐⭐⭐ Rutherford's Alpha Scattering Experiment (observations + conclusions)
• ⭐⭐⭐⭐⭐ Bohr's Atomic Model — postulates, energy shells, limitations
• ⭐⭐⭐⭐⭐ Electronic Configuration (write for any element Z=1 to 20)
• ⭐⭐⭐⭐⭐ Valency calculations from electronic configuration
• ⭐⭐⭐⭐⭐ Atomic Number, Mass Number, Number of Neutrons (numericals)
• ⭐⭐⭐⭐ Isotopes — definition, examples, applications
• ⭐⭐⭐⭐ Thomson's Plum Pudding Model — description + limitations
• ⭐⭐⭐ Discovery of subatomic particles — scientists, experiments, year
• ⭐⭐⭐ Comparison of atomic models (Thomson vs Rutherford vs Bohr)

10

NCERT Textbook Questions & Answers

📝 In-Text Questions

NCERT IN-TEXT Q1

What are canal rays?

Canal rays (also called anode rays or positive rays) are streams of positively charged particles moving from anode to cathode in a modified discharge tube with a perforated cathode. They were discovered by E. Goldstein in 1886 during his canal ray experiment. Unlike cathode rays, canal rays depend on the gas used — when hydrogen gas is used, the lightest positive particles (protons) are obtained.

NCERT IN-TEXT Q2

If an atom contains one electron and one proton, will it carry any charge or not?

No, it will not carry any charge. An atom with one electron (−1 charge) and one proton (+1 charge) has equal positive and negative charges. Since the total charge = (+1) + (−1) = 0, the atom is electrically neutral. This is the case with a hydrogen atom (¹H).

NCERT IN-TEXT Q3

On the basis of Thomson's atomic model, explain how the atom is neutral as a whole.

According to Thomson's model, an atom consists of a uniformly distributed positive charge throughout the atom's sphere, and electrons (negative charges) are embedded within this positive sphere. The total positive charge is exactly equal in magnitude to the total negative charge of all embedded electrons. Since the positive and negative charges cancel each other, the atom as a whole remains electrically neutral.

NCERT IN-TEXT Q4

Name three particles of an atom. What is the relative charge of each? What is the relative mass of each particle?

The three particles of an atom are:
1. Electron (e⁻): Relative charge = −1; Relative mass = 1/1840 (negligible)
2. Proton (p⁺): Relative charge = +1; Relative mass = 1 amu
3. Neutron (n): Relative charge = 0; Relative mass = 1 amu

NCERT IN-TEXT Q5

Write the distribution of electrons in carbon and sodium atoms.

Carbon (C), Z = 6: Electronic configuration = 2, 4 (Shell K: 2 electrons; Shell L: 4 electrons)
Sodium (Na), Z = 11: Electronic configuration = 2, 8, 1 (Shell K: 2 electrons; Shell L: 8 electrons; Shell M: 1 electron)

📝 NCERT Exercise Questions

EXERCISE Q1

Compare the properties of electrons, protons and neutrons.

Property	Electron	Proton	Neutron
Charge	−1 (negative)	+1 (positive)	0 (neutral)
Mass (amu)	~1/1840	~1	~1
Location	Shells outside nucleus	Inside nucleus	Inside nucleus
Discoverer	J.J. Thomson (1897)	E. Goldstein (1886)	J. Chadwick (1932)

EXERCISE Q2

What are the limitations of J.J. Thomson's model of the atom?

Thomson's model had these major limitations:
1. It could not explain the results of Rutherford's alpha scattering experiment (large-angle deflection of alpha particles).
2. It could not account for the concentration of positive charge at a single point (nucleus) inside the atom.
3. It failed to explain the stability of an atom and the discrete line spectra of elements.

EXERCISE Q3

What are the limitations of Rutherford's model of the atom?

Rutherford's model had two major limitations:
1. Atomic Instability: According to classical physics, a revolving electron continuously radiates energy, loses speed, and should spiral inward and collapse into the nucleus within a very short time (~10⁻⁸ seconds). This predicts that atoms should be unstable, which contradicts reality.
2. Atomic Spectra: If electrons spiral inward, they should emit radiation of all wavelengths (continuous spectrum). However, elements emit discrete line spectra, which Rutherford's model could not explain.

EXERCISE Q4

Describe Bohr's model of the atom.

Bohr's atomic model is based on these postulates:
1. Electrons revolve around the nucleus in definite, fixed circular paths called shells or orbits or energy levels (K, L, M, N...).
2. While in a permitted orbit, electrons do not emit energy. They are in a stationary state.
3. Each shell has a fixed energy. Electrons can jump to a higher shell by absorbing energy and return to a lower shell by emitting energy.
4. The maximum number of electrons in any shell is given by the formula 2n².

EXERCISE Q5

If the number of electrons in an atom is 8, what is the (i) number of shells, (ii) number of valence electrons?

If the atom has 8 electrons, it is Oxygen (Z = 8).
Electronic configuration: K = 2, L = 6 (i.e., 2, 6)
(i) Number of shells = 2 (K and L shells are occupied)
(ii) Number of valence electrons = 6 (electrons in the outermost L shell)

EXERCISE Q6

With the help of a table, differentiate between isotopes and isobars.

Property	Isotopes	Isobars
Atomic Number (Z)	Same	Different
Mass Number (A)	Different	Same
Number of Neutrons	Different	Different
Element	Same element	Different elements
Chemical Properties	Same	Different
Example	¹H, ²H, ³H (Hydrogen)	ⁿA and ⁿB (e.g., ⁴⁰₁₈Ar and ⁴⁰₂₀Ca)

HOTS QUESTION

Why do Helium (He) and Neon (Ne) not react with any other element under normal conditions?

Helium has electronic configuration 2 (K shell completely filled with 2 electrons). Neon has electronic configuration 2, 8 (K and L shells both completely filled). Since both elements have fully filled outermost shells, they have valency = 0. This means they have no tendency to gain, lose, or share electrons. Hence, they are chemically inert and do not form compounds under normal conditions. They are called noble gases.

11

Extra Exam-Oriented Questions

1-Mark Questions

1 MARK

Q1. Who discovered the neutron?

Ans: James Chadwick discovered the neutron in 1932.

1 MARK

Q2. What is the charge on a proton?

Ans: +1 (positive charge; +1.6 × 10⁻¹⁹ Coulombs).

1 MARK

Q3. What is the maximum number of electrons in the L shell?

Ans: 8 electrons (2n² = 2 × 2² = 8).

1 MARK

Q4. What is the valency of oxygen?

Ans: Valency of oxygen = 8 − 6 = 2 (since oxygen has 6 valence electrons in its outermost shell).

1 MARK

Q5. State the formula for mass number.

Ans: Mass Number (A) = Number of Protons + Number of Neutrons, i.e., A = P + N.

2-Mark Questions

2 MARKS

Q6. What are isotopes? Give two examples.

Ans: Atoms of the same element having the same atomic number but different mass numbers are called isotopes. They have the same number of protons but different numbers of neutrons.
Examples: (i) Hydrogen isotopes: Protium (¹H), Deuterium (²H), Tritium (³H) — all have Z=1 but A=1, 2, 3.
(ii) Carbon isotopes: Carbon-12 (¹²C) and Carbon-14 (¹⁴C) — both have Z=6 but A=12 and 14.

2 MARKS

Q7. Write the electronic configuration of sodium (Na, Z=11) and calculate its valency.

Ans: Electronic configuration of Na (Z=11): K=2, L=8, M=1, i.e., 2,8,1.
Valence electrons = 1 (electrons in outermost M shell).
Since sodium is a metal, Valency = number of valence electrons = 1.
Sodium can form bonds by donating 1 electron.

2 MARKS

Q8. Calculate the number of protons, neutrons, and electrons in ²³₁₁Na.

Ans: Given: Atomic Number (Z) = 11, Mass Number (A) = 23.
• Protons = Z = 11
• Electrons = Z = 11 (neutral atom)
• Neutrons = A − Z = 23 − 11 = 12

3-Mark Questions

3 MARKS

Q9. Describe Rutherford's alpha scattering experiment and state its conclusions.

Ans: Experiment: Rutherford bombarded a very thin gold foil (0.00004 cm thick) with fast-moving alpha particles from a radioactive source. A circular ZnS fluorescent screen surrounding the foil detected the scattered alpha particles as flashes of light.
Conclusions:
1. Most alpha particles passed straight through → most of the atom is empty space.
2. A few were deflected at small angles → a small concentrated positive charge exists inside the atom.
3. Very few bounced back (~180°) → there is a very small, dense, positively charged nucleus at the centre.
4. The size of the nucleus is extremely small compared to the atom (≈ 1/10,000 of atomic diameter).

3 MARKS

Q10. Write the electronic configurations and valencies of: (a) Chlorine (Z=17), (b) Magnesium (Z=12), (c) Nitrogen (Z=7).

Ans:
(a) Chlorine (Z=17): Config = 2,8,7. Valence e⁻ = 7. Non-metal → Valency = 8 − 7 = 1
(b) Magnesium (Z=12): Config = 2,8,2. Valence e⁻ = 2. Metal → Valency = 2
(c) Nitrogen (Z=7): Config = 2,5. Valence e⁻ = 5. Non-metal → Valency = 8 − 5 = 3

5-Mark Questions (Long Answer)

5 MARKS

Q11. Compare Thomson's, Rutherford's, and Bohr's atomic models with their limitations.

1. Thomson's Model (1904): Atom is a sphere of uniform positive charge with electrons embedded in it like plums in a pudding (Plum Pudding Model). Limitation: Could not explain Rutherford's scattering experiment results.

2. Rutherford's Model (1911): Based on the Gold Foil experiment. The atom has a tiny, dense, positively charged nucleus at the centre. Electrons revolve around it in circular orbits. Atom is mostly empty space. Limitation: Could not explain atomic stability (electrons should spiral into nucleus) and could not explain atomic spectra.

3. Bohr's Model (1913): Electrons revolve in fixed energy shells (K, L, M, N) without radiating energy. Energy is emitted/absorbed only when electrons jump between shells. Formula: 2n² for max electrons per shell. Limitation: Could not explain spectra of multi-electron atoms and wave nature of electrons.

Conclusion: Each model was an improvement on the previous, leading towards our modern quantum mechanical understanding of atomic structure.

12

MCQ Practice Zone (40 Questions)

🎯

Each MCQ is followed by the correct answer and a brief explanation. Attempt each question before reading the answer!

Q1

Who discovered the electron?

A. E. Goldstein

B. J.J. Thomson

C. J. Chadwick

D. Niels Bohr

Answer: B. J.J. Thomson discovered the electron in 1897 using the cathode ray tube experiment.

Q2

Canal rays carry:

A. Negative charge

B. Positive charge

C. No charge

D. Both positive and negative

Answer: B. Canal rays (anode rays) are positively charged particles deflected toward the negative plate.

Q3

The maximum number of electrons in the K shell is:

A. 2

B. 8

C. 18

D. 32

Answer: A. Maximum electrons in K shell (n=1) = 2n² = 2×1² = 2.

Q4

The atomic number of an element is equal to the number of:

A. Neutrons in the nucleus

B. Protons in the nucleus

C. Electrons and neutrons

D. Nucleons

Answer: B. Atomic number (Z) = number of protons. In a neutral atom, it also equals the number of electrons.

Q5

Rutherford's atomic model was based on:

A. Cathode ray tube experiment

B. Canal ray experiment

C. Alpha particle scattering experiment

D. Hydrogen spectrum

Answer: C. Rutherford bombarded gold foil with alpha particles in the famous alpha scattering experiment.

Q6

Thomson's atomic model is also called:

A. Planetary Model

B. Plum Pudding Model

C. Nuclear Model

D. Quantum Model

Answer: B. Thomson's model, where electrons are embedded in a positive sphere, resembles a plum pudding dessert.

Q7

The mass number of an element with 6 protons and 6 neutrons is:

A. 6

B. 12

C. 18

D. 3

Answer: B. Mass Number = Protons + Neutrons = 6 + 6 = 12. This is Carbon-12 (¹²C).

Q8

The electronic configuration of sodium (Na, Z=11) is:

A. 2, 9

B. 2, 6, 3

C. 2, 8, 1

D. 1, 8, 2

Answer: C. Na has 11 electrons distributed as: K=2, L=8, M=1, giving configuration 2,8,1.

Q9

Which of the following has valency 2?

A. Hydrogen

B. Sodium

C. Oxygen

D. Chlorine

Answer: C. Oxygen has 6 valence electrons; Valency = 8−6 = 2.

Q10

Isotopes have the same:

A. Mass number

B. Atomic number

C. Number of neutrons

D. Physical properties

Answer: B. Isotopes have the same atomic number (same protons/same element) but different mass numbers (different neutrons).

Q11

The neutron was discovered by:

A. Thomson

B. Goldstein

C. Chadwick

D. Rutherford

Answer: C. James Chadwick discovered the neutron in 1932, receiving the Nobel Prize in 1935.

Q12

In Rutherford's experiment, most alpha particles:

A. Were deflected by 90°

B. Bounced back

C. Passed straight through the gold foil

D. Were absorbed by gold foil

Answer: C. Most alpha particles passed straight through, proving that atoms are mostly empty space.

Q13

The number of neutrons in ²³₁₁Na is:

A. 11

B. 23

C. 12

D. 34

Answer: C. Neutrons = Mass Number − Atomic Number = 23 − 11 = 12.

Q14

The formula for the maximum number of electrons in a shell is:

A. n²

B. 2n²

C. 2n

D. n/2

Answer: B. According to Bohr-Bury rules, max electrons in nth shell = 2n².

Q15

Iodine-131 is used in the treatment of:

A. Cancer

B. Goitre (thyroid disorders)

C. Diabetes

D. Heart disease

Answer: B. Iodine-131 isotope is used medically in the treatment of goitre and thyroid disorders.

Q16

Which atomic model was proposed by Niels Bohr?

A. Plum Pudding Model

B. Nuclear Model

C. Planetary Model

D. Wave Mechanical Model

Answer: C. Bohr's Planetary Model shows electrons orbiting the nucleus like planets orbit the sun, in fixed shells.

Q17

The valency of nitrogen (N, Z=7, config: 2,5) is:

A. 5

B. 2

C. 3

D. 7

Answer: C. Nitrogen has 5 valence electrons. Non-metal → Valency = 8 − 5 = 3.

Q18

Carbon-14 is used for:

A. Cancer treatment

B. Nuclear energy

C. Radiocarbon dating of fossils

D. Treatment of thyroid

Answer: C. Carbon-14 (radioactive isotope) is used in radiocarbon dating to determine the age of ancient fossils and archaeological samples.

Q19

Which particle has zero mass and negative charge?

A. Electron

B. Proton

C. Neutron

D. Alpha particle

Answer: A. Electron has negligible/near-zero mass (1/1840 amu) and carries negative charge.

Q20

Noble gases have valency:

A. 1

B. 2

C. 8

D. 0

Answer: D. Noble gases have fully filled outermost shells and do not need to gain/lose electrons. Hence valency = 0.

Q21

The shells in an atom from inside to outside are:

A. K, L, M, N

B. N, M, L, K

C. A, B, C, D

D. 1, 2, 3, 4

Answer: A. Shells are named K, L, M, N from innermost to outermost. Mnemonic: "Kids Love Mangoes Naturally".

Q22

Which experiment led to the discovery of the nucleus?

A. Cathode ray tube experiment

B. Canal ray experiment

C. Gold foil alpha scattering experiment

D. Hydrogen spectrum experiment

Answer: C. Rutherford's gold foil experiment proved the existence of a small, dense, positively charged nucleus.

Q23

The atomic number of chlorine is 17. How many electrons are in its outermost shell?

A. 1

B. 2

C. 8

D. 7

Answer: D. Chlorine config = 2,8,7. Outermost shell (M) has 7 electrons.

Q24

Deuterium is an isotope of:

A. Carbon

B. Helium

C. Hydrogen

D. Oxygen

Answer: C. Deuterium (²H) is an isotope of hydrogen with 1 proton and 1 neutron.

Q25

According to Bohr's model, an electron emits energy when it:

A. Moves to a higher shell

B. Moves from a higher shell to a lower shell

C. Stays in the same shell

D. Revolves around the nucleus

Answer: B. When an electron falls from a higher energy shell to a lower shell, it releases energy in the form of radiation.

Q26

The number of protons in ¹⁶₈O is:

A. 16

B. 24

C. 8

D. 10

Answer: C. The subscript (8) gives the atomic number = number of protons = 8.

Q27

The maximum number of electrons in M shell is:

A. 2

B. 8

C. 18

D. 32

Answer: C. M shell: n=3, max electrons = 2n² = 2×9 = 18.

Q28

Uranium-235 is used as:

A. Cancer treatment

B. Radiocarbon dating

C. Thyroid treatment

D. Nuclear fuel in reactors

Answer: D. Uranium-235 is used as fuel in nuclear reactors to generate electricity.

Q29

The limitation of Bohr's model was that it could not explain:

A. Stability of atoms

B. Discovery of nucleus

C. Spectra of multi-electron atoms

D. Electronic configuration

Answer: C. Bohr's model could explain only hydrogen's line spectrum. It failed for atoms with more than one electron.

Q30

In Thomson's model, what is the atom compared to?

A. Plum pudding

B. Solar system

C. Billiard ball

D. Waterfall

Answer: A. Thomson's model is called the "Plum Pudding Model" — the positive sphere is pudding and electrons are plums embedded in it.

Q31

An atom has 8 protons and 10 neutrons. Its mass number is:

A. 10

B. 8

C. 2

D. 18

Answer: D. Mass Number = Protons + Neutrons = 8 + 10 = 18.

Q32

The valency of magnesium (Mg, Z=12) is:

A. 1

B. 2

C. 3

D. 8

Answer: B. Mg config = 2,8,2. Valence electrons = 2. Metal → Valency = 2.

Q33

Which of the following is NOT a subatomic particle?

A. Electron

B. Proton

C. Neutron

D. Molecule

Answer: D. A molecule is a group of atoms. Electrons, protons, and neutrons are the three subatomic particles of an atom.

Q34

The cathode ray particles were deflected towards:

A. The negative plate

B. No deflection

C. The positive plate

D. Both plates equally

Answer: C. Electrons (negative charge) are attracted toward the positive plate, proving they carry negative charge.

Q35

The electronic configuration of calcium (Ca, Z=20) is:

A. 2, 8, 10

B. 2, 18

C. 2, 8, 8, 2

D. 2, 8, 4, 6

Answer: C. Ca (Z=20): K=2, L=8, M=8, N=2, giving config 2,8,8,2. (Outermost shell can't exceed 8.)

Q36

Cobalt-60 is used in the treatment of:

A. Cancer

B. Goitre

C. Tuberculosis

D. Malaria

Answer: A. Cobalt-60 emits gamma radiation and is used in cancer radiation therapy.

Q37

Which model of the atom was proved wrong by Rutherford's experiment?

A. Bohr's model

B. Thomson's plum pudding model

C. Wave mechanical model

D. Dalton's model

Answer: B. Rutherford's alpha scattering experiment disproved Thomson's model by showing positive charge is concentrated in a nucleus, not spread uniformly.

Q38

The number of neutrons in ¹²₆C is:

A. 12

B. 18

C. 6

D. 2

Answer: C. Neutrons = A − Z = 12 − 6 = 6.

Q39

In an atom, which particles are present in the nucleus?

A. Electrons and protons

B. Protons and neutrons

C. Electrons and neutrons

D. Only protons

Answer: B. The nucleus contains protons (positive) and neutrons (neutral). Electrons are present outside the nucleus in shells.

Q40

Which of the following pairs represents isotopes?

A. ¹⁶₈O and ¹⁶₇N

B. ¹²₆C and ¹⁴₆C

C. ²³₁₁Na and ²³₁₂Mg

D. ⁴₂He and ⁴₃Li

Answer: B. ¹²C and ¹⁴C both have Z=6 (same element) but A=12 and 14 respectively (different mass numbers) → isotopes.

13

Previous Year Board Questions

ASSAM BOARD 2 MARKS

What is meant by atomic number? Write the atomic number of oxygen.

Atomic number (Z) is the number of protons present in the nucleus of an atom. It is unique for each element. The atomic number of oxygen is 8 (oxygen has 8 protons in its nucleus).

ASSAM BOARD 3 MARKS

State the postulates of Bohr's atomic model. What are its limitations?

Postulates: (1) Electrons revolve in fixed circular orbits (K, L, M, N) without radiating energy. (2) Each shell has a fixed energy level. (3) Electrons absorb energy to jump to higher shells and release energy when falling to lower shells. Max electrons = 2n².
Limitation: Could not explain spectra of multi-electron atoms; could not account for wave nature of electrons.

ASSAM BOARD 2 MARKS

Write two applications of isotopes.

1. Uranium-235 is used as fuel in nuclear reactors for generating electricity.
2. Cobalt-60 is used in radiotherapy for treatment of cancer.

CBSE 3 MARKS

Describe Rutherford's alpha particle scattering experiment and state its main conclusions.

Rutherford bombarded a thin gold foil with alpha particles. A ZnS screen surrounding the foil detected scattered alpha particles.
Observations: (1) Most particles passed straight through → atom is mostly empty. (2) A few were deflected → concentrated positive charge exists. (3) Very few bounced back 180° → tiny, dense nucleus at centre.
Conclusions: Atom has a tiny dense positively charged nucleus; rest of atom is empty space; electrons orbit outside nucleus.

CBSE 1 MARK

Who discovered the proton?

E. Goldstein discovered the proton in 1886 through the canal ray experiment.

ASSAM BOARD 5 MARKS

Write the electronic configurations of the following elements and determine their valencies: Na (Z=11), O (Z=8), Cl (Z=17), Mg (Z=12), N (Z=7).

Na (Z=11): 2,8,1 → Valence e⁻=1 (metal) → Valency = 1
O (Z=8): 2,6 → Valence e⁻=6 (non-metal) → Valency = 8−6 = 2
Cl (Z=17): 2,8,7 → Valence e⁻=7 (non-metal) → Valency = 8−7 = 1
Mg (Z=12): 2,8,2 → Valence e⁻=2 (metal) → Valency = 2
N (Z=7): 2,5 → Valence e⁻=5 (non-metal) → Valency = 8−5 = 3

14

One-Page Revision Sheet

⚡ Quick Revision — Chapter 4: Structure of the Atom

📐 Key Formulas

• Mass Number A = P + N
• Atomic Number Z = Protons
• Neutrons N = A − Z
• Max e⁻ in shell = 2n²
• Valency (metal) = valence e⁻
• Valency (non-metal) = 8 − valence e⁻

🔬 Scientists

• Thomson (1897) → e⁻
• Goldstein (1886) → p⁺
• Chadwick (1932) → n
• Rutherford (1911) → Nucleus
• Bohr (1913) → Energy shells

🪐 Shells

• K (n=1): max 2 e⁻
• L (n=2): max 8 e⁻
• M (n=3): max 18 e⁻
• N (n=4): max 32 e⁻
• Outermost: max 8 e⁻

⚛️ Atomic Models

• Thomson: Plum Pudding
• Rutherford: Nuclear Model
• Bohr: Planetary Model
• Each improved the last

🧪 Isotopes

• Same Z, different A
• Same protons, diff neutrons
• H: ¹H, ²H, ³H
• C: ¹²C, ¹⁴C
• Same chemical properties

🏥 Isotope Uses

• U-235 → Nuclear energy
• Co-60 → Cancer treatment
• I-131 → Goitre treatment
• C-14 → Carbon dating
• Deuterium → Heavy water

⚡ Valency Examples

• H → 1, He → 0
• O → 2, N → 3
• Na → 1, Mg → 2
• Cl → 1, Ca → 2
• Noble gases → 0

🧠 Memory Tricks

• EPN = e⁻, p⁺, n
• TGC = Thomson/Goldstein/Chadwick
• KLMN = "Kids Love Mangoes Naturally"
• A = P + N

15

Frequently Asked Questions

Q1. Who discovered the electron? ▼

J.J. Thomson discovered the electron in 1897 through the cathode ray tube (CRT) experiment. He found that cathode rays, when deflected toward a positive plate, confirmed they were made of negatively charged particles — later named electrons.

Q2. What is atomic number? How is it different from mass number? ▼

Atomic Number (Z) = number of protons in the nucleus of an atom. It identifies the element. Mass Number (A) = total number of protons + neutrons (nucleons) in the nucleus. Atomic number is unique for each element, while mass number can differ for isotopes of the same element.

Q3. What are isotopes? Give examples. ▼

Isotopes are atoms of the same element that have the same atomic number (same protons) but different mass numbers (different neutrons). Examples: Hydrogen isotopes — Protium (¹H), Deuterium (²H), Tritium (³H). Carbon isotopes — Carbon-12 (¹²C) and Carbon-14 (¹⁴C). They have identical chemical properties but different physical properties.

Q4. Why is Bohr's model important? ▼

Bohr's model is important because: (1) It explained the stability of atoms by proposing fixed energy shells where electrons don't radiate energy. (2) It successfully explained the line spectrum of hydrogen. (3) It introduced the concept of quantized energy levels (K, L, M, N shells). (4) It laid the foundation for quantum mechanics and modern atomic theory.

Q5. What is valency? How is it calculated? ▼

Valency is the combining capacity of an element. It depends on valence electrons (electrons in the outermost shell). For metals: Valency = number of valence electrons. For non-metals: Valency = 8 − valence electrons. Noble gases have valency = 0 (fully filled shells).

Q6. Why did Rutherford's model fail? ▼

Rutherford's model failed for two main reasons: (1) According to classical electrodynamics, an electron revolving around the nucleus continuously radiates energy, loses speed, and should spiral into the nucleus within 10⁻⁸ seconds — making atoms unstable. But atoms are stable. (2) The model predicted continuous electromagnetic radiation, but elements actually show discrete line spectra — which Rutherford couldn't explain.

Q7. What is the difference between atomic number and mass number? ▼

Atomic Number (Z) = number of protons only. Mass Number (A) = protons + neutrons. For Carbon-12: Z=6 (protons), A=12 (6 protons + 6 neutrons). Isotopes of the same element always have the same Z but different A.

Q8. Why are noble gases unreactive? ▼

Noble gases (He, Ne, Ar, etc.) have completely filled outermost shells — He has 2 electrons (K shell full), others have 8 electrons in the outermost shell. Since the octet (or duplet for He) is complete, they have no tendency to gain, lose, or share electrons. Therefore their valency = 0 and they do not form bonds under normal conditions.

Q9. What is the formula for maximum electrons in a shell? Give examples. ▼

Formula: Maximum electrons in the nth shell = 2n². Examples: K shell (n=1): 2×1²=2 | L shell (n=2): 2×4=8 | M shell (n=3): 2×9=18 | N shell (n=4): 2×16=32. However, the outermost shell never holds more than 8 electrons.

Q10. How are isotopes useful in medicine and industry? ▼

Medical uses: (1) Cobalt-60 is used in radiotherapy to treat cancer. (2) Iodine-131 is used to treat thyroid disorders (goitre). Industrial uses: (3) Uranium-235 is used as nuclear fuel in reactors to generate electricity. (4) Carbon-14 is used in radiocarbon dating of fossils. (5) Deuterium is used as a moderator in nuclear reactors (in heavy water D₂O).

16

Last-Minute Exam Tips

❌ Common Mistakes

• Confusing atomic number with mass number
• Forgetting Neutrons = A − Z (not A alone)
• Writing isotopes have same A (they have same Z)
• Mixing up Thomson (electron) and Goldstein (proton)
• Writing max L-shell = 10 instead of 8
• Forgetting noble gases have valency 0
• Using valence electrons as valency for non-metals (correct formula: 8 − valence e⁻)

✅ High-Scoring Strategy

• Always draw and label diagrams for atomic models
• Memorize the 3 observations and conclusions of Rutherford's experiment
• Practice writing electronic configurations up to Z=20
• Learn all isotope applications by heart (1-mark questions)
• Prepare a comparison table of Thomson/Rutherford/Bohr models
• Practice numericals on mass number and number of neutrons
• Attempt MCQs daily — 10 per day

📅 Revision Plan

• Day 1: Subatomic particles + discoveries
• Day 2: Atomic models + diagrams
• Day 3: Electronic configuration + valency
• Day 4: Atomic number, mass number + numericals
• Day 5: Isotopes + applications
• Day 6: NCERT exercises + board questions
• Day 7: MCQ revision + memory tricks

🎓

Conclusion

Understanding the Structure of the Atom is the foundation of Chemistry and Physics. Master this chapter thoroughly because it helps in higher classes and competitive examinations like NEET, JEE, and Board exams. Keep practising diagrams, numerical questions, and MCQs for excellent marks. You can do it!

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