Class 11 Chemistry Chapter 2 – Structure of Atom

1. Introduction

Every substance is made of atoms.
Earlier, Dalton believed atoms could not be divided.
Later experiments proved that atoms contain smaller particles called electrons, protons and neutrons.
Understanding atomic structure helps explain the physical and chemical properties of elements.

2. Discovery of Subatomic Particles

A. Electron

Scientist

J. J. Thomson (1897)

Experiment

Cathode Ray Discharge Tube

Observations

Rays travel from cathode to anode.
They move in straight lines.
They are attracted towards the positive plate.
Their properties do not depend on the gas used.

Conclusion

Cathode rays are made of negatively charged particles.
These particles are called electrons.
Electrons are present in every atom.

Charge-to-Mass Ratio (e/m)

Scientist:
J. J. Thomson

Value:

e/m = 1.76 × 10¹¹ C kg⁻¹

Charge of Electron

Scientist:
Robert Millikan

Experiment:
Oil Drop Experiment

Charge on electron

−1.602 × 10⁻¹⁹ C

Mass of electron

9.11 × 10⁻³¹ kg

3. Proton

Discovered using Canal Rays (Positive Rays).

Properties

Positive charge
Charge equal and opposite to electron
Mass nearly 1836 times greater than electron

Charge

+1.602 × 10⁻¹⁹ C

Mass

1.67 × 10⁻²⁷ kg

4. Neutron

Scientist

James Chadwick (1932)

Properties

No charge
Present inside nucleus
Mass almost equal to proton

5. Fundamental Particles

Particle	Charge	Relative Mass	Location
Electron	−1	1/1836	Outside nucleus
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus

6. Thomson’s Atomic Model

Also called

Plum Pudding Model
Watermelon Model

Main Ideas

Atom is a positively charged sphere.
Electrons are embedded inside it.
Positive charge is spread uniformly.

Merits

Explained electrical neutrality.

Drawbacks

Could not explain Rutherford’s experiment.
Could not explain atomic stability.

7. Rutherford’s Atomic Model

Experiment

Gold Foil (Alpha Particle Scattering Experiment)

Observations

Most α-particles passed straight.

Some were slightly deflected.

Very few bounced back.

Conclusions

Most of the atom is empty space.
Positive charge is concentrated in the nucleus.
Electrons revolve around the nucleus.

8. Drawbacks of Rutherford Model

Could not explain why electrons do not fall into nucleus.
Could not explain atomic spectra.
Could not explain arrangement of electrons.

9. Atomic Number (Z)

Definition

Number of protons in the nucleus.

For a neutral atom

Atomic Number = Number of Protons = Number of Electrons

Example

Carbon

Protons = 6

Electrons = 6

Atomic Number = 6

10. Mass Number (A)

Definition

Total number of protons and neutrons.

Formula

Mass Number = Protons + Neutrons

Example

Oxygen

Protons = 8

Neutrons = 8

Mass Number = 16

11. Isotopes

Definition

Atoms having

Same atomic number
Different mass numbers

Examples

Hydrogen
Carbon-12 and Carbon-14
Chlorine-35 and Chlorine-37

Important Point

Chemical properties remain almost the same because the number of electrons is the same.

12. Isobars

Definition

Atoms having

Same mass number
Different atomic numbers

Example

Carbon-14

Nitrogen-14

13. Electromagnetic Radiation

Produced by accelerating charged particles.

Travels at the speed of light.

Speed of light

c = 3 × 10⁸ m/s

Formula

c = νλ

where

c = speed of light
ν = frequency
λ = wavelength

14. Electromagnetic Spectrum

Order

Radio Waves

↓

Microwaves

↓

Infrared

↓

Visible Light

↓

Ultraviolet

↓

X-rays

↓

Gamma Rays

15. Planck’s Quantum Theory

Scientist

Max Planck

Main Idea

Energy is emitted or absorbed in small packets called quanta.

Formula

E = hν

where

E = Energy
h = Planck’s constant
ν = Frequency

Planck’s Constant

6.626 × 10⁻³⁴ J s

16. Photoelectric Effect

Scientist

Albert Einstein

Observation

Electrons are emitted when light of sufficient frequency falls on a metal surface.

Important Terms

Threshold Frequency

Minimum frequency needed to remove electrons.

Work Function

Minimum energy required to remove an electron.

Equation

KE = hν − W

17. Dual Nature of Light

Light behaves as

Wave
Particle (Photon)

This is called Wave-Particle Duality.

18. Atomic Spectrum

Atoms emit light of specific wavelengths.

Each element has its own unique spectrum.

Types

Emission Spectrum
Absorption Spectrum

Hydrogen shows line spectrum.

19. Bohr’s Atomic Model

Scientist

Niels Bohr (1913)

Postulates

Electrons move only in fixed circular orbits.
Electrons do not lose energy while moving in allowed orbits.
Energy is absorbed or emitted when electrons jump between energy levels.
Angular momentum is quantized.

20. Bohr’s Energy Equation

Energy of nth orbit

Eₙ = −2.18 × 10⁻¹⁸ / n² J

Ground State

n = 1

Highest stability

21. Radius of Orbit

Formula

rₙ = n²a₀

where

a₀ = 52.9 pm

22. Hydrogen Spectrum Series

Series	Final Orbit	Region
Lyman	n = 1	Ultraviolet
Balmer	n = 2	Visible
Paschen	n = 3	Infrared
Brackett	n = 4	Infrared
Pfund	n = 5	Infrared

23. Limitations of Bohr Model

Works only for hydrogen-like atoms.
Cannot explain spectra of multi-electron atoms.
Cannot explain fine spectral lines.
Fails under strong magnetic and electric fields.

Important Formula Sheet

Atomic Number = Protons
Mass Number = Protons + Neutrons
Neutrons = A − Z
c = νλ
E = hν
KE = hν − W
Eₙ = −2.18 × 10⁻¹⁸/n²
rₙ = n²a₀

One-Mark Questions

Who discovered the electron?
What is the charge on an electron?
Define atomic number.
Define mass number.
What are isotopes?
What are isobars?
State Planck’s quantum theory.
What is threshold frequency?
State Bohr’s first postulate.
Write the value of Planck’s constant.

Quick Revision

Electron → J. J. Thomson
Proton → Canal rays
Neutron → Chadwick
Gold foil experiment → Rutherford
Oil drop experiment → Millikan
Quantum theory → Planck
Photoelectric effect → Einstein
Bohr model → Niels Bohr
Speed of light → 3 × 10⁸ m/s
Planck constant → 6.626 × 10⁻³⁴ J s
Electron charge → −1.602 × 10⁻¹⁹ C
Electron mass → 9.11 × 10⁻³¹ kg

These notes are an original summary of the concepts in the chapter, rewritten in fresh language for Class 11 students rather than copied from the textbook.

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The remaining topics still need to be converted into notes, including:

✅ Dual nature of matter (de Broglie hypothesis)
✅ Heisenberg Uncertainty Principle
✅ Quantum Mechanical Model of Atom
✅ Atomic Orbitals
✅ Quantum Numbers
✅ Shapes of s, p, d orbitals
✅ Aufbau Principle
✅ Pauli Exclusion Principle
✅ Hund’s Rule
✅ Electronic Configuration
✅ Exceptions (Cr, Cu)
✅ Summary & Revision Notes
✅ Important Formula Sheet
✅ Previous Year/NCERT-based Questions
✅ Mind Maps
✅ One-shot Revision Notes

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📘 Chapter-wise headings
📌 Highlighted definitions
📊 Easy tables
🧠 Memory tricks
⭐ Exam-important points
❓Practice questions
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ok write all possible mcq, fill in the blank etc types of q from this chapter

I will prepare a complete question bank for Class 11 Chemistry Chapter 2: Structure of Atom from your uploaded chapter. It will include:

✅ MCQs (with answers)
✅ Assertion–Reason questions
✅ Fill in the blanks
✅ True/False
✅ Match the following
✅ Very short answer questions
✅ Short answer questions
✅ Long answer questions
✅ Numerical-based questions
✅ Important exam questions

Since the chapter is large, I will provide it in parts to keep it organized.

Class 11 Chemistry

Chapter 2: Structure of Atom

Question Bank (Part 1)

A. Multiple Choice Questions (MCQs)

1. The word atom is derived from the Greek word:

A) Atomus
B) A-tomio
C) Atomos
D) Anode

Answer: B) A-tomio

2. Dalton proposed his atomic theory in:

A) 1798
B) 1808
C) 1897
D) 1913

Answer: B) 1808

3. The discovery of electron was done by:

A) Rutherford
B) Bohr
C) J.J. Thomson
D) Chadwick

Answer: C) J.J. Thomson

4. Cathode rays are:

A) Positively charged particles
B) Neutral particles
C) Negatively charged particles
D) Electromagnetic waves

Answer: C) Negatively charged particles

5. The charge-to-mass ratio of electron was calculated by:

A) Millikan
B) Thomson
C) Planck
D) Bohr

Answer: B) Thomson

6. The charge on an electron was determined by:

A) Rutherford
B) Chadwick
C) Millikan
D) Dalton

Answer: C) Millikan

7. The oil drop experiment was performed by:

A) J.J. Thomson
B) R.A. Millikan
C) Goldstein
D) Bohr

Answer: B) R.A. Millikan

8. The charge on electron is:

A) +1.602 × 10⁻¹⁹ C
B) −1.602 × 10⁻¹⁹ C
C) 0 C
D) 1 C

Answer: B) −1.602 × 10⁻¹⁹ C

9. The mass of electron is:

A) 9.11 × 10⁻³¹ kg
B) 1.67 × 10⁻²⁷ kg
C) 1.008 kg
D) 0 kg

Answer: A) 9.11 × 10⁻³¹ kg

10. Proton was discovered through:

A) Cathode rays
B) Canal rays
C) X-rays
D) Gamma rays

Answer: B) Canal rays

11. Neutron was discovered by:

A) Rutherford
B) Chadwick
C) Thomson
D) Bohr

Answer: B) Chadwick

12. The charge on neutron is:

A) +1
B) −1
C) Zero
D) +2

Answer: C) Zero

13. Rutherford’s experiment used:

A) β-particles
B) α-particles
C) γ-rays
D) X-rays

Answer: B) α-particles

14. Rutherford used a thin foil of:

A) Silver
B) Copper
C) Gold
D) Aluminium

Answer: C) Gold

15. Most of the α-particles passed through gold foil because:

A) Atom is solid
B) Atom has empty space
C) Nucleus is large
D) Electrons are heavy

Answer: B) Atom has empty space

16. The nucleus contains:

A) Electrons only
B) Protons and neutrons
C) Neutrons only
D) Electrons and protons

Answer: B) Protons and neutrons

17. Atomic number represents the number of:

A) Neutrons
B) Electrons + neutrons
C) Protons
D) Nucleons

Answer: C) Protons

18. Mass number is equal to:

A) Protons + electrons
B) Protons + neutrons
C) Electrons + neutrons
D) Only neutrons

Answer: B) Protons + neutrons

19. Isotopes have:

A) Same mass number and different atomic number
B) Same atomic number and different mass number
C) Same number of neutrons
D) Different chemical symbols

Answer: B) Same atomic number and different mass number

20. Carbon-12 and Carbon-14 are:

A) Isobars
B) Isotopes
C) Isomers
D) Ions

Answer: B) Isotopes

B. Fill in the Blanks

The smallest particle of an element is called ______.
Answer: Atom
Electron was discovered by ______.
Answer: J.J. Thomson
The charge of electron is ______.
Answer: Negative
Millikan used ______ experiment to determine electron charge.
Answer: Oil drop
Proton is present in the ______.
Answer: Nucleus
Neutron was discovered by ______.
Answer: James Chadwick
Rutherford performed ______ scattering experiment.
Answer: Alpha particle
The central part of atom is called ______.
Answer: Nucleus
Atomic number is represented by symbol ______.
Answer: Z
Mass number is represented by symbol ______.
Answer: A
Atoms having same atomic number but different mass numbers are called ______.
Answer: Isotopes
Atoms having same mass number but different atomic numbers are called ______.
Answer: Isobars
The speed of electromagnetic radiation is ______ m/s.
Answer: 3 × 10⁸
Planck proposed the ______ theory.
Answer: Quantum
Energy of radiation is given by ______.
Answer: E = hν

C. True or False

Electron has positive charge.
❌ False
Proton is heavier than electron.
✅ True
Neutrons are present outside the nucleus.
❌ False
Rutherford model explained atomic stability completely.
❌ False
Atomic number equals number of protons in a neutral atom.
✅ True
Isotopes have identical chemical properties.
✅ True
Light shows only wave nature.
❌ False
Planck introduced quantum theory.
✅ True
Photoelectric effect was explained by Einstein.
✅ True
Bohr model was proposed in 1913.
✅ True

D. Match the Following

Column A	Column B
J.J. Thomson	Electron
Millikan	Oil drop experiment
Rutherford	Gold foil experiment
Chadwick	Neutron
Planck	Quantum theory
Einstein	Photoelectric effect
Bohr	Atomic mode

Question Bank (Part 2)

Answer: B

B. Assertion–Reason Questions

Choose:

A Both Assertion and Reason are true, and Reason is the correct explanation.
B Both are true, but Reason is not the correct explanation.
C Assertion is true, Reason is false.
D Assertion is false, Reason is true.

1.
Assertion: Most α-particles passed through the gold foil.
Reason: Most of the atom is empty space.

Answer: A

2.
Assertion: Isotopes have similar chemical properties.
Reason: They have the same atomic number.

Answer: A

3.
Assertion: Bohr model explains spectra of all elements.
Reason: Electrons revolve in fixed orbits.

Answer: C

4.
Assertion: Photoelectric effect supports the particle nature of light.
Reason: Light consists of photons.

Answer: A

5.
Assertion: Electrons lose energy while moving in Bohr’s allowed orbits.
Reason: They continuously emit radiation.

Answer: D

C. Fill in the Blanks

The Bohr model was proposed in ______.
The Balmer series lies in the ______ region.
The Lyman series lies in the ______ region.
The smallest packet of energy is called a ______.
The SI unit of wavelength is ______.
The SI unit of frequency is ______.
______ explained the photoelectric effect.
The first Bohr orbit has principal quantum number ______.
Electrons absorb energy to move to a ______ energy level.
Hydrogen spectrum consists of ______ lines.

Answers: 1913, visible, ultraviolet, quantum, metre (m), hertz (Hz), Einstein, 1, higher, spectral.

D. True / False

Bohr model explains the hydrogen spectrum. ✅
Energy of photons depends on frequency. ✅
Wavelength and frequency are directly proportional. ❌
Balmer series lies in the ultraviolet region. ❌
Threshold frequency differs for different metals. ✅
Photoelectric effect occurs with any frequency of light. ❌
Line spectrum is characteristic of an element. ✅
Electrons emit energy when jumping to lower energy levels. ✅
Ground state is the most stable state. ✅
Bohr model successfully explains all multi-electron atoms. ❌