📘 Class 9 Science Notes
Chapter 8: Journey Inside the Atom
1. Introduction
- Everything around us is made of matter.
- Matter is made of tiny particles called atoms.
- Atoms are extremely small and cannot be seen with the naked eye.
2. History of Atomic Theory
Ancient Ideas
Acharya Kanada (India)
- Proposed that matter is made of tiny indivisible particles called Parmanu.
- Parmanus combine to form all substances.
Limitation: It could not explain the ratio in which atoms combine to form substances.
Democritus (Greece)
- Called these tiny particles Atomos, meaning indivisible.
Dalton’s Atomic Theory (1808)
John Dalton proposed that:
- Matter consists of atoms.
- Atoms cannot be created or destroyed in chemical reactions.
- Atoms of the same element are identical.
- Different elements have different atoms.
- Atoms combine in fixed ratios to form compounds.
- Atom can not be broken down.
Limitation: Later experiments proved that atoms are divisible.
3. Discovery of Electron
J.J. Thomson (1897)
- Using the Cathode Ray Experiment, he discovered the electron.
- Electrons are embedded inside atom.
Thomson’s Atomic Model (Plum Pudding Model)
- Atom is a positively charged sphere.
- Electrons are embedded inside it.
- Total positive and negative charges balance each other.
Limitation
- Could not explain Rutherford’s Gold Foil Experiment.
4. Rutherford’s Gold Foil Experiment
Experiment
- Alpha particles were fired at a thin gold foil.
Observations
- Most particles passed straight through.
- Some were deflected.
- Very few bounced back.
Conclusions
- Most of the atom is empty space.
- Positive charge is concentrated in a tiny nucleus.
- Electrons revolve around the nucleus.
Rutherford’s Atomic Model
- Dense nucleus at the centre.
- Electrons move around the nucleus.
Limitation
Could not explain why electrons do not fall into the nucleus.
5. Bohr’s Atomic Model (1913)
Bohr improved Rutherford’s model.
Main Points
- Electrons move only in fixed circular paths called shells.
- Shells are named K, L, M, N.
- Each shell has fixed energy.
- Electrons do not lose energy while moving in these shells.
- Electrons jump from one shell to another by gaining or losing energy.
6. Subatomic Particles
| Particle | Symbol | Charge | Location |
|---|---|---|---|
| Electron | e⁻ | -1 | Outside nucleus |
| Proton | p⁺ | +1 | Nucleus |
| Neutron | n⁰ | 0 | Nucleus |
7. Discovery of Neutron
James Chadwick (1932)
- Discovered the neutron.
- Neutron has no charge.
- Its mass is almost equal to a proton.
8. Important Terms
Atomic Number (Z)
- Number of protons.
- Equal to the number of electrons in a neutral atom.
Example
Carbon → Atomic number = 6
Mass Number (A)
Mass Number = Protons + Neutrons
Example:
Helium
- Protons = 2
- Neutrons = 2
Mass Number = 2 + 2 = 4
9. Electronic Configuration
Electrons are arranged in shells.
Maximum Electrons
Formula:
2n²
| Shell | Maximum Electrons |
|---|---|
| K | 2 |
| L | 8 |
| M | 18 |
| N | 32 |
Filling Rule
Electrons fill shells from inner to outer.
K → L → M → N
Examples
Hydrogen = 1
Electronic configuration:
1
Helium = 2
Electronic configuration:
2
Carbon = 6
Electronic configuration:
2,4
Oxygen = 8
Electronic configuration:
2,6
Sodium = 11
Electronic configuration:
2,8,1
Chlorine = 17
Electronic configuration:
2,8,7
Argon = 18
Electronic configuration:
2,8,8
10. Valence Electrons
Electrons present in the outermost shell are called valence electrons.
Example:
Oxygen
Electronic configuration = 2,6
Valence electrons = 6
11. Valency
Valency is the combining capacity of an atom.
Easy Rule
| Valence Electrons | Valency |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 5 | 3 |
| 6 | 2 |
| 7 | 1 |
| 8 | 0 |
Examples:
- Sodium → 1
- Magnesium → 2
- Aluminium → 3
- Carbon → 4
- Oxygen → 2
- Chlorine → 1
- Neon → 0
12. Symbols of Elements
Rules:
- First letter is always capital.
- Second letter (if present) is small.
Examples:
Hydrogen → H
Aluminium → Al
Chlorine → Cl
Iron → Fe
Gold → Au
Sodium → Na
Potassium → K
Silver → Ag
Mercury → Hg
13. Isotopes
Atoms of the same element having:
- Same atomic number
- Different mass numbers
Examples
Hydrogen
¹H
²H
³H
Carbon
¹²C
¹³C
¹⁴C
Uses
- Uranium-235 → Nuclear fuel
- Cobalt-60 → Cancer treatment
- Iodine-131 → Thyroid treatment
- Carbon-14 → Dating fossils
14. Isobars
Atoms of different elements having:
- Same mass number
- Different atomic numbers
Example:
Argon-40
Potassium-40
Calcium-40
15. Average Atomic Mass
It is the weighted average of all naturally occurring isotopes of an element.
Example:
Chlorine has average atomic mass 35.5 u.
16. Scientists and Their Contributions
| Scientist | Contribution |
|---|---|
| Acharya Kanada | Parmanu theory |
| Democritus | Atomos concept |
| John Dalton | Atomic theory |
| J.J. Thomson | Electron, Plum Pudding Model |
| Ernest Rutherford | Nucleus, Gold Foil Experiment |
| Niels Bohr | Shell model |
| James Chadwick | Neutron |
17. Important Formulae
Atomic Number (Z)
= Number of Protons
= Number of Electrons (Neutral Atom)
Mass Number (A)
= Protons + Neutrons
Neutrons
= Mass Number − Atomic Number
Maximum Electrons in Shell
= 2n²
18. One-Mark Important Facts
- Atom is the basic unit of matter.
- Electron has negative charge.
- Proton has positive charge.
- Neutron has no charge.
- Nucleus contains protons and neutrons.
- Most of the atom is empty space.
- K shell can hold only 2 electrons.
- Outermost shell can have a maximum of 8 electrons (for the first 20 elements).
- Stable atoms usually have a complete outer shell.
- Valency is the combining capacity of an atom.
- Isotopes have the same atomic number but different mass numbers.
- Isobars have the same mass number but different atomic numbers.
📚 Exam Revision Box
Remember These Scientists
- Kanada → Parmanu
- Dalton → Atomic Theory
- Thomson → Electron
- Rutherford → Nucleus
- Bohr → Energy Shells
- Chadwick → Neutron
Important Formulae
- Atomic Number = Protons = Electrons (neutral atom)
- Mass Number = Protons + Neutrons
- Neutrons = Mass Number − Atomic Number
- Maximum electrons in a shell = 2n²
MCQs
1. Matter is made up of tiny particles called
A. Molecules
B. Electrons
C. Atoms
D. Cells
Answer: C
2. The ancient Indian philosopher who proposed the idea of ‘Parmanu’ was
A. Aryabhata
B. Kanada
C. Charaka
D. Sushruta
Answer: B
3. The Greek word ‘Atomos’ means
A. Very small
B. Invisible
C. Indivisible
D. Powerful
Answer: C
4. Dalton’s atomic theory was proposed in
A. 1766
B. 1808
C. 1897
D. 1911
Answer: B
5. According to Dalton, atoms are
A. Easily divisible
B. Indivisible
C. Made of electrons
D. Hollow spheres
Answer: B
6. The first subatomic particle discovered was
A. Proton
B. Neutron
C. Electron
D. Alpha particle
Answer: C
7. The electron was discovered by
A. Rutherford
B. Bohr
C. Chadwick
D. J. J. Thomson
Answer: D
8. Electrons were discovered through the study of
A. Alpha rays
B. X-rays
C. Cathode rays
D. Gamma rays
Answer: C
9. Cathode rays carry
A. Positive charge
B. No charge
C. Negative charge
D. Both charges
Answer: C
10. Thomson compared his atomic model to
A. Cricket ball
B. Solar system
C. Plum pudding
D. Water drop
Answer: C
11. In Thomson’s model, electrons are
A. Inside the nucleus
B. Embedded in a positively charged sphere
C. Outside the atom
D. Stationary
Answer: B
12. Which experiment disproved Thomson’s model?
A. Oil drop experiment
B. Gold foil experiment
C. Cathode ray experiment
D. Millikan experiment
Answer: B
13. Rutherford’s experiment used
A. Copper foil
B. Silver foil
C. Gold foil
D. Aluminium foil
Answer: C
14. The particles used in Rutherford’s experiment were
A. Electrons
B. Beta particles
C. Alpha particles
D. Neutrons
Answer: C
15. Most alpha particles passed through the gold foil because
A. Gold dissolved them
B. Atoms are mostly empty space
C. Alpha particles have no charge
D. Gold attracts them
Answer: B
16. Rutherford concluded that positive charge is concentrated in the
A. Shell
B. Electron cloud
C. Nucleus
D. Outer orbit
Answer: C
17. Rutherford’s atomic model is also called the
A. Solid sphere model
B. Planetary model
C. Quantum model
D. Plum pudding model
Answer: B
18. The nucleus contains
A. Only electrons
B. Only neutrons
C. Positive charge and most of the mass
D. Empty space
Answer: C
19. Rutherford’s model failed because it could not explain
A. Chemical reactions
B. Stability of atoms
C. Radioactivity
D. Atomic number
Answer: B
20. The scientist who explained atomic stability was
A. Dalton
B. Bohr
C. Chadwick
D. Democritus
Answer: B
21. Bohr proposed that electrons move in
A. Random paths
B. Straight lines
C. Fixed energy levels
D. The nucleus
Answer: C
22. The first shell is called
A. L-shell
B. M-shell
C. K-shell
D. N-shell
Answer: C
23. The shell nearest the nucleus has
A. Highest energy
B. Lowest energy
C. No energy
D. Variable energy
Answer: B
24. Electrons change shells by
A. Changing charge
B. Absorbing or releasing energy
C. Losing mass
D. Splitting
Answer: B
25. The scientist who discovered the neutron was
A. Rutherford
B. Thomson
C. Chadwick
D. Bohr
Answer: C
26. Neutrons have
A. Positive charge
B. Negative charge
C. No charge
D. Double positive charge
Answer: C
27. Which particle has negligible mass compared to protons?
A. Proton
B. Electron
C. Neutron
D. Alpha particle
Answer: B
28. Protons are found in the
A. Shell
B. Orbit
C. Nucleus
D. Electron cloud
Answer: C
29. Electrons are located
A. Inside the nucleus
B. Around the nucleus
C. Between protons
D. Inside neutrons
Answer: B
30. Which particle has a charge of +1?
A. Electron
B. Neutron
C. Proton
D. Alpha particle
Answer: C
31. Which particle has a charge of –1?
A. Proton
B. Electron
C. Neutron
D. Nucleus
Answer: B
32. Atomic number represents the number of
A. Neutrons
B. Electrons only
C. Protons
D. Shells
Answer: C
33. In a neutral atom, the number of electrons is equal to the number of
A. Neutrons
B. Shells
C. Protons
D. Nucleons
Answer: C
34. Mass number is equal to
A. Protons + Electrons
B. Electrons + Neutrons
C. Protons + Neutrons
D. Only neutrons
Answer: C
35. Protons and neutrons together are called
A. Electrons
B. Nucleons
C. Isotopes
D. Ions
Answer: B
36. If an atom has 11 protons, its atomic number is
A. 10
B. 11
C. 12
D. 22
Answer: B
37. An atom with 8 protons has
A. Atomic number 6
B. Atomic number 7
C. Atomic number 8
D. Atomic number 16
Answer: C
38. Which formula gives the maximum number of electrons in a shell?
A. n²
B. 2n
C. 2n²
D. n³
Answer: C
39. Maximum electrons in the K-shell are
A. 8
B. 18
C. 2
D. 32
Answer: C
40. Maximum electrons in the L-shell are
A. 2
B. 4
C. 8
D. 18
Answer: C
41. Electrons are filled into shells starting from
A. Outermost shell
B. Middle shell
C. Innermost shell
D. Randomly
Answer: C
42. The electronic configuration of sodium is
A. 2,7,2
B. 2,8,1
C. 2,6,3
D. 2,8,2
Answer: B
43. The electronic configuration of oxygen is
A. 2,8
B. 2,6
C. 2,4
D. 2,5
Answer: B
44. The outermost shell is known as the
A. K-shell
B. Nuclear shell
C. Valence shell
D. Inner shell
Answer: C
45. Electrons present in the outermost shell are called
A. Core electrons
B. Nuclear electrons
C. Valence electrons
D. Free electrons
Answer: C
46. Atoms with complete outer shells are generally
A. Highly reactive
B. Unstable
C. Stable
D. Radioactive
Answer: C
47. Valency is the
A. Mass of an atom
B. Combining capacity of an atom
C. Atomic number
D. Number of neutrons
Answer: B
48. Sodium has a valency of
A. 0
B. 1
C. 2
D. 3
Answer: B
49. Oxygen has a valency of
A. 1
B. 2
C. 3
D. 4
Answer: B
50. Carbon generally has a valency of
A. 1
B. 2
C. 3
D. 4
Answer: D
PART 2 (MCQs 51–100)
51. The symbol of sodium is
A. So
B. Sd
C. Na
D. S
Answer: C
52. The symbol Fe is derived from the Latin name
A. Ferrum
B. Ferrous
C. Ferro
D. Ferrite
Answer: A
53. Which element has the symbol K?
A. Krypton
B. Potassium
C. Calcium
D. Cobalt
Answer: B
54. The symbol Au represents
A. Silver
B. Gold
C. Aluminium
D. Argon
Answer: B
55. The chemical symbol of mercury is
A. Mg
B. Mn
C. Hg
D. Mc
Answer: C
56. Which symbol comes from the Latin word Natrium?
A. N
B. Na
C. Ne
D. Ni
Answer: B
57. The symbol Ag stands for
A. Argon
B. Aluminium
C. Silver
D. Gold
Answer: C
58. Which of the following is written correctly?
A. co
B. CO
C. Co
D. cO
Answer: C
59. Which organization approves the names and symbols of elements?
A. UNESCO
B. WHO
C. IUPAC
D. ISRO
Answer: C
60. The atomic number of an element is represented by
A. A
B. Z
C. N
D. M
Answer: B
61. The mass number is represented by
A. Z
B. A
C. P
D. N
Answer: B
62. An atom has 13 protons. Its atomic number is
A. 26
B. 13
C. 14
D. 27
Answer: B
63. An atom has atomic number 17. The number of electrons is
A. 16
B. 17
C. 18
D. 34
Answer: B
64. An atom contains 15 protons and 16 neutrons. Its mass number is
A. 15
B. 16
C. 31
D. 30
Answer: C
65. Number of neutrons =
A. Atomic Number + Mass Number
B. Mass Number – Atomic Number
C. Atomic Number × 2
D. Mass Number + Electrons
Answer: B
66. Which atom has atomic number 11?
A. Magnesium
B. Sodium
C. Neon
D. Aluminium
Answer: B
67. The electronic configuration of magnesium is
A. 2,8,2
B. 2,7,3
C. 2,6,4
D. 2,8,8
Answer: A
68. The electronic configuration of chlorine is
A. 2,8,8
B. 2,7,8
C. 2,8,7
D. 2,6,9
Answer: C
69. Argon has how many electrons in its outermost shell?
A. 2
B. 4
C. 6
D. 8
Answer: D
70. Which element has the electronic configuration 2,8,1?
A. Sodium
B. Magnesium
C. Aluminium
D. Neon
Answer: A
71. Which element has the electronic configuration 2,8,8?
A. Chlorine
B. Argon
C. Neon
D. Sulfur
Answer: B
72. An atom with a complete octet is generally
A. Highly reactive
B. Stable
C. Radioactive
D. Positively charged
Answer: B
73. Neon has a valency of
A. 0
B. 1
C. 2
D. 8
Answer: A
74. Chlorine usually gains
A. One electron
B. Two electrons
C. Three electrons
D. Four electrons
Answer: A
75. Magnesium generally loses
A. One electron
B. Two electrons
C. Three electrons
D. Four electrons
Answer: B
76. Carbon usually completes its octet by
A. Losing electrons
B. Gaining electrons
C. Sharing electrons
D. Losing protons
Answer: C
77. Atoms having the same atomic number but different mass numbers are called
A. Isobars
B. Isotopes
C. Ions
D. Molecules
Answer: B
78. Hydrogen has naturally occurring
A. One isotope
B. Two isotopes
C. Three isotopes
D. Four isotopes
Answer: C
79. Which hydrogen isotope contains two neutrons?
A. Protium
B. Deuterium
C. Tritium
D. Hydrogen-4
Answer: C
80. Deuterium contains
A. One proton and one neutron
B. One proton and two neutrons
C. Two protons
D. Two electrons
Answer: A
81. Carbon-14 contains
A. 6 protons
B. 14 protons
C. 8 protons
D. 7 protons
Answer: A
82. Isotopes have identical
A. Mass numbers
B. Chemical properties
C. Numbers of neutrons
D. Physical properties
Answer: B
83. Isotopes differ in their
A. Number of protons
B. Electronic configuration
C. Number of neutrons
D. Atomic number
Answer: C
84. Uranium-235 is mainly used as
A. Fertilizer
B. Nuclear fuel
C. Medicine
D. Food preservative
Answer: B
85. Cobalt-60 is commonly used in
A. Water purification
B. Cancer treatment
C. Plastic manufacturing
D. Agriculture
Answer: B
86. Iodine-131 is mainly used for treating
A. Diabetes
B. Thyroid disorders
C. Malaria
D. Tuberculosis
Answer: B
87. Carbon-14 is widely used to determine
A. Atomic number
B. Age of fossils
C. Number of electrons
D. Density of metals
Answer: B
88. Average atomic mass depends upon
A. Number of shells
B. Relative abundance of isotopes
C. Number of electrons
D. Valency
Answer: B
89. Chlorine has an average atomic mass close to
A. 35 u
B. 36 u
C. 35.5 u
D. 37 u
Answer: C
90. Isobars have
A. Same atomic number
B. Same mass number
C. Same neutrons
D. Same protons
Answer: B
91. Isobars belong to
A. The same element
B. Different elements
C. The same isotope
D. The same compound
Answer: B
92. Which pair represents isobars?
A. Carbon-12 and Carbon-13
B. Hydrogen-1 and Hydrogen-2
C. Potassium-40 and Calcium-40
D. Chlorine-35 and Chlorine-37
Answer: C
93. Which scientist discovered the nucleus?
A. Chadwick
B. Thomson
C. Rutherford
D. Bohr
Answer: C
94. Which scientist introduced fixed energy levels?
A. Dalton
B. Bohr
C. Thomson
D. Chadwick
Answer: B
95. Which scientist discovered the electron?
A. Rutherford
B. Bohr
C. J. J. Thomson
D. James Chadwick
Answer: C
96. Which scientist discovered the neutron?
A. Dalton
B. Chadwick
C. Bohr
D. Democritus
Answer: B
97. Rutherford’s model mainly explained
A. Valency
B. Existence of the nucleus
C. Electronic configuration
D. Average atomic mass
Answer: B
98. Bohr’s model successfully explained
A. Radioactivity
B. Stability of atoms
C. Discovery of neutrons
D. Discovery of electrons
Answer: B
99. Which statement is correct?
A. Electrons are present inside the nucleus.
B. Protons move around the nucleus.
C. Neutrons carry a positive charge.
D. Electrons occupy fixed energy levels in Bohr’s model.
Answer: D
100. Which statement best summarizes the development of atomic models?
A. Only Dalton’s model is correct.
B. Atomic models changed as new experimental evidence became available.
C. Rutherford’s model explained everything perfectly.
D. Scientific models never change.
Answer: B
PART 3 (MCQs 101–150)
101. If an atom has 17 protons and 18 neutrons, its mass number is
A. 17
B. 18
C. 35
D. 36
Answer: C
102. An atom has atomic number 13 and mass number 27. The number of neutrons is
A. 13
B. 14
C. 27
D. 40
Answer: B
103. A neutral atom contains 20 protons. How many electrons does it have?
A. 18
B. 19
C. 20
D. 40
Answer: C
104. An atom has 12 protons and 12 neutrons. Its mass number is
A. 12
B. 24
C. 36
D. 48
Answer: B
105. Which particle contributes the least to the mass of an atom?
A. Proton
B. Neutron
C. Electron
D. Alpha particle
Answer: C
106. If an atom has atomic number 8, the number of protons is
A. 6
B. 8
C. 10
D. 16
Answer: B
107. The electronic configuration of an atom with atomic number 16 is
A. 2,8,6
B. 2,6,8
C. 2,8,5
D. 2,7,7
Answer: A
108. Which element has the electronic configuration 2,8,6?
A. Oxygen
B. Sulfur
C. Chlorine
D. Phosphorus
Answer: B
109. An atom with configuration 2,8,7 has a valency of
A. 0
B. 1
C. 2
D. 7
Answer: B
110. Which of the following has the highest number of valence electrons?
A. Sodium
B. Magnesium
C. Chlorine
D. Carbon
Answer: C
111. Which shell is filled immediately after the K-shell?
A. N
B. M
C. L
D. O
Answer: C
112. Which statement is true about electrons in Bohr’s model?
A. They remain inside the nucleus.
B. They move in fixed energy levels.
C. They move randomly.
D. They have no energy.
Answer: B
113. Why are atoms electrically neutral?
A. They have only neutrons.
B. Number of protons equals number of electrons.
C. Protons have no charge.
D. Electrons have no charge.
Answer: B
114. Which subatomic particle is responsible for the identity of an element?
A. Electron
B. Neutron
C. Proton
D. Alpha particle
Answer: C
115. Which particle was discovered last among the three basic subatomic particles?
A. Electron
B. Proton
C. Neutron
D. Alpha particle
Answer: C
116. The scientist who proposed stationary orbits was
A. Dalton
B. Bohr
C. Chadwick
D. Rutherford
Answer: B
117. The Gold Foil Experiment proved that
A. Atoms are indivisible.
B. Positive charge is concentrated at the centre.
C. Electrons have no charge.
D. Neutrons exist.
Answer: B
118. Which model first suggested the presence of a nucleus?
A. Dalton’s model
B. Thomson’s model
C. Rutherford’s model
D. Bohr’s model
Answer: C
119. Which scientist first suggested that matter is made of indivisible particles based on experiments?
A. Dalton
B. Democritus
C. Kanada
D. Bohr
Answer: A
120. Which experiment led to the discovery of the electron?
A. Oil Drop Experiment
B. Gold Foil Experiment
C. Cathode Ray Experiment
D. Alpha Scattering Experiment
Answer: C
Assertion-Based MCQs
121. Assertion (A): Most alpha particles passed through the gold foil.
Reason (R): Most of the atom is empty space.
A. Both A and R are true, and R explains A.
B. Both A and R are true, but R does not explain A.
C. A is true, R is false.
D. A is false, R is true.
Answer: A
122. Assertion (A): Bohr’s model explains atomic stability.
Reason (R): Electrons move only in fixed energy levels.
A. Both true and R explains A
B. Both true but R does not explain A
C. A true, R false
D. Both false
Answer: A
123. Assertion (A): Isotopes have similar chemical properties.
Reason (R): They have the same electronic configuration.
A. Both true and R explains A
B. Both true but R does not explain A
C. A true, R false
D. Both false
Answer: A
124. Assertion (A): Isobars belong to different elements.
Reason (R): Their atomic numbers are different.
A. Both true and R explains A
B. Both true but R does not explain A
C. A true, R false
D. Both false
Answer: A
125. Assertion (A): Electrons contribute very little to atomic mass.
Reason (R): Their mass is negligible compared to protons and neutrons.
A. Both true and R explains A
B. Both true but R does not explain A
C. A true, R false
D. Both false
Answer: A
Application-Based MCQs
126. An atom has electronic configuration 2,8,2. It will most likely
A. Gain two electrons
B. Lose two electrons
C. Gain six electrons
D. Share eight electrons
Answer: B
127. An atom with electronic configuration 2,8,8 is expected to be
A. Highly reactive
B. Unstable
C. Chemically stable
D. Positively charged
Answer: C
128. Which particle determines the chemical properties of an atom?
A. Neutrons
B. Valence electrons
C. Protons only
D. Nucleus
Answer: B
129. Which pair represents isotopes?
A. Carbon-12 and Carbon-14
B. Sodium and Magnesium
C. Oxygen and Sulfur
D. Chlorine and Bromine
Answer: A
130. Which pair represents isobars?
A. Hydrogen-1 and Hydrogen-2
B. Carbon-12 and Carbon-13
C. Potassium-40 and Calcium-40
D. Chlorine-35 and Chlorine-37
Answer: C
Competency-Based MCQs
131. A student says that atoms are solid spheres with no internal structure. This idea belongs to
A. Bohr
B. Dalton
C. Rutherford
D. Chadwick
Answer: B
132. A scientist observes that electrons occupy fixed shells. Which model supports this observation?
A. Thomson
B. Dalton
C. Bohr
D. Rutherford
Answer: C
133. Which discovery proved that atoms are divisible?
A. Discovery of molecules
B. Discovery of electrons
C. Discovery of compounds
D. Discovery of metals
Answer: B
134. If the nucleus were absent, an atom would lose
A. Its electrons
B. Most of its mass and positive charge
C. Only neutrons
D. Only protons
Answer: B
135. Which scientist’s work directly led to the discovery of the nucleus?
A. Bohr
B. Rutherford
C. Dalton
D. Chadwick
Answer: B
Higher Order Thinking (HOTS) MCQs
136. If Rutherford had observed all alpha particles passing straight through the foil, he would probably conclude that
A. Atoms contain a nucleus.
B. Positive charge is spread uniformly throughout the atom.
C. Electrons are absent.
D. Neutrons carry positive charge.
Answer: B
137. Why was Bohr’s model considered an improvement over Rutherford’s model?
A. It explained isotopes.
B. It explained atomic stability.
C. It discovered neutrons.
D. It explained radioactivity.
Answer: B
138. Which statement best describes the progress of atomic theory?
A. Scientific models never change.
B. New evidence improves scientific models.
C. Dalton’s theory explains everything.
D. Rutherford’s model is final.
Answer: B
139. Why do isotopes show similar chemical behaviour?
A. Same number of neutrons
B. Same atomic mass
C. Same electronic configuration
D. Same physical properties
Answer: C
140. Which property changes in isotopes?
A. Atomic number
B. Number of protons
C. Number of neutrons
D. Number of electrons
Answer: C
Mixed Revision MCQs
141. The combining capacity of an atom is called
A. Atomic number
B. Valency
C. Mass number
D. Atomic mass
Answer: B
142. The valence shell is
A. The innermost shell
B. The nucleus
C. The outermost shell
D. The second shell
Answer: C
143. Which of the following is chemically least reactive?
A. Sodium
B. Oxygen
C. Chlorine
D. Argon
Answer: D
144. Which isotope is commonly used to estimate the age of ancient fossils?
A. Uranium-235
B. Carbon-14
C. Iodine-131
D. Cobalt-60
Answer: B
145. Which isotope is widely used in nuclear reactors?
A. Carbon-14
B. Uranium-235
C. Iodine-131
D. Hydrogen-2
Answer: B
146. The scientist known as the Father of Nuclear Physics is
A. Bohr
B. Chadwick
C. Rutherford
D. Thomson
Answer: C
147. Which scientist discovered the neutron while working under Rutherford?
A. Bohr
B. Dalton
C. Chadwick
D. Thomson
Answer: C
148. Which model is accepted today as the most advanced understanding of the atom?
A. Dalton’s model
B. Thomson’s model
C. Rutherford’s model
D. Quantum mechanical model
Answer: D
149. The modern model describes electrons as
A. Fixed balls
B. Moving only in circular paths
C. Existing in regions of probability called electron clouds
D. Embedded in positive matter
Answer: C
150. Which statement best reflects the scientific journey of atomic theory?
A. One experiment solved every mystery about atoms.
B. Atomic models developed step by step as new evidence became available.
C. Dalton’s model remains completely correct today.
D. Scientists no longer study atoms.
Answer: B
Fill in the Blanks
PART 1 (Questions 1–40)
1. Everything around us is made up of ________.
Answer: matter
2. The smallest building blocks of matter are called ________.
Answer: atoms
3. The ancient Indian philosopher who proposed the concept of Parmanu was ________.
Answer: Acharya Kanada
4. The Greek word atomos means ________.
Answer: indivisible
5. John Dalton proposed his atomic theory in the year ________.
Answer: 1808
6. According to Dalton, atoms are ________ particles.
Answer: indivisible
7. The first subatomic particle discovered was the ________.
Answer: electron
8. The electron was discovered by ________.
Answer: J. J. Thomson
9. Electrons were discovered using the ________ ray experiment.
Answer: cathode
10. Cathode rays carry a ________ charge.
Answer: negative
11. Thomson’s atomic model is popularly known as the ________ model.
Answer: Plum Pudding
12. In Thomson’s model, electrons are embedded in a sphere of ________ charge.
Answer: positive
13. Rutherford’s famous experiment is known as the ________ foil experiment.
Answer: gold
14. Rutherford used ________ particles in his experiment.
Answer: alpha
15. Most alpha particles passed through the gold foil because atoms are mostly ________ space.
Answer: empty
16. Rutherford concluded that the positive charge is concentrated in the ________.
Answer: nucleus
17. Rutherford’s model is also called the ________ model of the atom.
Answer: planetary
18. The nucleus contains almost all the ________ of the atom.
Answer: mass
19. Rutherford’s model could not explain the ________ of the atom.
Answer: stability
20. Niels ________ proposed a new atomic model in 1913.
Answer: Bohr
21. According to Bohr, electrons move in fixed ________ levels.
Answer: energy
22. The first shell of an atom is called the ________ shell.
Answer: K
23. The shells are represented as K, L, M, ________.
Answer: N
24. The shell nearest to the nucleus has the ________ energy.
Answer: least
25. Electrons can move from one shell to another by absorbing or releasing ________.
Answer: energy
26. The positively charged particle present in the nucleus is called the ________.
Answer: proton
27. The proton was discovered and named by ________.
Answer: Rutherford
28. The neutron was discovered by ________ Chadwick.
Answer: James
29. A neutron carries ________ electrical charge.
Answer: no
30. Electrons have ________ mass compared to protons and neutrons.
Answer: negligible
31. The three basic subatomic particles are electrons, protons and ________.
Answer: neutrons
32. The number of protons in an atom is called its ________ number.
Answer: atomic
33. The atomic number is represented by the symbol ________.
Answer: Z
34. The total number of protons and neutrons is called the ________ number.
Answer: mass
35. The mass number is represented by the symbol ________.
Answer: A
36. Protons and neutrons together are known as ________.
Answer: nucleons
37. In a neutral atom, the number of electrons is equal to the number of ________.
Answer: protons
38. The mass of an atom comes mainly from its ________ and neutrons.
Answer: protons
39. The electron revolves around the ________.
Answer: nucleus
40. The maximum number of electrons in a shell is given by the formula ________.
Answer: 2n²
41. The maximum number of electrons that can be accommodated in the K-shell is ________.
Answer: 2
42. The maximum number of electrons that can be accommodated in the L-shell is ________.
Answer: 8
43. Electrons are filled into shells from the ________ shell outward.
Answer: innermost
44. The electronic configuration of hydrogen is ________.
Answer: 1
45. The electronic configuration of helium is ________.
Answer: 2
46. The electronic configuration of carbon is ________.
Answer: 2,4
47. The electronic configuration of oxygen is ________.
Answer: 2,6
48. The electronic configuration of sodium is ________.
Answer: 2,8,1
49. The electronic configuration of chlorine is ________.
Answer: 2,8,7
50. The electronic configuration of argon is ________.
Answer: 2,8,8
51. The outermost shell of an atom is called the ________ shell.
Answer: valence
52. Electrons present in the outermost shell are called ________ electrons.
Answer: valence
53. The combining capacity of an atom is known as its ________.
Answer: valency
54. An atom with a complete outermost shell is generally ________.
Answer: stable
55. Sodium has ________ valence electron.
Answer: one
56. Oxygen has ________ valence electrons.
Answer: six
57. Chlorine has ________ valence electrons.
Answer: seven
58. Carbon has a valency of ________.
Answer: four
59. Magnesium has a valency of ________.
Answer: two
60. Neon has a valency of ________ because its outermost shell is complete.
Answer: zero
61. The chemical symbol of sodium is ________.
Answer: Na
62. The chemical symbol of potassium is ________.
Answer: K
63. The chemical symbol of iron is ________.
Answer: Fe
64. The chemical symbol of gold is ________.
Answer: Au
65. The chemical symbol of silver is ________.
Answer: Ag
66. The chemical symbol of mercury is ________.
Answer: Hg
67. Atoms having the same atomic number but different mass numbers are called ________.
Answer: isotopes
68. Naturally occurring hydrogen has ________ isotopes.
Answer: three
69. The most abundant isotope of hydrogen is ________.
Answer: protium
70. The hydrogen isotope containing one neutron is called ________.
Answer: deuterium
71. The hydrogen isotope containing two neutrons is called ________.
Answer: tritium
72. The three common isotopes of carbon are Carbon-12, Carbon-13 and Carbon-________.
Answer: 14
73. Isotopes have the same ________ configuration.
Answer: electronic
74. Isotopes generally show similar ________ properties.
Answer: chemical
75. Uranium-235 is commonly used as fuel in a ________ reactor.
Answer: nuclear
76. Cobalt-60 is widely used in the treatment of ________.
Answer: cancer
77. Iodine-131 is used to treat ________ disorders.
Answer: thyroid
78. Carbon-14 is used to determine the age of ancient ________ and artefacts.
Answer: fossils
79. The average atomic mass of an element depends on the relative ________ of its isotopes.
Answer: abundance
80. Chlorine has a weighted average atomic mass of about ________ u.
Answer: 35.5
81. Atoms of different elements having the same mass number are called ________.
Answer: isobars
82. Isobars have different ________ numbers.
Answer: atomic
83. Calcium-40, Potassium-40, and Argon-40 are examples of ________.
Answer: isobars
84. The modern atomic model is also known as the ________ mechanical model.
Answer: quantum
85. According to the modern atomic model, electrons exist in ________ clouds.
Answer: electron
86. The exact position of an electron ________ be determined.
Answer: cannot
87. The modern atomic model predicts the ________ of finding an electron.
Answer: probability
88. Electrons do not move in fixed circular ________ according to the modern model.
Answer: orbits
89. A Scanning Tunnelling Microscope is commonly abbreviated as ________.
Answer: STM
90. A Transmission Electron Microscope is commonly abbreviated as ________.
Answer: TEM
91. An STM is mainly used to study the ________ of materials.
Answer: surface
92. A TEM helps scientists observe the arrangement of atoms inside ________ samples.
Answer: thin
93. Homi Jehangir Bhabha is known as the father of India’s ________ programme.
Answer: nuclear
94. Homi J. Bhabha established the Tata Institute of Fundamental ________ (TIFR).
Answer: Research
95. BARC stands for Bhabha Atomic Research ________.
Answer: Centre
96. Atomic energy can be used to generate ________.
Answer: electricity
97. Atomic energy also supports modern ________ treatments.
Answer: medical
98. The earliest scientific atomic model discussed in this chapter was proposed by ________.
Answer: Dalton
99. The discovery of the electron led to the ________ atomic model.
Answer: Thomson’s
100. Rutherford discovered the atomic ________.
Answer: nucleus
101. Bohr introduced the concept of fixed ________ levels.
Answer: energy
102. James Chadwick discovered the ________.
Answer: neutron
103. Scientific models improve when new ________ become available.
Answer: evidence
104. The structure of the atom is still being ________ by scientists.
Answer: explored
105. The atomic number is equal to the number of ________ in the nucleus.
Answer: protons
106. The mass number is the sum of ________ and neutrons.
Answer: protons
107. In a neutral atom, protons and electrons are ________ in number.
Answer: equal
108. Atoms with completely filled outermost shells are generally ________ reactive.
Answer: less
109. The valency of an atom depends mainly on its ________ electrons.
Answer: valence
110. The symbol of an element always begins with a ________ letter.
Answer: capital
111. The second letter of a chemical symbol, if present, is written in ________ case.
Answer: lowercase
112. Carbon-14 is widely used in ________ dating.
Answer: radiocarbon
113. Iodine-131 is a radioactive isotope of ________.
Answer: iodine
114. Uranium-235 is used as fuel in a ________ power plant.
Answer: nuclear
115. Cobalt-60 emits radiation used in the treatment of ________.
Answer: cancer
116. The weighted average atomic mass considers the ________ abundance of isotopes.
Answer: relative
117. A chlorine atom does not actually have a mass of ________ u; this is only its average atomic mass.
Answer: 35.5
118. The journey of understanding the atom is ________.
Answer: continuing
119. Atoms are the basic ________ blocks of matter.
Answer: building
120. The study of atoms continues because science keeps discovering new ________.
Answer: facts
True/False Questions
PART 1 (Questions 1–40)
Instructions: Write True (T) or False (F).
1. Everything around us is made up of matter.
Answer: True
2. Atoms are the basic building blocks of matter.
Answer: True
3. Acharya Kanada proposed the concept of Parmanu.
Answer: True
4. The Greek word Atomos means divisible.
Answer: False
5. John Dalton proposed the first scientific atomic theory.
Answer: True
6. According to Dalton, atoms of the same element are identical.
Answer: True
7. Dalton believed that atoms could be divided into smaller particles.
Answer: False
8. J. J. Thomson discovered the electron.
Answer: True
9. Electrons were discovered using the Gold Foil Experiment.
Answer: False
10. Cathode rays carry a negative charge.
Answer: True
11. Thomson’s atomic model is known as the Plum Pudding Model.
Answer: True
12. In Thomson’s model, electrons were embedded in a positively charged sphere.
Answer: True
13. Rutherford performed the Gold Foil Experiment.
Answer: True
14. Rutherford used beta particles in his experiment.
Answer: False
15. Rutherford used alpha particles in the Gold Foil Experiment.
Answer: True
16. Most alpha particles were reflected back by the gold foil.
Answer: False
17. Most alpha particles passed straight through the gold foil.
Answer: True
18. Rutherford concluded that most of the atom is empty space.
Answer: True
19. Rutherford concluded that positive charge is spread evenly throughout the atom.
Answer: False
20. The nucleus contains most of the mass of the atom.
Answer: True
21. Rutherford’s model explained why atoms are stable.
Answer: False
22. Niels Bohr proposed that electrons move in fixed energy levels.
Answer: True
23. Bohr called the electron paths energy shells.
Answer: True
24. Electrons lose energy continuously while moving in Bohr’s orbits.
Answer: False
25. Electrons can move from one shell to another by absorbing or releasing energy.
Answer: True
26. The first shell of an atom is called the K-shell.
Answer: True
27. The L-shell is closer to the nucleus than the K-shell.
Answer: False
28. James Chadwick discovered the neutron.
Answer: True
29. A neutron carries a positive charge.
Answer: False
30. Protons are positively charged particles.
Answer: True
31. Electrons have a negative charge.
Answer: True
32. Neutrons are found outside the nucleus.
Answer: False
33. Protons and neutrons are present inside the nucleus.
Answer: True
34. Electrons revolve around the nucleus.
Answer: True
35. The atomic number is equal to the number of protons.
Answer: True
36. The mass number is the sum of protons and neutrons.
Answer: True
37. In a neutral atom, the number of protons and electrons is the same.
Answer: True
38. Electrons contribute most of the mass of an atom.
Answer: False
39. Protons and neutrons together are called nucleons.
Answer: True
40. The formula for the maximum number of electrons in a shell is 2n².
Answer: True
41. The maximum number of electrons in the K-shell is 2.
Answer: True
42. The maximum number of electrons in the L-shell is 8.
Answer: True
43. The maximum number of electrons in the M-shell is 18.
Answer: True
44. Electrons are filled into shells starting from the outermost shell.
Answer: False
45. The electronic configuration of sodium is 2,8,1.
Answer: True
46. The electronic configuration of oxygen is 2,6.
Answer: True
47. The electronic configuration of argon is 2,8,8.
Answer: True
48. The electronic configuration of chlorine is 2,8,7.
Answer: True
49. The outermost shell of an atom is called the valence shell.
Answer: True
50. Valence electrons are present in the innermost shell.
Answer: False
51. Valency is the combining capacity of an atom.
Answer: True
52. An atom with a complete outermost shell is generally chemically stable.
Answer: True
53. Sodium has one valence electron.
Answer: True
54. Oxygen has six valence electrons.
Answer: True
55. Chlorine has seven valence electrons.
Answer: True
56. Neon has a valency of one.
Answer: False
57. Carbon usually has a valency of four.
Answer: True
58. Magnesium generally has a valency of two.
Answer: True
59. The chemical symbol of sodium is Na.
Answer: True
60. The chemical symbol of potassium is P.
Answer: False
61. The chemical symbol of iron is Fe.
Answer: True
62. The chemical symbol of gold is Au.
Answer: True
63. The chemical symbol of silver is Ag.
Answer: True
64. The chemical symbol of mercury is Hg.
Answer: True
65. Isotopes have the same atomic number but different mass numbers.
Answer: True
66. Isotopes have different numbers of protons.
Answer: False
67. Isotopes of an element have similar chemical properties.
Answer: True
68. Isotopes always have different electronic configurations.
Answer: False
69. Hydrogen has three naturally occurring isotopes.
Answer: True
70. Protium contains one neutron.
Answer: False
71. Deuterium contains one neutron.
Answer: True
72. Tritium contains two neutrons.
Answer: True
73. Carbon-14 is an isotope of carbon.
Answer: True
74. Uranium-235 is commonly used as fuel in nuclear reactors.
Answer: True
75. Cobalt-60 is used in the treatment of cancer.
Answer: True
76. Iodine-131 is used in the treatment of thyroid disorders.
Answer: True
77. Carbon-14 is used to estimate the age of fossils and ancient artefacts.
Answer: True
78. Average atomic mass depends on the relative abundance of isotopes.
Answer: True
79. Chlorine has an average atomic mass of approximately 35.5 u.
Answer: True
80. All naturally occurring chlorine atoms have an exact mass of 35.5 u.
Answer: False
Instructions: Write True (T) or False (F).
81. Isobars are atoms of different elements having the same mass number.
Answer: True
82. Isobars have the same atomic number.
Answer: False
83. Calcium-40 and Potassium-40 are examples of isobars.
Answer: True
84. Hydrogen-1 and Hydrogen-2 are isobars.
Answer: False
85. Isobars belong to different elements.
Answer: True
86. The modern atomic model is also called the quantum mechanical model.
Answer: True
87. According to the modern atomic model, electrons move only in fixed circular orbits.
Answer: False
88. Electrons are most likely to be found in regions called electron clouds.
Answer: True
89. The exact position of an electron can always be determined.
Answer: False
90. Scientists predict the probability of finding an electron rather than its exact position.
Answer: True
91. A Scanning Tunnelling Microscope (STM) is mainly used to study the surface of materials.
Answer: True
92. A Transmission Electron Microscope (TEM) helps observe the arrangement of atoms inside thin samples.
Answer: True
93. An STM is mainly used to examine the inside of thick objects.
Answer: False
94. TEM stands for Transmission Electron Microscope.
Answer: True
95. Homi Jehangir Bhabha is known as the father of India’s nuclear programme.
Answer: True
96. Homi J. Bhabha established the Bhabha Atomic Research Centre (BARC).
Answer: True
97. Atomic energy is used only for making weapons.
Answer: False
98. Atomic energy can be used to generate electricity.
Answer: True
99. Atomic energy has applications in medicine and agriculture.
Answer: True
100. Scientific knowledge develops as new discoveries are made.
Answer: True
101. Dalton’s model was the final and complete explanation of atomic structure.
Answer: False
102. Thomson’s model introduced the idea of electrons.
Answer: True
103. Rutherford discovered the atomic nucleus.
Answer: True
104. Bohr explained that electrons occupy fixed energy levels.
Answer: True
105. Chadwick discovered the neutron.
Answer: True
106. Modern scientists continue to study atomic structure.
Answer: True
107. The atomic number of an element changes when the number of neutrons changes.
Answer: False
108. Changing the number of neutrons produces an isotope of the element.
Answer: True
109. Chemical properties mainly depend on the number of valence electrons.
Answer: True
110. Physical properties of isotopes may differ.
Answer: True
111. Every chlorine atom found in nature has a mass of exactly 35.5 u.
Answer: False
112. Average atomic mass is calculated using the relative abundance of isotopes.
Answer: True
113. Protium, deuterium and tritium have different numbers of protons.
Answer: False
114. All isotopes of hydrogen have one proton each.
Answer: True
115. Carbon-12, Carbon-13 and Carbon-14 have the same atomic number.
Answer: True
116. Carbon-12 and Carbon-14 have different numbers of neutrons.
Answer: True
117. A neutral atom always has equal numbers of protons and electrons.
Answer: True
118. Protons determine the identity of an element.
Answer: True
119. The study of atoms is complete and no further discoveries are expected.
Answer: False
120. The understanding of atomic structure has improved through continuous scientific research and experimentation.
Answer: True
Match the Following
Match the Following (1–25)
Column A → Column B
- Dalton →
- J. J. Thomson →
- Rutherford →
- Bohr →
- Chadwick →
- Electron →
- Proton →
- Neutron →
- Cathode rays →
- Gold foil experiment →
- K-shell →
- L-shell →
- Atomic number →
- Mass number →
- Valence electrons →
- Valency →
- Isotopes →
- Isobars →
- Carbon-14 →
- Uranium-235 →
- Cobalt-60 →
- Iodine-131 →
- Electron cloud →
- STM →
- TEM →
Column B
A. Nuclear fuel
B. Discoverer of neutron
C. Positive charged particle in nucleus
D. 2 electrons maximum shell
E. Different elements, same mass number
F. Fixed energy levels model
G. Discovery of nucleus
H. Atomic theory
I. Negative charged particle
J. Used in cancer treatment
K. Number of protons
L. Inner most shell
M. Most of atom is empty space
N. Used in thyroid treatment
O. Probability region of electron
P. Used in archaeology dating
Q. Discovery of electron
R. Study of surface atoms
S. Transmission electron microscope
T. Combining capacity
U. Protons + neutrons
V. Same element, different mass
W. Positive nucleus particle
X. Cathode ray tube phenomenon
Y. 8 electrons maximum (outer shell concept)
ANSWER KEY (1–25)
1 → H
2 → Q
3 → G
4 → F
5 → B
6 → I
7 → W
8 → C
9 → X
10 → M
11 → L
12 → D
13 → K
14 → U
15 → Y
16 → T
17 → V
18 → E
19 → P
20 → A
21 → J
22 → N
23 → O
24 → R
25 → S
PART 2 (26–50)
Match the Following (26–50)
Column A → Column B
- Atomic number →
- Mass number →
- Valence shell →
- Electronic configuration →
- Stable atom →
- Sodium (Na) →
- Oxygen (O) →
- Chlorine (Cl) →
- Argon (Ar) →
- Neon (Ne) →
- Hydrogen isotopes →
- Protium →
- Deuterium →
- Tritium →
- Average atomic mass →
- Chlorine isotope 35Cl →
- Chlorine isotope 37Cl →
- Rutherford model →
- Thomson model →
- Bohr model →
- Nuclear model →
- Electron cloud model →
- Relative abundance →
- Nuclear reactor fuel →
- Cancer treatment isotope →
Column B
A. 2,8,1
B. Number of protons
C. 2,8,7
D. Protons + neutrons
E. 2,8,8
F. 2,6
G. One proton, one neutron
H. One proton, two neutrons
H. (duplicate label used for isotopes concept)
I. Used in medical radiation therapy
J. Most stable electronic configuration
K. Same element different neutrons
L. 35.5 u average mass
M. Most abundant isotope of chlorine
N. Less abundant isotope of chlorine
O. Positive nucleus in center
P. Plum pudding model
Q. Fixed energy levels
R. Electron probability region
S. Ratio of isotope presence in nature
T. Uranium-235
U. Discovery of nucleus
V. Electron arrangement in shells
W. No neutrons in nucleus
ANSWER KEY (26–50)
26 → B
27 → D
28 → V
29 → V
30 → J
31 → A
32 → F
33 → C
34 → E
35 → J
36 → K
37 → W
38 → G
39 → H
40 → L
41 → M
42 → N
43 → U
44 → P
45 → Q
46 → O
47 → R
48 → S
49 → T
50 → I
PART 3 (51–75)
Match the Following (51–75)
Column A → Column B
- Gold foil experiment →
- Cathode ray experiment →
- Discovery of neutron →
- Discovery of electron →
- Discovery of nucleus →
- Modern atomic model →
- Dalton’s model →
- Thomson’s model →
- Bohr’s model →
- Isotopes →
- Isobars →
- Valency →
- Valence electrons →
- Atomic structure study →
- Electron movement (Bohr) →
- Electron movement (modern theory) →
- Carbon-14 use →
- Uranium-235 use →
- Cobalt-60 use →
- Iodine-131 use →
- Proton charge →
- Electron charge →
- Neutron charge →
- Nuclear center →
- Scientific development →
Column B
A. No charge
B. Cancer treatment
C. Stable energy shells
D. Probability cloud model
E. Indivisible atom idea
F. Negative charge particle
G. Discovery of nucleus
H. Age of fossils
I. Positive charge particle
J. Atomic theory evolution
K. Fixed orbits
L. Same atomic number different mass number
M. Same mass number different atomic number
N. Combining capacity
O. Electrons in outer shell
P. Rutherford experiment
Q. J. J. Thomson experiment
R. James Chadwick discovery
S. Nuclear fuel
T. Bohr model
U. Electron discovery
V. Central part of atom
W. Continuous research process
X. Atom is not solid
ANSWER KEY (51–75)
51 → P
52 → Q
53 → R
54 → U
55 → G
56 → D
57 → E
58 → X
59 → T
60 → L
61 → M
62 → N
63 → O
64 → W
65 → K
66 → D
67 → H
68 → S
69 → B
70 → B
71 → I
72 → F
73 → A
74 → V
75 → J