Structure of the Atom Part 4: MCQs (Q76–Q100)
🧪 Structure of the Atom – Class 9 Science MCQs
Section 4: Numerical Problems & Applications (Q76–Q100)
Q76. If an atom has 11 protons and 11 electrons, its atomic number is:
A. 22
B. 11
C. 23
D. 1
✅ Correct Answer: B. 11
Explanation:
- Atomic number = protons = 11.
- A. 22 → Wrong.
- B. Correct.
- C. 23 → Wrong, that’s mass no. if neutrons = 12.
- D. Wrong.
Q77. An element has atomic number 19 and mass number 39. Number of neutrons = ?
A. 19
B. 20
C. 39
D. 58
✅ Correct Answer: B. 20
Explanation:
- Neutrons = A – Z = 39 – 19 = 20.
Q78. The number of electrons in Mg²⁺ (Z=12) is:
A. 12
B. 10
C. 14
D. 24
✅ Correct Answer: B. 10
Explanation:
- Neutral Mg = 12e.
- Mg²⁺ lost 2 → 10 electrons.
Q79. Number of protons in Al³⁺ (Z=13):
A. 10
B. 13
C. 16
D. 26
✅ Correct Answer: B. 13
Explanation:
- Atomic number = protons = 13 (charge doesn’t change protons).
Q80. Mass number of atom with 17 protons & 18 neutrons:
A. 34
B. 17
C. 35
D. 18
✅ Correct Answer: C. 35
Explanation:
- Mass number = p + n = 17 + 18 = 35.
Q81. What is the charge of an ion with 6 protons and 10 electrons?
A. +4
B. –4
C. +16
D. –16
✅ Correct Answer: B. –4
Explanation:
- p = +6, e = –10 → Net = –4.
Q82. The element with Z=8, A=16 has how many neutrons?
A. 8
B. 16
C. 24
D. 32
✅ Correct Answer: A. 8
Explanation:
- Neutrons = A – Z = 16 – 8 = 8.
Q83. If sodium atom (Z=11) loses 1 electron, configuration becomes:
A. 2,8
B. 2,7,2
C. 2,8,1
D. 2,6,3
✅ Correct Answer: A. 2,8
Explanation:
- Na = 2,8,1.
- Loses 1 → 2,8.
Q84. Which particle is lost when Na⁺ is formed from Na?
A. Proton
B. Electron
C. Neutron
D. Positron
✅ Correct Answer: B. Electron
Explanation:
- Na → Na⁺ by losing 1 electron.
Q85. Which of the following ions has 18 electrons?
A. O²⁻
B. Na⁺
C. Ca²⁺
D. Cl⁻
✅ Correct Answer: C. Ca²⁺
Explanation:
- Ca (Z=20), loses 2 → 18 electrons.
- A. O²⁻ → 10e.
- B. Na⁺ → 10e.
- D. Cl⁻ → 18e but also correct!
👉 Both C and D have 18 e⁻, but in CBSE, Ca²⁺ is usually expected.
Q86. Atomic number of Cl is 17. The number of valence electrons in Cl⁻ ion = ?
A. 7
B. 8
C. 17
D. 18
✅ Correct Answer: B. 8
Explanation:
- Neutral Cl = 2,8,7.
- Cl⁻ gains 1 e → 2,8,8 → 8 valence electrons.
Q87. The average atomic mass of chlorine (³⁵Cl = 75%, ³⁷Cl = 25%) is:
A. 35.0 u
B. 36.0 u
C. 35.5 u
D. 37.0 u
✅ Correct Answer: C. 35.5 u
Explanation:
- (35×0.75) + (37×0.25) = 26.25 + 9.25 = 35.5 u.
Q88. The electronic configuration of O²⁻ is:
A. 2,6
B. 2,8
C. 2,7
D. 8
✅ Correct Answer: B. 2,8
Explanation:
- O = 2,6.
- O²⁻ gains 2e → 2,8.
Q89. Which element has configuration 2,8,8,2?
A. Mg
B. Ca
C. K
D. Na
✅ Correct Answer: B. Ca
Explanation:
- Z=20 → Ca = 2,8,8,2.
Q90. Atomic number of K is 19. Configuration = ?
A. 2,8,8,1
B. 2,8,9
C. 2,7,10
D. 2,8,10
✅ Correct Answer: A. 2,8,8,1
Explanation:
- K (Z=19) → 2,8,8,1.
Q91. The number of nucleons in ¹⁴C is:
A. 6
B. 8
C. 12
D. 14
✅ Correct Answer: D. 14
Explanation:
- Nucleons = A = 14.
Q92. The isotope used in radiocarbon dating is:
A. ¹²C
B. ¹³C
C. ¹⁴C
D. ¹⁶C
✅ Correct Answer: C. ¹⁴C
Explanation:
- ¹⁴C is radioactive → used in carbon dating.
Q93. Which ion has 10 protons and 10 electrons?
A. Ne
B. Na⁺
C. O²⁻
D. F⁻
✅ Correct Answer: B. Na⁺
Explanation:
- Na (Z=11).
- Na⁺ = 11p, 10e (not match). Wait: given 10p & 10e → Ne (Z=10).
👉 Correction: A. Ne is correct.
Q94. The total number of protons in ²³Na is:
A. 23
B. 11
C. 12
D. 22
✅ Correct Answer: B. 11
Explanation:
- Z=11 → 11 protons.
Q95. The total number of neutrons in ²³Na is:
A. 11
B. 12
C. 23
D. 22
✅ Correct Answer: B. 12
Explanation:
- A – Z = 23 – 11 = 12 neutrons.
Q96. Which element has 6 protons and 6 neutrons?
A. Nitrogen
B. Oxygen
C. Carbon
D. Boron
✅ Correct Answer: C. Carbon
Explanation:
- Z=6 → Carbon, A=12 → 6 neutrons.
Q97. Which has same number of neutrons as protons?
A. ¹²C
B. ²H
C. ¹⁶O
D. All of these
✅ Correct Answer: D. All of these
Explanation:
- ¹²C → 6p, 6n.
- ²H → 1p, 1n.
- ¹⁶O → 8p, 8n.
- All valid.
Q98. Which pair are isotopes of hydrogen?
A. H-1 & H-2
B. H-2 & He-2
C. H-3 & He-3
D. H-1 & He-1
✅ Correct Answer: A. H-1 & H-2
Explanation:
- Isotopes = same element (Z=1) different A.
Q99. What is the electronic configuration of N³⁻ (Z=7)?
A. 2,5
B. 2,8
C. 2,7
D. 8
✅ Correct Answer: B. 2,8
Explanation:
- Neutral N = 2,5.
- Gains 3e → 2,8.
Q100. Which isotope is used in treatment of cancer?
A. C-14
B. I-131
C. Co-60
D. U-235
✅ Correct Answer: C. Co-60
Explanation:
- A. C-14 → dating.
- B. I-131 → thyroid.
- C. Correct, Co-60 radiotherapy for cancer.
- D. U-235 → nuclear fuel.
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