Questions on Mass Number
20 Multiple-Choice Questions on Mass Number
1. What is the mass number of an atom?
A) The total number of protons and electrons
B) The number of neutrons
C) The number of electrons only
D) The total number of protons and neutrons
E) The number of protons only
2. If an atom has 11 protons and 12 neutrons, what is its mass number?
A) 11
B) 12
C) 22
D) 23
E) 24
3. Which of the following contributes most to the mass number?
A) Electrons
B) Protons and neutrons
C) Protons only
D) Neutrons only
E) None of the above
4. The mass number is represented by which symbol?
A) Z
B) A
C) N
D) M
E) m
5. An atom has 8 protons and 8 neutrons. What is its mass number?
A) 8
B) 10
C) 12
D) 14
E) 16
6. Which of the following correctly expresses the mass number in nuclear notation for carbon-14?
A) ₆¹⁴C
B) ¹⁴₈C
C) ₈¹⁴C
D) ₆¹²C
E) ¹⁴₆N
7. The mass number of an element changes when:
A) The number of electrons changes
B) The number of protons changes
C) The number of neutrons changes
D) The element becomes an ion
E) The atomic number changes
8. What is the mass number of an isotope of lithium with 3 protons and 4 neutrons?
A) 3
B) 4
C) 6
D) 7
E) 8
9. The isotope symbol ³⁷₁₇Cl indicates a mass number of:
A) 17
B) 20
C) 37
D) 54
E) 24
10. What is the difference between atomic number and mass number?
A) Atomic number includes neutrons; mass number doesn’t
B) Mass number = atomic number – neutrons
C) Mass number = atomic number + neutrons
D) They are always the same
E) Atomic number = mass number × 2
11. Which of these particles is not counted in the mass number?
A) Proton
B) Neutron
C) Electron
D) Nucleon
E) None of the above
12. If an atom has a mass number of 35 and 17 protons, how many neutrons does it have?
A) 35
B) 18
C) 17
D) 52
E) 19
13. What information does the mass number give that the atomic number does not?
A) The number of electrons
B) The number of protons
C) The number of neutrons
D) The atomic mass
E) The element's symbol
14. What would be the mass number of an atom with 15 protons and 16 neutrons?
A) 15
B) 16
C) 30
D) 31
E) 32
15. Which isotope has a mass number of 1?
A) Deuterium
B) Tritium
C) Protium
D) Helium
E) Oxygen
16. Two isotopes of an element differ in:
A) Atomic number
B) Proton number
C) Electron number
D) Mass number
E) Element name
17. Which of the following can change the mass number of an atom?
A) Loss of electrons
B) Gain of protons
C) Loss of neutrons
D) Change in energy levels
E) Ionization
18. What is the mass number of an atom with 29 protons and 35 neutrons?
A) 29
B) 64
C) 35
D) 58
E) 14
19. If an atom’s mass number is 40 and its atomic number is 18, how many neutrons does it have?
A) 18
B) 20
C) 22
D) 40
E) 58
20. In which case is the mass number incorrectly determined?
A) ₁₁²³Na → 11 protons + 12 neutrons = 23 ✔
B) ₆¹²C → 6 protons + 6 neutrons = 12 ✔
C) ₈¹⁵O → 8 protons + 7 neutrons = 15 ✔
D) ₁₂²⁵Mg → 12 protons + 14 neutrons = 26 ❌
E) ₇¹⁴N → 7 protons + 7 neutrons = 14 ✔
Answer Key with Explanations
1. D – Mass number = total number of protons + neutrons.
2. D – 11 + 12 = 23.
3. B – Protons and neutrons (nucleons) contribute to mass; electrons are negligible.
4. B – A represents mass number.
5. E – 8 + 8 = 16.
6. A – Mass number = superscript, atomic number = subscript.
7. C – Adding or removing neutrons changes mass number.
8. D – 3 + 4 = 7.
9. C – The superscript shows mass number: 37.
10. C – Mass number = atomic number (protons) + neutrons.
11. C – Electrons are too light to affect mass number.
12. B – 35 – 17 = 18 neutrons.
13. C – Mass number shows how many neutrons are present.
14. D – 15 + 16 = 31.
15. C – Protium has 1 proton, 0 neutrons → mass number = 1.
16. D – Isotopes have same atomic number, different mass numbers.
17. C – Neutron loss changes mass number.
18. B – 29 + 35 = 64.
19. C – 40 – 18 = 22 neutrons.
20. D – 12 + 14 = 26, not 25 → incorrect.
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