physics : class12 Atomic Mass and Nuclear Mass : Mass Defect : Atoms and Nuclei MCQ Questions and Answers
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Atomic Mass and Nuclear Mass
MCQ questions and answers.
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MCQ Questions
1. The neutral atomic mass of helium-4 is 4.002603 u. Taking the mass of an electron as 0.0005486 u and neglecting electronic binding energy, what is the nuclear mass of helium-4?
A.
4.001505 u
B.
4.002054 u
C.
4.003152 u
D.
4.001957 u
Correct Answer:
4.001505 u
Explanation:
The atomic mass includes the nucleus and two orbital electrons. Therefore, nuclear mass = atomic mass − 2m_e = 4.002603 − 2(0.0005486) = 4.0015058 u, approximately 4.001505 u.
2. The atomic mass of oxygen-16 is 15.994915 u. Using m_e = 0.0005486 u, what is its nuclear mass, neglecting electronic binding energy?
A.
15.990526 u
B.
15.994366 u
C.
15.999304 u
D.
15.991623 u
Correct Answer:
15.990526 u
Explanation:
Oxygen has eight electrons in its neutral atom. Hence, nuclear mass = 15.994915 − 8(0.0005486) = 15.9905262 u, approximately 15.990526 u.
3. For a nuclide with atomic number Z and mass number A, which expression correctly relates its atomic mass M_a to its nuclear mass M_n, when the electronic binding energy is neglected?
A.
M_a = M_n + Zm_e
B.
M_a = M_n + (A − Z)m_e
C.
M_a = M_n − Zm_e
D.
M_a = M_n + Am_e
Correct Answer:
M_a = M_n + Zm_e
Explanation:
A neutral atom contains the nucleus plus Z electrons. Thus, its atomic mass is the nuclear mass plus the mass of Z electrons: M_a = M_n + Zm_e.
4. A neutral atom has atomic mass 40.078 u and atomic number 20. If the electron mass is 0.0005486 u, what is the corresponding nuclear mass?
A.
40.067028 u
B.
40.077451 u
C.
40.089972 u
D.
40.078000 u
Correct Answer:
40.067028 u
Explanation:
The neutral calcium atom has 20 electrons. Therefore, M_n = M_a − 20m_e = 40.078 − 20(0.0005486) = 40.067028 u.
5. Two neutral isotopes of the same element have atomic masses M_1 and M_2 and contain the same number of electrons. What is the difference between their nuclear masses?
A.
M_1 − M_2
B.
M_1 + M_2
C.
(M_1 − M_2)/Z
D.
Z(M_1 − M_2)
Correct Answer:
M_1 − M_2
Explanation:
For isotopes of the same element, the number of electrons Z is identical. Since M_a = M_n + Zm_e for both atoms, subtracting the two equations gives M_n1 − M_n2 = M_1 − M_2.
6. The atomic mass of iron-56 is 55.934936 u. Taking Z = 26 and m_e = 0.0005486 u, what is its nuclear mass?
A.
55.920672 u
B.
55.934387 u
C.
55.949200 u
D.
55.906408 u
Correct Answer:
55.920672 u
Explanation:
Iron-56 has 26 electrons in its neutral atom. Thus, M_n = 55.934936 − 26(0.0005486) = 55.9206724 u, approximately 55.920672 u.
7. A singly ionized atom X+ has nuclear charge +Ze and contains Z − 1 electrons. If its measured mass is M_+, what is the neutral atomic mass of X, neglecting ionization energy?
A.
M_+ + m_e
B.
M_+ − m_e
C.
M_+ + (Z − 1)m_e
D.
M_+ + Zm_e
Correct Answer:
M_+ + m_e
Explanation:
The singly ionized ion lacks one electron compared with the neutral atom. Therefore, the neutral atomic mass is the ion mass plus one electron mass: M_a = M_+ + m_e.
8. A neutral atom has nuclear mass 23.985042 u and atomic number 11. What is its atomic mass if m_e = 0.0005486 u?
A.
23.979008 u
B.
23.985591 u
C.
23.991076 u
D.
23.985042 u
Correct Answer:
23.991076 u
Explanation:
A neutral atom with atomic number 11 contains 11 electrons. Hence, M_a = M_n + 11m_e = 23.985042 + 11(0.0005486) = 23.9910766 u, approximately 23.991076 u.
9. When the mass defect is calculated using atomic masses rather than nuclear masses, which expression gives the mass defect of a nuclide containing Z protons and A − Z neutrons?
A.
Δm = Zm_H + (A − Z)m_n − M_a
B.
Δm = Zm_p + (A − Z)m_n − M_a
C.
Δm = Zm_H + Am_n − M_a
D.
Δm = M_a − Zm_H − (A − Z)m_n
Correct Answer:
Δm = Zm_H + (A − Z)m_n − M_a
Explanation:
Using atomic masses, the proton mass is replaced by the hydrogen-atom mass because the Z electron masses on the separated side cancel the Z electron masses included in the neutral atomic mass. Thus, Δm = Zm_H + (A − Z)m_n − M_a.
10. A neutral atom has atomic mass 100.000000 u and nuclear mass 99.970000 u. Assuming the electron mass is 0.0005486 u, what is the atomic number of the atom?
A.
45
B.
50
C.
55
D.
60
Correct Answer:
55
Explanation:
The difference between atomic and nuclear mass is the total mass of the electrons: M_a − M_n = Zm_e. Therefore, Z = (100.000000 − 99.970000)/0.0005486 = 0.030000/0.0005486 ≈ 54.69, which corresponds to the integer atomic number 55 after considering the rounded masses.
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