chemistry : class11 Orbital Energy Levels : Concept of orbitals : Structure of Atom MCQ Questions and Answers
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Orbital Energy Levels
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MCQ Questions
1. In an isolated hydrogen atom, which group of orbitals has exactly the same energy?
A.
2s, 2p, and 3s
B.
3s, 3p, and 3d
C.
2p, 3p, and 4p
D.
3d, 4s, and 4p
Correct Answer:
3s, 3p, and 3d
Explanation:
For a hydrogen atom, orbital energy depends only on the principal quantum number n. Therefore, all orbitals with n = 3, namely 3s, 3p, and 3d, are degenerate.
2. According to the (n + l) rule for multi-electron atoms, which orbital is expected to have lower energy?
A.
3d
B.
4s
C.
4p
D.
5s
Correct Answer:
4s
Explanation:
For 4s, n + l = 4 + 0 = 4, whereas for 3d, n + l = 3 + 2 = 5. The orbital with the lower n + l value generally has lower energy, so 4s lies below 3d.
3. Between the 4p and 5s orbitals in a multi-electron atom, which is expected to have lower energy?
A.
4p
B.
5s
C.
Both have exactly the same energy
D.
Their order cannot be predicted using orbital energy rules
Correct Answer:
4p
Explanation:
Both orbitals have n + l = 5: 4p gives 4 + 1, and 5s gives 5 + 0. When n + l values are equal, the orbital with lower n has lower energy; hence 4p is lower than 5s.
4. Which sequence represents the increasing order of orbital energies in a typical multi-electron atom?
A.
1s < 2p < 2s < 3p < 3s < 4s < 3d
B.
1s < 2s < 2p < 3s < 3p < 4s < 3d
C.
1s < 2s < 3s < 2p < 3p < 3d < 4s
D.
1s < 2s < 2p < 3s < 4s < 3p < 3d
Correct Answer:
1s < 2s < 2p < 3s < 3p < 4s < 3d
Explanation:
In multi-electron atoms, orbital energies generally follow the (n + l) rule. The resulting order is 1s, 2s, 2p, 3s, 3p, 4s, and then 3d.
5. Which comparison is correct for the indicated orbitals in hydrogen-like species, ignoring fine-structure effects?
A.
The 2p orbital of He+ has higher energy than the 2s orbital of H
B.
The 2p orbital of He+ and the 2s orbital of H have the same energy
C.
The 2p orbital of He+ has lower energy than the 2s orbital of H
D.
The relative energies depend only on whether the orbital is s or p
Correct Answer:
The 2p orbital of He+ has lower energy than the 2s orbital of H
Explanation:
For hydrogen-like species, orbital energy is proportional to -Z²/n² and is independent of l. Both orbitals have n = 2, but He+ has Z = 2, so its 2p orbital is more strongly bound and has lower energy than the 2s orbital of hydrogen.
6. Why does the 4s orbital generally lie lower in energy than the 3d orbital in a neutral multi-electron atom?
A.
The 4s orbital has a smaller principal quantum number
B.
The 4s orbital has a lower n + l value and greater penetration toward the nucleus
C.
The 3d orbital has no angular momentum
D.
The 4s orbital always contains fewer electrons
Correct Answer:
The 4s orbital has a lower n + l value and greater penetration toward the nucleus
Explanation:
The 4s orbital has n + l = 4, while 3d has n + l = 5. In addition, an s orbital penetrates more effectively toward the nucleus than a d orbital, experiencing a greater effective nuclear attraction and usually having lower energy.
7. Among the following orbitals, which is expected to have the highest energy in a multi-electron atom?
A.
3p
B.
4s
C.
3d
D.
4p
Correct Answer:
4p
Explanation:
The values of n + l are 4 for 3p, 4 for 4s, 5 for 3d, and 5 for 4p. Between 3d and 4p, the n + l values are equal, so the orbital with higher n, 4p, has higher energy. It is therefore the highest-energy orbital listed.
8. Which sequence correctly gives the increasing energy order of the orbitals 5s, 4d, 5p, and 4f in a multi-electron atom?
A.
4f < 5p < 4d < 5s
B.
5s < 5p < 4d < 4f
C.
5s < 4d < 5p < 4f
D.
5s < 4f < 4d < 5p
Correct Answer:
5s < 4d < 5p < 4f
Explanation:
The n + l values are 5 for 5s, 6 for 4d, 6 for 5p, and 7 for 4f. Since 4d and 5p have equal n + l values, 4d has lower energy because it has the smaller n. Thus the order is 5s < 4d < 5p < 4f.
9. Which statement correctly distinguishes orbital energy patterns in hydrogen-like atoms from those in multi-electron atoms?
A.
In both types of atoms, orbital energy depends only on the azimuthal quantum number
B.
In hydrogen-like atoms, orbitals with the same n are degenerate, whereas in multi-electron atoms their energies can differ with l
C.
In hydrogen-like atoms, 2s is always lower than 2p, but in multi-electron atoms they are always degenerate
D.
In multi-electron atoms, orbital energy depends only on n, just as in hydrogen
Correct Answer:
In hydrogen-like atoms, orbitals with the same n are degenerate, whereas in multi-electron atoms their energies can differ with l
Explanation:
For hydrogen-like atoms, energy depends only on n, so orbitals such as 2s and 2p have the same energy. In multi-electron atoms, shielding and penetration make energy depend on both n and l, removing this degeneracy in general.
10. Which statement about the energy difference between 2s and 2p orbitals is correct?
A.
They are always separated by the same energy in every atom
B.
They are degenerate in hydrogen but generally have different energies in multi-electron atoms
C.
The 2p orbital is always lower in energy than the 2s orbital
D.
The 2s orbital does not exist in hydrogen-like atoms
Correct Answer:
They are degenerate in hydrogen but generally have different energies in multi-electron atoms
Explanation:
In hydrogen, both 2s and 2p have the same principal quantum number and therefore the same energy. In multi-electron atoms, the 2s orbital penetrates more effectively and experiences less shielding, so it generally lies lower in energy than 2p.
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