1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d10 (b) [Kr] 5s2 4d10 5p6 6s1 (c) 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 (d) [Kr] 5s2 4d10 5p6 6s2 4f7
The Correct Answer and Explanation is:
Correct Answer: (b) [Kr] 5s2 4d10 5p6 6s1
Textbook-Style Explanation:
To determine the correct electron configuration, we must follow the Aufbau principle, which states that electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. Additionally, the Pauli exclusion principle and Hund’s rule govern electron distribution within orbitals.
Option (b) shows the electron configuration:
[Kr] 5s² 4d¹⁰ 5p⁶ 6s¹
This matches the electron configuration of Cesium (Cs), which has an atomic number of 55. Cesium has 55 electrons, and this configuration correctly accounts for all of them:
- [Kr] = 36 electrons (up to krypton)
- 5s² = 2 electrons (36 + 2 = 38)
- 4d¹⁰ = 10 electrons (38 + 10 = 48)
- 5p⁶ = 6 electrons (48 + 6 = 54)
- 6s¹ = 1 electron (54 + 1 = 55)
Thus, this configuration is both correct and complete.
Let’s briefly review the other options:
- (a) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d¹⁰
❌ Incorrect: The 4d subshell should not be completely filled before the 5p. Also, it gives 48 electrons, which is for Cadmium (Cd), but the sequence is incorrect. - (c) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s²
❌ Incomplete: This adds up to only 38 electrons, which corresponds to Strontium (Sr), but lacks noble gas shorthand or higher subshells. - (d) [Kr] 5s² 4d¹⁰ 5p⁶ 6s² 4f⁷
❌ Incorrect for any ground-state neutral element. The 4f block starts after 6s and belongs to the lanthanides; 4f⁷ would be Gadolinium (Gd), atomic number 64. But the 5d¹ orbital should also be included.
Therefore, (b) is the only electron configuration that is both correct in order and complete for its element (Cs).
