Curium (Cm) belongs to 5f series. Its atomic number is 96. Which of the following is the correct electronic configuration of Curium?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \([Rn]5f^76d^17s^2\)
In the actinoids (Th to Lr) the 5f orbitals are filled after the 7s orbital, and one electron may sit in 6d instead of 5f. The 5f, 6d and 7s levels have comparable energies, and a half-filled (5f7) or full 5f subshell is extra stable.
Curium has Z = 96, and radon (Z = 86) supplies the core, so 10 electrons remain. Two go into 7s. Of the other eight, seven half-fill the 5f subshell and the eighth goes into 6d rather than spoil the stable 5f7 arrangement:
\( \displaystyle \text{Cm}\ (Z = 96): \mathrm{[Rn]\,5f^{7}\,6d^{1}\,7s^{2}} \quad (86 + 7 + 1 + 2 = 96) \)
This matches its lanthanoid partner gadolinium, \( \mathrm{[Xe]\,4f^{7}\,5d^{1}\,6s^{2}} \).
Why the other options are incorrect
Option (2) \([Rn]5f^76d^3\) — Incorrect. It leaves the 7s orbital empty, though 7s is filled before 5f and 6d are occupied.
Option (3) \([Rn]5f^76d^27s^1\) — Incorrect. It moves an electron out of the filled 7s orbital into 6d, which gives no extra stability here.
Option (4) \([Rn]5f^{10}\) — Incorrect. It has the right count of 10 electrons but places all of them in 5f, with an empty 7s orbital and no half-filled subshell.
NCERT (2026-27 edition):
Electronic Configurations – …
