JEE Main 2022 · 29 June · morning · Single correct · +4 / −1
The electronic configuration of Pt (atomic number 78) is :
- (a) [Xe] 4f¹⁴ 5d⁹ 6s¹
- (b) [Kr] 4f¹⁴ 5d¹⁰
- (c) [Xe] 4f¹⁴ 5d¹⁰
- (d) [Xe] 4f¹⁴ 5d⁸ 6s²
Answer and solution
Answer (a) [Xe] 4f¹⁴ 5d⁹ 6s¹
Solution
Chemistry ma’am · 3 steps
Count to 78 first, then remember Pt breaks the Aufbau order by moving one 6s electron into 5d.
- Expected filling. After [Xe] (54 electrons), Pt has 24 more. The Aufbau order fills 6s, then 4f, then 5d, which predicts [Xe] 4f¹⁴ 5d⁸ 6s².
- What is observed. In the 5d series, the 5d and 6s levels are very close in energy. Moving one electron from 6s to 5d gives a more stable arrangement, so the observed configuration of Pt is [Xe] 4f¹⁴ 5d⁹ 6s¹.
Option Total electrons Verdict (a) [Xe] 4f¹⁴ 5d⁹ 6s¹ 54 + 14 + 9 + 1 = 78 observed configuration of Pt (b) [Kr] 4f¹⁴ 5d¹⁰ 36 + 14 + 10 = 60 wrong count, and it skips 5s, 5p and 4d (c) [Xe] 4f¹⁴ 5d¹⁰ 54 + 14 + 10 = 78 right count, but 6s is not empty in Pt (d) [Xe] 4f¹⁴ 5d⁸ 6s² 54 + 14 + 8 + 2 = 78 Aufbau prediction, not what is observed - So. Platinum is [Xe] 4f¹⁴ 5d⁹ 6s¹, which is option (a).
The idea to keep. For transition metals, first check that the electrons add up, then recall the known exceptions: Cr, Cu, Pd, Ag, Pt and Au do not follow the plain Aufbau order.
Answer from the NTA final answer key. Solution by PrepWiser.