JEE Main 2024 · 9 April · evening · Single correct · +4 / −1

The electronic configuration of Einsteinium is :
(Given atomic number of Einsteinium =99)

  1. (a) [Rn] 5 f¹³ 6 d° 7 s²
  2. (b) [Rn] 5 f¹² 6 d° 7 s²
  3. (c) [Rn] 5 f¹¹ 6 d° 7 s²
  4. (d) [Rn] 5 f¹⁰ 6 d° 7 s²
Answer and solution

Answer (c) [Rn] 5 f¹¹ 6 d° 7 s²

Solution

Chemistry ma’am · 4 steps

Subtract radon's 86, fill 7s first, and whatever is left goes into 5f.

  1. Core electrons. Radon has atomic number 86, so [Rn] accounts for 86 electrons. Einsteinium has 99 - 86 = 13 electrons left to place.
  2. Fill 7s first. After radon, the 7s orbital fills first and takes 2 electrons. That leaves 13 - 2 = 11 electrons.
  3. Rest into 5f. Einsteinium is an actinide, so the remaining 11 electrons go into 5f. Like most later actinides, it keeps 6d empty.

    Es (Z = 99): [Rn] 5f¹¹ 6d⁰ 7s²

  4. So. The configuration is [Rn] 5f¹¹ 6d⁰ 7s², which is option (c). Check: 86 + 11 + 2 = 99.

The idea to keep. For any actinide, subtract 86 for radon, put 2 in 7s, and the rest goes into 5f. Always add the electrons back up to check you get Z.

Answer from the NTA final answer key. Solution by PrepWiser.

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Asked about 170 times in JEE Main since 2020. This question is from 2024.