JEE Chemistry 2024 Question with Solution

Given: The atomic number of Einsteinium is $99$.

Find: Its electronic configuration and the correct option.

Einsteinium has $99$ electrons. Using the noble gas core, [Rn] represents $86$ electrons.

After [Rn], electrons fill the $7s$ orbital first, giving $7s^{2}$. The remaining electrons enter the $5f$ subshell because Einsteinium belongs to the actinide series.

Thus, beyond [Rn]:

$$99 - 86 = 13$$

Out of these $13$ electrons, $2$ go to $7s$, and the remaining $11$ go to $5f$.

So the configuration becomes:

$$[Rn] \, 5f^{11} \, 6d^{0} \, 7s^{2}$$

Therefore, the correct option is B.

Given: Einsteinium is an actinide with atomic number $99$.

Find: The correct electronic configuration.

The general order of orbital filling is:

$$1s \to 2s \to 2p \to 3s \to 3p \to 4s \to 3d \to 4p \to 5s \to 4d \to 5p \to 6s \to 4f \to 5d \to 6p \to 7s \to 5f \to 6d \to 7p$$

The noble gas core [Rn] accounts for $86$ electrons.

Now fill the remaining electrons after [Rn]:

1. $7s^{2}$
2. Then electrons enter $5f$.
3. For Einsteinium, this gives $5f^{11}$ and $6d^{0}$.

Hence, the electronic configuration is:

$$[Rn] \, 5f^{11} \, 6d^{0} \, 7s^{2}$$

This matches option (2), so the correct option is B.