Question
Explain why transition elements with electronic configuration $3d^{44}s^2$ and $3d^{94}s^2$ do not exist.
✓
Answer
$(1) \ d-$orbitals are degenerate orbitals and they acquire extra stability when half$-$filled $\left(3 d^5\right)$ or completely filled $\left(3 d^{10}\right)$. Hence $3 d^4$ and $3 d^9$ electronic configurations are less stable.
$(2)$ The energy difference between $3 d$ and $4 . s \ ^{\prime}$ subshells is very low, hence there arises a transfer of one electron from $45$ orbital to $3 d$ orbital.
The electronic configuration changes as,
$ 3 d^4, 4 s^2 \rightarrow 3 d^5 4 s^1$
$ 3 d^9{ }^9 4 s^2 \rightarrow 3 d^{10} 45^1$
Therefore transition elements, with electronic configurations $3d^4, 4s^2$ and $3d^9, 4s^2$ do not exist.
$(2)$ The energy difference between $3 d$ and $4 . s \ ^{\prime}$ subshells is very low, hence there arises a transfer of one electron from $45$ orbital to $3 d$ orbital.
The electronic configuration changes as,
$ 3 d^4, 4 s^2 \rightarrow 3 d^5 4 s^1$
$ 3 d^9{ }^9 4 s^2 \rightarrow 3 d^{10} 45^1$
Therefore transition elements, with electronic configurations $3d^4, 4s^2$ and $3d^9, 4s^2$ do not exist.
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