Question

Which of the following ions is NOT isoelectronic to a noble gas?

• A. $A{l}^{3+}$
• B. $K^{+}$
• C. $B{r}^{-}$
• D. $C{o}^{2+}$

Answer 1

Answer: (D)

$C{o}^{2+}$

Cobalt has an atomic number of $27$. It's electronic configuration can be written as $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^7$. Or it can also be written as $\left[Ar\right]4s^{2}3d^7$, where $\left[Ar\right]$ represents the electronic configuration of Argon.

If two electrons are taken out from Cobalt, we are left with $C{o}^{2+}$, that is $25$ electrons, whose electronic configuration is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^7$, or it can also be written as $\left[Ar\right]3d^7$, which is clearly not the electronic configuration of a noble gas. Thus, $C{o}^{2+}$ is not isoelectronic to a noble gas.

Whereas, clearly all the other ions like $A{l}^{3+}$, $K^{+}$, $B{r}^{-}$ respectively have $10$, $18$, and $36$ which represent the electron count of noble gases Neon, Argon, and Krypton respectively

Option D is the correct answer.