Question

Which of the following species does not have eight valence electrons?

• A. $C{a}^{+}$
• B. $R{b}^{+}$
• C. $Xe$
• D. $B{r}^{-}$

Answer 1

The correct option is A $C{a}^{+}$

Finding the number of valence electrons.
(a) $C{a}^{+}$
Z$\left(Ca\right)$ = 20
The electronic configuration of
$Ca$: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$ or $\left[Ar\right]4s^2$
$C{a}^{+}$: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$ or $\left[Ar\right]4s^1$
Valence shell electronic configuration of $C{a}^{+}$ : $4s^1$
Hence, $C{a}^{+}$ does not have 8 electrons in its valence shell.

(b) $R{b}^{+}$
Z$\left(Rb\right)$ = 37
The electronic configuration of
$Rb$: $\left[Ar\right]3d^{10}4s^{2}4p^{6}5s^1$
$R{b}^{+}$: $\left[Ar\right]3d^{10}4s^{2}4p^6$
Valence shell electronic configuration of $R{b}^{+}$ : $4s^{2}4p^6$
Hence, $R{b}^{+}$ has 8 electrons in its valence shell.
(c) $Xe$
Z$\left(Xe\right)$ = 54
The electronic configuration of
$Xe$: $\left[Kr\right]4d^{10}5s^{2}5p^6$
So valence shell electronic configuration of $Xe$ : $5s^{2}5p^6$
Hence, $Xe$ has 8 electrons in its valence shell.

(d) $B{r}^{-}$
Z$\left(Br\right)$= 35
The electronic configuration of
$Br$: $\left[Ar\right]3d^{10}4s^{2}4p^5$
$B{r}^{-}$: $\left[Ar\right]3d^{10}4s^{2}4p^6$
So, valence shell electronic configuration of $B{r}^{-}$ : $4s^{2}4p^6$
Hence, $B{r}^{-}$ has 8 electrons in its valence shell.