Question

Which of the following 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 in:
(a) $C{a}^{+}$
Calcium has an atomic number of 20.
The electronic configuration of $Ca$ is $\left[He\right]2s^{2}2p^{6}3s^{2}3p^{6}4s^2$

In the case of $C{a}^{+}$ (with 19 electrons):
The electronic configuration will be: $\left[He\right]2s^{2}2p^{6}3s^{2}3p^{6}4s^1$
Configuration of the valence shell: $4s^1$
It has only one electron in its valence shell. Hence, $C{a}^{+}$ does not have 8 electrons in its valence shell.

To attain 8 electrons in its valence shell, it should lose two electrons from its valence shell and attain the configuration $\left[He\right]2s^{2}2p^{6}3s^{2}3p^6$
Configuration of the valence shell: $3s^{2}3p^6$ Total number of valence electrons = 8

(b) $R{b}^{+}$
Rubidium has an atomic number of 37
The electronic configuration of $R{b}^{+}$ will be: $\left[Ar\right]3d^{10}4s^{2}4p^6$
Since $R{b}^{+}$ has 8 electrons in its valence shell and resembles the configuration of a noble gas, Krypton, it has a complete octet.

(c) $Xe$
The atomic number of Xenon is 54
The electronic configuration of Xe is $\left[Kr\right]4d^{10}5s^{2}5p^6$
It has 8 electrons in its valence shell, and thus is a noble gas.

(d) $B{r}^{-}$
The atomic number of Bromine is 35
The electronic configuration of $B{r}^{-}$ is $\left[Ar\right]3d^{10}4s^{2}4p^6$
It has 8 electrons in its valence shell.

Hence, $C{a}^{+}$ does not have eight electrons in its valence shell.