Question

${\text{K}}^{+}\text{ and C}{\text{l}}^{-}$ ions are isoelectronic. Which of the following statements is not correct?
A. Both ${\text{K}}^{+}\text{ and C}{\text{l}}^{-}$ ions contain 18 electrons
B. Both ${\text{K}}^{+}\text{ and C}{\text{l}}^{-}$ ions have the same configuration
C. ${\text{K}}^{+}$ is bigger than $\text{C}{\text{l}}^{-}$ ion in ionic size
D. $\text{C}{\text{l}}^{-}$ ion is bigger than ${\text{K}}^{+}$ ion in size

Answer 1

Hint: Those species of the elements which have the same number of electrons in the shell of an atom or they have the same electronic configuration are known as isoelectronic species. Cation consists of positive charge and they formed after the loss of electrons.

Complete answer:
-As we know that the isoelectronic elements are those species which have the same electronic species.
-So, the electronic configuration of ${\text{K}}^{+}\text{ and C}{\text{l}}^{-}$ions is:
$\begin{array}{rl}& {\text{K}}^{+}\text{ (18) = 1}{\text{s}}^2\text{ 2}{\text{s}}^2\text{ 2}{\text{p}}^6\text{ 3}{\text{s}}^2\text{ 3}{\text{p}}^6\\ & \text{C}{\text{l}}^{-}\text{ (18) = 1}{\text{s}}^2\text{ 2}{\text{s}}^2\text{ 2}{\text{p}}^6\text{ 3}{\text{s}}^2\text{ 3}{\text{p}}^6\end{array}$
-That’s why they are considered as isoelectronic species and also they have the same number of electrons that is 18.
-That's why option A and B are the correct statements.
-Now, we know that the size of the cation is smaller than its parent atom because the loss of one electron takes place due to which the effect of nuclear charge increases on the rest of the atoms and hence, decreases the size of the atom.
-Whereas in the anion, there is the addition of an extra electron on the outermost shell of the atom due to which the size of the shell increases as compared to its parent atom.
-Hence, we can say that the size of anion i.e. $\text{C}{\text{l}}^{-}$is bigger than the size of the cation i.e. ${\text{K}}^{+}$.
-So, option D is the correct statement.
Therefore, Option C is the correct answer because this statement is wrong.
So, the correct answer is “Option C”.

Note: We can easily identify the pair of isoelectronic species by calculating the number of electrons of the elements as they will be the same but they have a different number of protons and neutrons. For, example, in ${\text{K}}^{+}\text{ and C}{\text{l}}^{-}$ ions, the no. of protons are 19 and 17 but the no. of electrons are 18 for both.