Question

Which of the following pairs are atoms and/or ions are isoelectronic?

a) F-, Cl-
b) K+, Ar
c) O 2-, N 3+
d) Xe, Br-

Answer 1

Isoelectronic means that they have the same number of electrons.

b) K atom has 19 electrons (atomic number -18), K⁺ has one less electrons,  18.
Ar atom has 18 electrons (atomic number 18).

Correct answer is B.

Answer 2

Answer: The correct answer is Option b.

Explanation:

Isoelectronic atoms are defined as the atoms which have same number of  electrons.

In the calculation, the positive charge is subtracted from the total number of electrons and negative charge is added to the total number of electrons.

For the given options:

• Option a:  $F^-\text{ and }Cl^-$

For $F^-$

Number of electrons in fluorine are 9

Number of electrons in fluorine ion = $[9+1]=10$

For $Cl^-$

Number of electrons in chlorine are 17

Number of electrons in chlorine ion = $[17+1]=18$

Thus, they are not isoelectronic.

• Option b:  $K^+\text{ and }Ar$

For $K^+$

Number of electrons in potassium are 19

Number of electrons in fluorine ion = $[19-1]=18$

For $Ar$

Number of electrons in argon are 18

Thus, they are isoelectronic.

• Option c:  $O^{2-}\text{ and }N^{3+}$

For $O^{2-}$

Number of electrons in oxygen are 8

Number of electrons in oxygen ion = $[8+2]=10$

For $N^{3+}$

Number of electrons in nitrogen are 7

Number of electrons in nitrogen ion = $[7-3]=4$

Thus, they are not isoelectronic.

• Option d:  $Xe\text{ and }Br^-$

For $Xe$

Number of electrons in xenon are 54

For $Br^-$

Number of electrons in bromine are 35

Number of electrons in bromine ion = $[35+1]=36$

Thus, they are not isoelectronic.

Hence, the correct answer is Option b.