Select the correct statement. a) na+ and k+,are isoelectronic because they both lost one electron and are in group 1a. b) ar and k+ are isoelectronic because they both have 18 electrons. c) na and mg are isoelectronic because they both are in the third period. d) ca+ and ca2+ are isoelectronic because they both are charged atoms of calcium and lost electrons.

B is the correct answer.

Explanation:

When particles or substances are isoelectronic, this mean that they contain the same number of electrons.

(B) is correct

Atomic Argon has 18 electrons. While potassium atom has 19 electrons, the potassium ion has 18 electrons since it has lost one electron to make it a univalent positive ion.

(A) is wrong.

Although sodium and potassium are in the same group on the periodic table, the do not contain the same number of electrons.

Atomic sodium has 11 electrons, losing one electron turns it into an ion and hence it has net 10 electrons.

Atomic potassium has 19 electrons, losing one electron turns it into an ion and hence it has net 18 electrons.

(C) is wrong.

Although they both belong to the same period, atomic magnesium has 12 electrons while atomic sodium has 11 electrons.

(D) is wrong.

Atomic calcium has 20 electrons. The first ion had lost only one electron making it have 19 electrons.

The second electron has lost 2 electrons, making it have 18 electrons