6) D

Remember mass number = no. of protons + neutrons

1 – Wrong
This ion contains 16 protons.
No. of neutrons = 34 – 16 = 18. We need 20 neutrons.

2 – Correct
This ion contains 17 protons.
No. of neutrons = 37 – 17 = 20.
Number of electrons in a Cl atom = 17
Since this ion has a charge of (-1), number of electrons = 17 + 1 = 18

3 – Wrong
This atom contains 18 protons.
No. of neutrons = 40 – 18 = 22. We need 20 neutrons.


4 – Correct
This ion contains 19 protons.
No. of neutrons = 39 – 19 = 20.
Number of electrons in a K atom = 19
Since this ion has a charge of (+1), number of electrons = 19 – 1 = 18


5 – Wrong
This atom contains 20 protons.
No. of neutrons = 40 – 20 = 20.
However, number of electrons in a Ca atom = 20. We need 18 electrons

Since only 2 and 4 are correct, D is the answer.