Classification of Elements Part 1

The electronic configuration of given ions are as follows:
K19=2,8,8,1 K+=2,8,8
Ca20=2,8,8,2 Ca2+=2,8,8
P15=2,8,5 P3−=2,8,8
S16=2,8,6 S2−=2,8,8
Cl17=2,8,7 Cl−=2,8,8
As the effective nuclear charge increases from P to Ca, the atomic size decreases in the same order.
Order of ionic radii

P3−

15

>

S2−

16

>

Cl−

17

>

K+

19

>

Ca2+

20

Therefore, option (a) is correct.