Write electron configurations for these atoms and ions, and comment on the result.

a.) ar
b.) cl-
c.) s2-
d.) p3-

A) Ar, Z = 18 => 18 electrons => 1s2 2s2 2p6 3s2 3p6

b) Cl-

Cl, Z = 17 => Cl- = 17e + 1e = 18 electrons => same electron configuration as Ar

c) S2-

S, Z = 16 => S(2-) = 16e + 2e = 18 electrons => same electron configuration as Ar 

d) P(3-), Z= 15 => P(3-) = 15e + 3e  = 18 electrons => same electron configuration as Ar

Cl- ion has one more electron than Cl atom, S2- ion has two more electrons than S atom, and P3- has three more electrons than P atom.

Then the three ions have 18 electrons, which is the number electrons of Ar atoms, and therefore the 4 species have the same electron configuration of Ar.