the electron configuration:
1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁶, etc
Ar = [Ne] 3s² 3p⁶ ( 18)
Iodine = 53 = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁵
if use Ar (18) ⇒ [Ar] 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁵
if use Kr (36) ⇒ [Kr] 5s², 4d¹⁰, 5p⁵