Ions: Electron Configuration

Question

Write the complete electron configuration for the chromium(III) ion.

Using NOBLE GAS notation write the electron configuration for the iron(II) ion.

Answer 1

Chromium atomic number 24.

Period 4, Group 6

d-block transition element

Electron configuration of a neutral Cr atom: [Ar]3d⁵ 4s¹ (see note below)

Electron configuration of Cr(III) ion: [Ar]3d³

Note: You would normally want to write [Ar]3d⁴ 4s² but half full and full subshells are more stable, so an electron from the 4s² jumps to the 3d to make both the 3d and 4s subshells half-full.

Looking at neutral iron (no charge), we have atomic number 26 and it is in period 4, group 8. It is a d-block transition metal and so the electron configuration is [Ar] 3d⁶ 4s². Now, to become Fe3+, it has to lose 3 electrons and those will be the outermost, so it will lose the 4s² and one electron from 3d. The resulting electron configuration for Fe(III) ion is [Ar]3d⁵