Which of these has the electron configuration [Ar]3d4?

Question

Mn+2

Cr+2

V+3

Fe+3

K+

Answer 1

Cr+2. Cr has a normal electron configuration of [Ar]3d54s1. To give it the +2 charge you'd remove two electrons, first getting rid of the 4s1, then bringing the 3d5 to 3d4.

Answer 2

Cr+2, because the original config. of Cr = [Ar]3d4 4s2, it has lost its two outer electrons from the 4 s-orbital (hence +2 charge), leaving electron config. [Ar]3d4