We’re being asked to determine the element of a +2 cation with an electron configuration of [Ar]3d9
Let’s determine the chemical symbol this cation is isoelectronic with.
Isoelectronic means that two species have the same number of electrons.
The cation has an electronic configuration of [Ar]3d9:
What element has a +2 cation with the electron configuration [Ar]3d9?