HW 2A.9 part (a)

[Charysa Santos 4G]

Why would the M^2+ metal for [Ar]3d^7 be Co^2+? Shouldn't it be Cu^2+ since the metal in question supposedly lost 2 electrons and now has the electron configuration [Ar]3d^7?

[Charysa Santos 4G]

And also, can someone explain why this also applies to part (b), and why it is Fe^2+?

[Alex Tchekanov Dis 2k]

This is because the electron configuration for neutral Co is [Ar]3d74s2 and when you take off electrons, you take them from the 4s level instead of the 3d because the 4s is technically in a lower energy state. For these problems, it always helps to write the neutral configuration first and then take off electrons.

[Charysa Santos 4G]

Ah, so because it's asking for a +2 metal, the ion's electron configuration for the d orbital is still the same as the neutral atom's because you're taking the 2 electrons from the s orbital... Understood!