Chapter 2, Problem 2.22P

Provide explanations of the following phenomena:
a. The electron configuration of Cr is $\left[\text{Ar}\right]{\text{4s}}^{\text{1}}{\text{3d}}^{\text{5}}$ rather than $\left[\text{Ar}\right]{\text{4s}}^{\text{2}}{\text{3d}}^{\text{4}}$
b. The electron configuration of Ti is $\left[\text{Ar}\right]{\text{4s}}^{\text{1}}{\text{3d}}^{\text{2}}$ , but that of ${\text{Cr}}^{\text{2}+}$ is $\left[\text{Ar}\right]{\text{3d}}^{\text{4}}$ .