Why do some parts of the transition metal block seem to violate our established relationship between the number of d orbital electrons and metallic bond strength? Use 2 groups of elements and describe their properties to support your explanation

The iron (II) ion has 24 electrons in this configuration: [Ar] 3d6 The neutral chromium atom also has 24 electrons, but in this configuration: [Ar] 3d5 4s1 I understand that empty, half, and full shells are preferred, but I don't under stand why two atoms with the same number of electrons would have different configurations. I would assume it must be due to the different number of protons, but I don't understand what the reason is.