Which of the following transition metals in its ground state have unpaired electrons in an s-orbital?

Explanation

Right from the start, you can guess that the unknown element must be a d-block element, or a transition metal. That must be the case because in order to be able to accommodate six unpaired electrons you need to have access to the five d-orbitals transition metals have access to. As you know, each orbital can hold a maximum of two electrons of different spin. When degenerate atomic orbitals are filled, they follow Hund's Rule, which states that one electron is added to each degenerate orbital in a subshell before two electrons can be added to the same orbital in a subshell. This implies that you can have a maximum of five unpaired electrons, one in each of the five d-orbitals. You can have six unpaired electrons if you have five unpaired electrons in the 3d-orbitals and one unpaired electron in the 4s-orbital. The element that matches this electron configuration is chromium, CrCr: [Ar] 3d5 4s1