Which subshell is filled in transition metals? What is unique about its order of filling?

A subshell is a set of orbitals with the same principal quantum number (n) and the same azimuthal quantum number (l). Subshells are denoted by a combination of the principal quantum number (n) and a lowercase letter, which represents the value of l (s, p, d, f). The s subshell has l = 0, the p subshell has l = 1, the d subshell has l = 2, and the f subshell has l = 3.

Transition metals are a group of elements found in the d-block of the periodic table. They are known for their ability to form various coordination compounds and have variable oxidation states. These elements have partially filled d orbitals in their ground state or in their stable oxidation states.

In transition metals, the d subshell is being filled. As we move across a period in the periodic table, we see that the elements in the d-block have a partially filled d subshell.

The order of filling in transition metals is unique due to the energy levels of the orbitals. According to the Aufbau principle, electrons fill the lowest energy orbitals first. However, the energy levels of orbitals are not constant and can change as electrons are added to an atom. In transition metals, the energy difference between the d and s orbitals becomes smaller, and the 4s orbital is filled before the 3d orbital. This unique order of filling leads to the formation of a partially filled d subshell in transition metals. For example, in Chromium (\(Z=24\)), the electron configuration is [Ar] 4s1 3d5 instead of the expected [Ar] 4s2 3d4. This is because having a half-filled d subshell provides additional stability. In summary, the d subshell is filled in transition metals, and the order of filling is unique due to the energy levels and stability of the orbitals.