Transition Elements (A2)

What is a transition element?

A transition element is a d-block element that forms at least one stable ion with a partially filled d sub-shell. (Sc and Zn are d-block but not transition: $\text{Sc}^{3+}$ is $d^0$ and $\text{Zn}^{2+}$ is $d^{10}$.) Characteristic properties:

variable oxidation states (3d and 4s electrons are close in energy);
coloured ions and compounds;
form complex ions with ligands;
act as catalysts (heterogeneous and homogeneous).

Recall the anomalies: $\text{Cr} = [\text{Ar}]3d^5 4s^1$ and $\text{Cu} = [\text{Ar}]3d^{10}4s^1$ (extra stability of half/fully-filled d). On ionising, the 4s electrons are lost first.