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There are two major definitions of transition metal: one includes most d-block elements, but excludes group 12 metals (zinc, cadmium, mercury); the other includes all d-block elements. Transition metals are notable for their coordination chemistry, which includes numerous colorful complexes, and their rich redox chemistry.
What is a transition metal?
The simplest definition of transition metal includes the entirety of the d-block. However, lutetium and lawrencium, which are considered lanthanides and actinides respectively, though they both belong in the d-block due to their s2f14d1 electron configurations, which contain a completely filled f-shell that does not participate in reactions. Sometimes lanthanum and actinium are grouped as transition metals because their electron configurations are s2d1, resembling scandium and yttrium. However, the expected electron configuration, according to the Aufbau principle, is s2f1. Other definitions include all of the lanthanides with the rest of the transition metals, grouping them as inner transition metals.
Whether zinc, cadmium, and mercury are transition metals is up for debate. None of these elements lose d electrons (save for mercury, but this has only been observed under extreme conditions). However, they do tend to show some properties that resemble transition metals, such as a willingness to coordinate to ligands.