Many electron configurations of elements are simple and straightforward. We can look at the outer shell and easily understand how that set of elements will react in terms of electron gain or loss.
However, there are sets of elements that are more complex in their behavior. One such group is called the transition elements.
Transition elements are the elements that are found in Groups 3-12 (old groups IIA-IIB) on the periodic table (salmon-colored block in the middle of the table). The term refers to the fact that the d sublevel, which is in the process of being filled, is in a lower principal energy level than the s sublevel filled before it.
Compounds of many transition elements are distinctive for being widely and vividly colored. As visible light passes through a transition metal compound dissolved in water, the d-orbitals absorb light of various energies.
The transition elements are found in groups IIIA-IIB (new groups 3-12).
These elements are characterized by having unfilled d sublevels.
In general, the next higher s sublevel is already filled or has one electron missing.
Many transition element compounds are brightly colored due to the inner-level d electron transitions.