Transition metals are located in the d-block of the periodic table of elements. There are a total of 38 transition metals and they are present in groups 3 to 12, located in the center of the periodic table. Transition metals (including iron) show multiple valence states or oxidation states. The reason for this is the presence of a d-orbital, which is absent from alkali and alkaline earth metals. Transition elements have d orbitals in addition to s orbitals. These orbitals are very close in energy to each other. The atom has the option of losing electrons from both the s and d orbitals. For example, in the case of iron, there are two electrons in the 4s orbital and 6 electrons in the 3d orbital. An iron atom could lose the two electrons from the 4s orbital and show +2 valency or lose an additional electron from the 3d orbital (and have higher stability due to five 3d orbitals, each containing 1 electron) and exhibit a valence of +3.
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