A characteristic of the transition metals is that they exhibit variable oxidation states in their compounds Figure 23.2 show the oxidation stated from scandium to copper. Note that the common oxidation states for each element include +2, +3, or both. The +3 oxidation states are more stable at the beginning of the series, whereas toward the end the +2 oxidation states are more stable. The reason for this trend can be understood by examining the ionization energy plots in Figure 23.3. In general, the ionization energies increase gradually from left to right. However, the third ionization energy (when an electron is removed from the 3d orbital) increases more rapidly than the fist and second ionization energy. Because it takes more energy to remove the third electron from the metals near the and of the row than from those near the beginning, the metals near the end tend to form M2+ ions rather than M3+ ions.