This group of metals is distinguished from other metals not by their physical properties, but by their electronic structure. Transition metals are elements characterized by a partially filled subshell. The First Transition Series comprises scandium (Sc), titanium (Ti), vanadium chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni) and copper (Cu). The Second and Third Transition Series include the lanthanides and actinides, respectively. The transition metals are noted for their variability in oxidation state. Thus, manganese has two electrons in its outside shell and five electrons in the next shell down, and exhibits oxidation states of +1, +2, +3, +4, +5, +6, and +7. They are also characterized by the fact that well into the series, going from left to right, the properties of succeeding metals do not differ greatly from preceding ones.