Transition metals exhibit many
different oxidation states, more than for any other class of elements.
For reasons that we shall see in the next section, ions in many
of these states are brightly colored, which indicates the electronic
absorption of energy in the visible spectrum. The name "chromium"
came from "chroma" for color. In principle, the maximum oxidation
number for a transition metal would be the total number of outer
s and d electrons, and this maximum indeed is found
from scandium (Sc) to manganese (Mn), with 3 to 7 outer electrons
Each d electron can go into
a separate orbital in these elements, so pairing of two electrons
with opposite spin in the same orbital is not necessary. Pairing
to fill an orbital must begin with iron (Fe), and this pairing effectively
removes both electrons from bonding activity. The maximum oxidation
number decreases in the progression from iron to zinc (Zn) at the
end of the first transition-metal series. The +2 and +3 states are
most common for these latter transition metals.