The transition metals in Row 5 are larger than those in Row 4,
as expected from their additional shell of electrons. But those
in Row 6 are almost identical in size to the corresponding ones
of Row 5, even though orbitals of higher principal quantum number
are being filled. This phenomenon is known as the lanthanide
contraction after lanthanum, the first metal in the series.
The insertion of the fourteen f-orbital elements causes an
increase in the nuclear charge of +14 by the time hafnium (Z = 72)
is reached, thereby pulling all the electrons closer in. In Group
VIB, molybdenum (Mo) has a nuclear charge +18 greater than chromium
(Cr), but tungsten (W), below Mo, has a +32 greater nuclear charge
than molybdenum. The atomic radii accordingly are Cr, 1. 18Å;
Mo, 1.30Å; and W, 1.30Å.
This near-identity in size between the second and third series of
transition metals causes them to have similar chemical properties,
which often are quite different than those of the metals above them
in the first series. Niobium (Nb) and tantalum (Ta) are almost identical
in behavior 3 and are less like vanadium (V). Ruthenium (Ru) and
osmium (0s) are similar, but are quite different from iron (Fe).
Iron, cobalt (Co), and nickel (Ni) in the first transition-metal
series have more properties in common than with the six metals below
Titanium (Ti), manganese, iron, and nickel make up nearly 2.5%
of the crust of the Earth, and the other transition metals are rarer.
All of the Row 4 transition metals except Sc, Ti, and Ni are essential
to living organisms, as is only one metal from later rows, molybdenum
(Mo). Iron is used along with a delocalized aromatic ring as the
oxygen-binding agent in hemoglobin, and as an oxidation-reduction
carrier in the cytochromes, where it accepts and gives up an electron
and fluctuates between the +2 and +3 oxidation states. Copper also
is found in oxidation-reduction proteins, and goes from the +1 to
the +2 state and back again. The other six essential transition
metals (V, Cr, Mn, Co, Zn, and Mo) generally are bound in the +2
state to enzymes, in which they push and pull electrons in other
molecules during catalysis.