common transition metals can be thought of in four classes: the
pre-iron elements in Groups IIIB through VIIB, the iron triad Fe,
Co, Ni, and the elements below them in Group VIIIB, the coinage
metals (Cu, Ag, Au), and the zinc triad in Group IIB (Zn, Cd, Hg).
The pre-iron metals all are tough, hard, and high-melting. They
are used mainly as catalysts for chemical reactions in industry,
and as additives to iron to make steels with special hardness, elasticity,
strength at high temperatures, and corrosion resistance. The iron
triad share the unusual property of being magnetic. They also begin
a downward trend in melting point and hardness, and an upward trend
in size, caused by the pairing of electrons in d orbitals.
This trend continues with the soft and malleable coinage metals.