transition metals generally are hard, brittle, and have high melting
points. Hardness and melting point correlate well with the number
of impaired d electrons, as can be seen from the melting point curve
below. This correlation arises because bonding between atoms in
these metals is partially covalent. Potassium and calcium at the
beginning of the fourth row of the table lose all their outer electrons,
to become +1 and +2 ions in the metal. Their electrons are free
to wander through the solid. The first transition metal, scandium,
also loses all three of its outer electrons, but as the number of
electrons in the d orbitals increases in successive elements, complete
loss of electrons becomes more difficult. Chromium does not become
a ion completely
in the metal, but retains a partial hold on its outer-orbital electrons,
sharing them to an extent with neighboring ions. If we could watch
the electrons in a block of chromium, we might see them moving through
the metal, but spending a disproportionate amount of time in positions
that correspond to covalent bonds between adjacent Cr atoms. This
partial covalent behavior creates stronger attractions between atoms
in chromium and makes it harder, tougher, and higher melting than
potassium or calcium. In spite of this tendency toward covalency,
the transition metals are good conductors of electricity because
the outer-shell electrons are still mobile, and there are relatively
many of them per atom.
Electron-shell diagram for transition metals.
The s orbital is the outer shell, and the d orbitals from the preceeding
principle quantum number are buried more deeply.
The A-group representative elements in the periodic table are interrupted
to accommodate the transition metals (B group).