Transition metals in Row 4 favor lower oxidation numbers because
the atoms are smaller and their electrons are held more tightly.
The elements in Rows 5 and 6 show more of the higher oxidation states
because their larger atomic sizes loosens their grip on the outer
d and s electrons. As an example, Fe usually is found
in the +2 (ferrous) and +3 (ferric) oxidation states. Osmium, two
rows below it, has oxidation states up to +8, as in osmium tetroxide.
The elements are more metallic in their lower oxidation states,
and their compounds are more ionic. Compounds in higher oxidation
states are more covalent, with electrons being shared rather than
lost entirely. Osmiurn tetroxide is a covalently bonded, volatile,
tetrahedral molecule Of .
(Osmiurn in this high oxidation state is so unstable that even weakly
reducing organic molecules can reduce it to a black precipitate
of lower-oxidation-state compounds and the metal. This, plus the
volatility Of ,
makes it useful as a tissue stain in electron microscopy. The vapor
is potentially dangerous because it stains human tissue equally
well, especially the eyes.)
Metallic and covalent radii of the elements in Rows 4-6 are shown
above. If you cut out the transition metals from this plot and pushed
calcium (Ca) and gallium (Ga) together, the size profile would not
look radically different from that encountered in Rows 2 and 3;
this is what we did in the radius drawings at the beginning of the
chapter. Within the transition metals, shrinkage is unusually rapid
from Sc to Mn, for which electrons are being added one at a time
into empty d orbitals without pairing. The attractive charge
on the nucleus increases steadily for these elements, yet the additional
electrons are being placed in half-buried d orbitals where
they have little compensating effect in increasing the atomic size.
Only when electrons have to be paired with others, from Fe to Zn,
does the repulsion between electrons in filled orbitals begin to
increase the size of the atoms and bring the transitionmetal radii
back up to the main track at Ga and In.