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.