Background, Occurrence, & Isolation
Aluminum, the most abundant metal in the earth's crust, is isolated on an enormous scale from the mineral bauxite. Aluminum is very easily oxidized, but it resists excessive corrosion due to the formation of a tough oxide coating. If this coating is removed, then the metal is quite reactive (it will even react with water). Although aluminum is a metal, its compounds involve primarily covalent bonding. Compounds containing group 13 elements often behave as Lewis acids.
The post transition elements Ga, In, and Tl exhibit two stable oxidation states, +1 and +3. The +3 state is more stable for Ga and In, but the +1 state is more stable for Tl. The tendency for post transition elements to exhibit two stable oxidation states, one equal to the units digit of the group number and the other two less than that, is due to the inert pair effect. The inert pair effect is due to a relativistic effect which contracts the valence s orbital thereby lowering its energy (For an interesting disscusion of this relativistic effect, see the article Why Is Mercury Liquid, Norby, L. J. J. Chem. Educ. 1991, 68, 110).
There are two notable deviations from the normal trend for atomic size progressing down this group. At first, we might expect that an atom of Ga would be larger than an atom of Al based on their positions in group. Gallium, however, is considerably smaller than Al due to a phenomenon known as the d-block contraction. Atoms get smaller as we progress from left to right across a given period, thus in period three Na is larger than Mg, Mg is larger than Al, Al is larger than Si, etc., until we arrive at Ar, the smallest atom in the row. The next element, K, is in period four and is, as expected, much larger than any period three atom. Again, as we progress to the right atomic radius decreases due to increasing Zeff. It is in period four, however, that we first encounter the transition elements. Atomic radii decrease as we move from Sc to Ti to V, etc., as electrons are added to the d-subshell. By the time we completely fill the d-subshell and reach the p-block elements, the atomic radius has decreased so much that Ga is actually smaller than Al. Because of the d-block contraction, the post transition elements of period four are about the same size as their period three congeners. Similarly, the occurrence of the f-block elements between the period six s- and p-blocks causes the period six p-block elements to be about the same size as their period five congeners. This effect is called the lanthanide contraction, or f-block contraction.
The following pictures are from "Chemistry Comes Alive," Vol. 4, published by the Journal of Chemical Education Software. Click on a thumbnail for a larger picture.
TERMS/CONCEPTS TO KNOW
inert pair effect
d-block & lanthanide contraction