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The element gallium was predicted, as eka-aluminum, by Mendeleev in 1870, and subsequently discovered by Lecoq de Boisbaudran in 1875; in fact de Boisbaudran had been searching for the missing element for some years, based on his own independent theory. The first experimental indication of gallium came with the observation of two new violet lines in the spark spectrum of a sample deposited on zinc. Within a month of these initial results de Boisbaudran had isolated 1 g of the metal starting from several hundred kilograms of crude zinc blende ore. The new element was named in honor of France (Latin Gallia ), and the striking similarity of its physical and chemical properties to those predicted by Mendeleev ( [link] ) did much to establish the general acceptance of the periodic Law; indeed, when de Boisbaudran first stated that the density of Ga was 4.7 g/cm 3 rather than the predicted 5.9 g/cm 3 , Mendeleev wrote to him suggesting that he redetermine the value (the correct value is 5.904 g/cm 3 ).
| Property | Mendeleev's prediction (1871) for eka-aluminum, M | Observed properties of gallium (discovered 1875) |
| Atomic weight | ca. 68 | 69.72 |
| Density, g.cm -3 | 5.9 | 5.904 |
| Melting point | Low | 29.78 |
| Vapor pressure | Non-volatile | 10 -3 mmHg, 1000 °C |
| Valence | 3 | 3 |
| Oxide | M 2 O 3 | Ga 2 O 3 |
| Density of oxide (g/cm 3 ) | 5.5 | 5.88 |
| Properties of metal | M should dissolve slowly in acids and alkalis and be stable in air | Ga metal dissolves slowly in acids and alkalis and is stable in air |
| Properties of hydroxide | M(OH) 3 should dissolve in both acids and alkalis | Ga(OH) 3 dissolves in both acids and alkalis |
| Properties of salts | M salts will tend to form basic salts; the sulfate should form alums; M 2 S 3 should be precipitated by H 2 S or (NH 4 ) 2 S; anhydrous MCl 3 should be more volatile than ZnCl 2 | Ga salts readily hydrolyze and form basic salts; alums are known; Ga 2 S 3 can be precipitated under special conditions by H 2 S or (NH 4 ) 2 S, anhydrous GaCl 3 is more volatile than ZnCl 2 . |
Gallium has a beautiful silvery blue appearance; it wets glass, porcelain, and most other surfaces (except quartz, graphite, and Teflon ® ) and forms a brilliant mirror when painted on to glass. The atomic radius and first ionization potential of gallium are almost identical with those of aluminum and the two elements frequently resemble each other in chemical properties. Both are amphoteric, but gallium is less electropositive as indicated by its lower electrode potential. Differences in the chemistry of the two elements can be related to the presence of a filled set of 3d orbitals in gallium.
Gallium is very much less abundant than aluminum and tends to occur at low concentrations in sulfide minerals rather than as oxides, although gallium is also found associated with aluminum in bauxite. The main source of gallium is as a by-product of aluminum refining. At 19 ppm of the earth's crust, gallium is about as abundant as nitrogen, lithium and lead; it is twice as abundant as boron (9 ppm), but is more difficult to extract due to the lack of any major gallium-containing ore. Gallium always occurs in association either with zinc or germanium, its neighbors in the periodic table, or with aluminum in the same group. Thus, the highest concentrations (0.1 - 1%) are in the rare mineral germanite (a complex sulfide of Zn, Cu, Ge, and As); concentrations in sphalerite (ZnS), bauxite, or coal, are a hundred-fold less.
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