A metal that melts in the hand

Gal­li­um is a sil­very-gray met­al with a bluish tinge, and is quite brit­tle. It is not en­coun­tered in na­ture in pure form and is a trace el­e­ment. The av­er­age con­tent of gal­li­um in the earth’s crust is 19 g/t. Gal­li­um is con­tained in min­er­als, pri­mar­i­ly spha­lerite, mag­netite, cas­si­terite, gar­net, beryl, tour­ma­line, spo­dumene, phl­o­go­pite, bi­otite, mus­covite, sericite, lep­i­do­lite, chlo­rite, feldspars, nephelite, hack­man­ite and na­tro­lite. Gal­lite, Cu­GaS₂ is a rather rare met­al that is used to ex­tract pure gal­li­um, Ad­di­tion­al­ly, gal­li­um can be ob­tained as a sec­ondary prod­uct in pro­cess­ing baux­ites.

Gal­li­um melts at just 29.76 °C, so it even melts in the hand. At around room tem­per­a­ture, an­oth­er three met­als also melt: mer­cury, cae­sium and ru­bid­i­um. But be­cause of their high tox­i­c­i­ty or re­ac­tiv­i­ty, un­like gal­li­um they must not be held in the hand.

How gal­li­um was dis­cov­ered

The ex­is­tence of gal­li­um was pre­dict­ed by D. I. Mendeleev in 1871 on the ba­sis of the Pe­ri­od­ic Law that he for­mu­lat­ed.

Mendeleev named this el­e­ment “ekaa­lu­minum” and pre­dict­ed such prop­er­ties as its den­si­ty and melt­ing point. Mendeleev also pre­dict­ed:

  • the na­ture of its ox­ide,
  • its bond in com­pounds with chlo­rine
  • that the met­al would slow­ly dis­solve in acids and al­ka­lis;
  • it would not re­act with air
  • eka-alu­minum ox­ide M₂O₃ should re­act with acids with the for­ma­tion of the salts MX₃;
  • that it should form base salts;
  • its chlo­ride would have greater volatil­i­ty than Zn­Cl₂;
  • that the el­e­ment would be dis­cov­ered by spec­tro­scope.

Mendeleev proved to be a Nos­tradamus of chem­istry: when gal­li­um was ob­tained, all the prop­er­ties that he had pre­dict­ed were con­firmed!

In 1875, the French chemist Paul-Émile Lecoq de Bois­bau­dran stud­ied spha­lerite us­ing spec­troscopy and found two pur­ple lines that be­longed to a new el­e­ment. A year lat­er the sci­en­tist ex­tract­ed a new el­e­ment us­ing elec­trol­y­sis. Bois­bau­dran named this el­e­ment af­ter the Latin name of France – Gal­lia. There is a leg­end that the sci­en­tist also placed an­oth­er mean­ing in this name. Lecoq sounds like le coq in French, i.e. “roost­er” (gal­lus in Latin). So as if by chance, Bois­bau­dran im­mor­tal­ized his name in the name of the new el­e­ment.

Study­ing the gal­li­um ob­tained, Bois­bau­dran de­ter­mined that the den­si­ty dif­fered from Mendeleev’s pre­dic­tions. When Mendeleev found out about this, he wrote to his French col­league, rec­om­mend­ing that he check his re­sults again. And as it turned out, he was quite right to do so: Bois­bau­dran’s ini­tial find­ings were in­deed in­cor­rect.

Field of ap­pli­ca­tion of gal­li­um

Most gal­li­um pro­duced is used for the man­u­fac­tur­er of semi­con­duc­tors. Ar­senide (GaAs) and gal­li­um ni­tride (GaN) are used in elec­tron­ic com­po­nents of many de­vices, to make in­te­grat­ed cir­cuits, high-per­for­mance pro­ces­sors and mi­crowave am­pli­fiers. Gal­li­um ar­senide is used in var­i­ous elec­troop­tic in­fra-red de­vices. Gal­li­um-alu­minum ar­senide is used to make in­fra-red laser diodes of high pow­er. On the ba­sis of gal­li­um ni­tride and in­di­um-gal­li­um ni­tride, blue and pur­ple laser diodes are man­u­fac­tured. In­ci­den­tal­ly, a laser with gal­li­um ni­tride is used in the Blu-ray disc drives.
Pho­toele­ments on the ba­sis of gal­li­um ar­senide and in­di­um-gal­li­um phos­phide and ar­senide are used in satel­lites and Mars rovers.

Gal­li­um has an in­ter­est­ing fea­ture: it great­ly re­duces the melt­ing point of al­loys in which it is con­tained. The tem­per­a­ture be­comes low­er than each com­po­nent of the al­loy in­di­vid­u­al­ly (eu­tec­tic prop­er­ties). The al­loy Gal­lis­tan (68.5% gal­li­um, 21.5% in­di­um and 10% tin) has a melt­ing point of -19°С and is used in some ther­mome­ters in­stead of mer­cury.

Gal­li­um is also used in medicine. In gen­er­al, the met­al is char­ac­ter­ized by a low tox­i­c­i­ty and does not per­form any nat­u­ral bi­o­log­i­cal func­tion. So medicine con­tain­ing gal­li­um can be used in treat­ing and di­ag­nos­ing can­cer­ous dis­eases (the iso­topes gal­li­um-67 and -68). Gal­li­um is also used in treat­ing sev­er­al bac­te­ri­al in­fec­tions: the ion Ga³⁺ dis­places Fe³⁺ on the meta­bol­ic path­ways of bac­te­ri­al res­pi­ra­tion, caus­ing the bac­te­ria to die. Medicine con­tain­ing gal­li­um can also be used to treat malar­ia. Gal­li­um helps to de­tect neu­tri­no par­ti­cles em­a­nat­ing from the sun. De­tect­ing these par­ti­cles is usu­al­ly a very com­plex and la­bo­ri­ous process. Gal­li­um in the reg­is­tra­tion mix­ture in­creas­es the sen­si­tiv­i­ty of the anal­y­sis, and ac­cord­ing­ly helps to de­tect neu­tri­nos. The GALLEX de­tec­tors at the Lab­o­ra­tori Nazion­ali del Gran-Sas­so con­tain 12.2 tons of gal­li­um-71. They de­tect neu­tri­nos em­anat­ed by the sun, and trans­form them into ra­dioac­tive iso­topes, the ra­di­a­tion of which can be record­ed. Sim­i­lar stud­ies are also car­ried out at the Bak­san neu­tri­no ob­ser­va­to­ry (Kabardi­no-Balka­ria), where the neu­tri­no de­tec­tors con­tain 5 tons of liq­uid gal­li­um.

Ther­mome­ters can be test­ed us­ing the melt­ing tem­per­a­ture of gal­li­um! This fig­ure - 302.9146 K (29.7646 °C) – is used as a stan­dard by the In­ter­na­tion­al Bu­reau of Weights and Mea­sures. In 2007, a fo­cused gal­li­um ion beam with a thick­ness of 7 nm was used at Si­mon Fras­er Uni­ver­si­ty to print the small­est book in the world – “Tee­ny Ted from Turnip Town”. The size of the book is 0.07 by 0.10 mm.

Gal­li­um has an­oth­er amus­ing use: gal­li­um spoon, which look iden­ti­cal to alu­minum ones, are used to play a trick with a dis­ap­pear­ing spoon. In hot tea or cof­fee, this spoon sim­ply melts!