76 | The History and Use of Our Earth’s Chemical Elements The radioactive isotope calcium-45 is deposited in bones and teeth as well as other plant

76 | The History and Use of Our Earth’s Chemical Elements The radioactive isotope calcium-45 is deposited in bones and teeth as well as other plant

and animal tissues. Because our bodies cannot distinguish between Ca-45 and the stable Ca-

40, the radioactive isotope Ca-45 is used as a tracer to study diseased bone and tissue. At the same time, a massive overexposure to Ca-45 can displace the stable form of Ca-40 in animals and can cause radiation sickness or even death.

A few calcium compounds, when in powder or vapor form, are toxic when ingested or

inhaled.

STRONTIUM SYMBOL:฀Sr฀ PERIOD:฀5฀ GROUP:฀2฀(IIA)฀ ATOMIC฀NO:฀38

ATOMIC฀MASS:฀87.62฀amu฀ VALENCE:฀2฀ OXIDATION฀STATE:฀+2฀ ฀ NATURAL฀STATE:฀Solid ORIGIN฀OF฀NAME:฀Strontium฀was฀named฀after฀the฀town฀Strontian,฀located฀in฀Scotland฀in฀ the฀British฀Isles. ISOTOPES:฀There฀are฀29฀isotopes฀of฀strontium,฀ranging฀from฀Sr-75฀to฀Sr-102.฀The฀four฀ natural฀forms฀of฀strontium฀are฀stable฀and฀not฀radioactive.฀These฀stable฀isotopes฀are฀Sr- 84,฀which฀constitutes฀0.56%฀of฀the฀element’s฀existence฀on฀Earth;฀Sr-86,฀which฀makes฀ up฀9.86%;฀Sr-87,฀which฀accounts฀for฀7.00%฀of฀the฀total;฀and฀Sr-88,฀which฀makes฀up฀ 82.58%฀of฀strontium฀found฀on฀Earth.฀The฀remaining฀isotopes฀are฀radioactive฀with฀half- lives฀ranging฀from฀a฀few฀microseconds฀to฀minutes,฀hours,฀days,฀or฀years.฀Most,฀but฀not฀ all,฀are฀produced฀in฀nuclear฀reactors฀or฀nuclear฀explosions.฀Two฀important฀radioisotopes฀ are฀Sr-89฀and฀Sr-90.

ELECTRON฀CONFIGURATION ฀ Energy฀Levels/Shells/Electrons฀ Orbitals/Electrons

s2,฀p6

฀ 3-M฀=฀18฀

s2,฀p6,฀d10

฀ 4-N฀=฀8฀

s2,฀p6

฀ 5-O฀=฀2฀

s2

Properties In its elemental state, strontium is a relatively soft, pale yellow metal somewhat similar to

elemental calcium. When freshly cut, strontium has a silvery shine to its surface that soon turns grayish as it is oxidized by atmospheric oxygen (2Sr + O 2 → 2SrO) and nitrogen (3Sr +

Guide to the Elements | 77 N 2 → Sr 3 N 2 ), which prevents further oxidation. Strontium’s melting point is 769°C, its boil-

ing point is 1348°C, and its density is 2.54 g/cm 3 . Characteristics

When strontium metal is exposed to water, it releases hydrogen, as do the other earth met- als (Sr + 2H 2 O → Sr(OH) 2 +H 2 ↑). Strontium can ignite when heated above its melting point. When in a fine powder form, it will burn spontaneously in air. It must be stored in an inert atmosphere or in naphtha. Several of its salts burn with a bright red flame, making it useful in signal flares and fireworks.

Abundance฀and฀Source Strontium metal is not found in its elemental state in nature. Its salts and oxide compounds

constitute only 0.025% of the Earth’s crust. Strontium is found in Mexico and Spain in the min- eral ores of strontianite (SrCO 3 ) and celestite (SrSO 4 ). As these ores are treated with hydrochloric acid (HCl), they produce strontium chloride (SrCl 2 ) that is then used, along with potassium chloride (KCl), to form a eutectic mixture to reduce the melting point of the SrCl 2 , as a molten electrolyte in a graphite dish-shaped electrolysis apparatus. This process produces Sr cations col- lected at the cathode, where they acquire electrons to form strontium metal. At the same time, Cl - anions give up electrons at the anode and are released as chlorine gas Cl

2 ↑. Two other methods of producing strontium are by thermal reduction of strontium oxide and by the distillation of strontium in a vacuum.

History In 1787 William Cruikshank (1745–1795) isolated, but did not identify, strontium from

the mineral strontianite he examined. In 1790 Dr. Adair Crawford (1748–1794), an Irish chemist, discovered strontium by accident as he was examining barium chloride. He found a substance other than what he expected and considered it a new mineral. He named the new element “strontium” and its mineral “strontianite” after a village in Scotland. In 1808 Sir Humphry Davy treated the ore with hydrochloric acid, which produced strontium chloride. He then mixed mercury oxide with the strontium chloride to form an amalgam alloy of the two metals that collected at the cathode of his electrolysis apparatus. He heated the resulting substance to vaporize the mercury, leaving the strontium metal as a deposit.

Common฀Uses Strontium does not have as many practical uses as do some of the other alkali earth metals.

Strontium nitrate [Sr(NO 3 ) 2 ], when burned, produces a bright red flame, and it is used in fireworks. During military combat, it is used to make “tracer bullets” so that their paths can be tracked at night. Strontium is also used in making specialty metals when alloyed with other metals and in the manufacture of soaps, greases, and similar materials that are resistant to extreme high or low temperatures.

Examples฀of฀Compounds Strontium-90, a radioactive strontium isotope with a half-life of 29 years, is a dangerous

fallout source of radiation from atmospheric nuclear bombs. If a person is exposed to it, it will rapidly accumulate in bone tissue and interfere with the production of new red blood cells