Chemical Properties
Pale-yellow, soft metal; chemically similar
to calcium. Soluble
in alcohol and acids, decomposes water on contact.
Description
Strontium has the symbol Sr, the atomic number 38 and an atomic weight of 87.623 g/mol. As an alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. Due to its extreme reactivity with oxygen and water, this element occurs naturally only in compounds with other elements. The metal turns yellow when exposed to air. It occurs naturally in the minerals Celestine(SrSO4) and strontianite(SrCO3). The isotope, 90Sr, is present in radioactive fallout and has a half-life of 28.90 years. The following table presents the abundance of strontium. Strontium commonly occurs in nature, the 15th most abundant element on earth, averaging 0.034% in all igneous rock. It is found chiefly as the form of the sulfate mineral Celestite(SrSO4) and the carbonate Strontianite (SrCO3). Of the two, Celestite occurs much more frequently in sedimentary deposits of sufficient size to make the development of mining facilities attractive. Strontianite is more useful of the two common minerals because strontium is used most often in the carbonate form, but few deposits have been discovered that are suitable for development.
Physical properties
In its elemental state, strontium is a relatively soft, pale yellow metal somewhat similar toelemental calcium. When freshly cut, strontium has a silvery shine to its surface that soonturns grayish as it is oxidized by atmospheric oxygen (2Sr + O2 → 2SrO) and nitrogen (3Sr +N2 → Sr3N2), which prevents further oxidation. Strontium’s melting point is 769°C, its boiling point is 1348°C, and its density is 2.54 g/cm3.
Isotopes
Strontium has four naturally occurring isotopes 84Sr
(0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%), but
there are 33 known isotopes (Tables 1.14 and 1.15).
This element (Sr) has four stable, naturally occurring
isotopes: 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr
(82.58%). Only 87Sr is radiogenic since it is produced
by decay from the radioactive alkali metal 87Rb, which
has a half-life of 4.88×1010 years. Thus, in any material,
there are two sources of 87Sr. That formed during
primordial nucleosynthesis along with 84Sr, 86Sr and
88Sr, and that formed by radioactive decay of 87Rb. The
ratio 87Sr/86Sr is the parameter typically reported in
geologic investigations. The ratios reported in minerals
and rocks have values ranging from 0.7 to greater than
4.0. Because strontium has an electronic configuration
similar to that of calcium, it readily substitutes for Ca
in minerals.
Sixteen unstable isotopes are known to exist. Of greatest
importance are strontium-89 (89Sr) with a half-life of
50.57 days, and strontium-90 (90Sr) with a half-life of
28.78 years. They decay by emitting an electron and an
anti-neutrino (ne) in beta-minus decay (b
decay) to
become yttrium, 90Y (half-life ? 64 h). 89Sr is an artificial
radioisotope that is used in the treatment of bone cancer.
In circumstances where cancer patients have widespread
and painful bony metastases, the administration
of 89Sr results in the delivery of b-particles directly to the
area of the bony problem, where calcium turnover is
greatest. 90Sr is a by-product of nuclear fission found
in “nuclear fallout” and presents a health problem since
it substitutes for calcium in bone, preventing its expulsion
from the body. Significant absorption usually
results in death.
Because it is a long-lived high-energy beta-emitter,
90Sr is used in SNAP (Systems for Nuclear Auxiliary
Power) devices. These devices hold promise for use in
spacecraft, remote weather stations, navigational buoys,
etc., where a lightweight, long-lived, nuclear-electric
power source is required.
Isotopes
There are 29 isotopes of strontium, ranging from Sr-75 to Sr-102. The fournatural 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 makesup 9.86%; Sr-87, which accounts for 7.00% of the total; and Sr-88, which makes up82.58% of strontium found on Earth. The remaining isotopes are radioactive with halflives ranging from a few microseconds to minutes, hours, days, or years. Most, but notall, are produced in nuclear reactors or nuclear explosions. Two important radioisotopesare Sr-89 and Sr-90.
Origin of Name
Strontium was named after the town Strontian, located in Scotland in
the British Isles.
Occurrence
Strontium metal is not found in its elemental state in nature. Its salts and oxide compoundsconstitute only 0.025% of the Earth’s crust. Strontium is found in Mexico and Spain in the mineral ores of strontianite (SrCO3) and celestite (SrSO4). As these ores are treated with hydrochloricacid (HCl), they produce strontium chloride (SrCl2) that is then used, along with potassiumchloride (KCl), to form a eutectic mixture to reduce the melting point of the SrCl2, as a moltenelectrolyte in a graphite dish-shaped electrolysis apparatus. This process produces Sr cations collected 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 Cl2↑.Two other methods of producing strontium are by thermal reduction of strontium oxideand by the distillation of strontium in a vacuum.
Characteristics
When strontium metal is exposed to water, it releases hydrogen, as do the other earth metals (Sr + 2H2O → Sr(OH)2 + H2↑). 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 inertatmosphere or in naphtha. Several of its salts burn with a bright red flame, making it usefulin signal flares and fireworks.
Flammability and Explosibility
Substances and mixtureswhichincontactwithwateremitflammablegases
Carcinogenicity
The carcinogenicity of stable
(nonradioactive) strontium chromate was attributed solely
to intracellular soluble chromium. 90Sr has been examined
in long-term studies in four species, involving beagles,
mice, monkeys, and pigs. A summary of the findings of these
studies can be found in Ref.. Following intravenous
injection of 90Sr at doses ranging from 0.027 to
362×104 Bq/kg, the most prominent 90Sr -induced endpoint
was bone sarcoma. Neoplasms involving the soft tissues near
bone in the oronasopharynx and paranasal sinuses and bone
marrow dysplasia were also significantly elevated over controls.
Feeding studies in beagles extending from the in utero
period to age 540 days resulted in the development of the
same array of tumors, and, in addition, myeloproliferative
disorders. Inhalation exposure to 90Sr Cl2 was associated with multiple carcinogenic and non-neoplastic lesions in
dogs, with an excess of bone tumors reported as the major
finding. Interestingly, inhalation exposure of dogs to insoluble
forms of 90Sr was associated with lung tumors, but not bone
tumors. In an additional study in which beagle dogs were
injected with lowlevels of 90Sr (21.1 kBq/kg, or five times the
maximum permissible (retained) body burden), 90Sr was not
associated with a decrease in survival time in dogs. It has
been estimated that 90Sr is approximately two orders of
magnitude less toxic than radium. Two monkey studies
were also summarized by the Council on Radiation Protection
and Measurements. One of these studies involved
administration of single intravenous injections of 90Sr
(0.10–6.21 MBq) to rhesus monkeys. These monkeys had
no symptoms or disease attributable to 90Sr 20 years after
exposure. In another study, administration of 1.85 or 3.7 MBq
of 90Sr as a single oral dose resulted in bone sarcomas.
Environmental Fate
Most stable strontium and some radioactive strontium
compounds exist as dust in air, which eventually settles over
land and water. Stable strontium dissolves in water and moves
deeper in soil to underground water. The solid is found suspended
in water. Strontium is also found naturally in soil due
to the release of coal ash and industrial wastes. Soluble strontium
compounds, through chemical reactions, can transform to
insoluble and vice versa. The long half-life of strontium-90 (29
years) can allow airborne particles to move all around the
world.
Toxicity evaluation
Strontium’s inherent toxicity and that of its compounds
resembles that of calcium. The state of calcium nutrition of
exposed individuals is a major determinant of toxicity. The
radioactive isotope, when ingested or inhaled, is processed by
the body and resides in bones. Strontium ionizes molecules in
the body by the emission of beta particles. It increases the risk
of cancer.
