STRONTIUM: HISTORY, STRUCTURE, PROPERTIES, REACTIONS AND USES - CHEMISTRY - 2025

The Strontium is an alkaline earth metal whose chemical symbol is Mr. Freshly cut is white with a silvery sheen, but when exposed to air oxidizes and acquires a yellowish color. For this reason, it must be protected from oxygen during storage.

Strontium is extracted from its veins in the form of the minerals celestite or celestine (SrSO ₄) and strontianite (SrCO ₃). However, celestite is the main form in which strontium mining occurs, its deposits being in sedimentary soils and in association with sulfur.

Metallic strontium sample protected by an argon atmosphere. Source: Strontium unter Argon Schutzgas Atmosphäre.jpg Matthias Zepperderivative work: Materialscientist

Celestite occurs in the form of rhombic crystals, it is usually colorless, vitreous and transparent. Although strontium is extracted in this way, it must be transformed into its respective carbonate, from which it is finally reduced.

In 1790, strontium was identified as a new element by Adair Crawford and William Cruickshank, in a mineral from a lead mine near the town of Strontion in Argyll, Scotland. Strontium was isolated in 1807 by Humphry Davy, through the use of electrolysis.

Strontium is a malleable, ductile metal and a good conductor of electricity; but it has little industrial and commercial use. One of its applications is the formation of alloys with aluminum and magnesium, improving the handling and fluidity of these metals.

In the periodic table, strontium is located in group 2, between calcium and barium, finding that some of its physical properties, such as density, melting point and hardness, have intermediate values ​​in relation to those shown for calcium and barium.

Strontium occurs in nature as four stable isotopes: ⁸⁸ Sr with.82.6% abundance; the ⁸⁶ Sr, with 9.9% abundance; the ⁸⁷ Sr, with 7.0% abundance; and ⁸⁴ Sr, with 0.56% abundance.

⁹⁰ Sr is a radioactive isotope that constitutes the most harmful component of radioactive fallout, a product of nuclear explosions and leaks from nuclear reactors, since due to the similarity between calcium and strontium, the isotope is incorporated into bones, producing bone cancer and leukemia.

History

A mineral from a lead mine near Strontian village in Argyll, Scotland, was studied. It was originally identified as a type of barium carbonate. But Adair Crawford and William Cruickshank, in 1789, noted that the substance studied was another in question.

Chemist Thomas Charles Hope named the new mineral strontite, and the corresponding “earth” (strontium oxide, SrO) named it strontia.

In 1790, Crawford and Cruickshank burned the studied substance and observed that the flame was crimson-red in color, different from the flames observed up to that time in known elements. They concluded that they were in front of a new element.

In 1808, Sir William Humphry Davy, subjected to electrolysis to a humid mixture of hydroxide or chloride of strontium with oxide of mercury, using a cathode of mercury. Then, the mercury from the formed amalgam was evaporated, leaving the strontium free.

Davy named the isolated element strontium (strontium).

Structure and electron configuration of strontium

Metallic strontium crystallizes at room temperature into a face centered cubic (fcc) structure.

In this structure, the Sr atoms are located at the vertices and on the cube faces of the unit cell. It is relatively more dense than other structures (such as cubic or bcc) for having a total of four atoms of Mr.

The Sr atoms remain united thanks to the metallic bond, product of the overlapping of their atomic valence orbitals in all directions within the crystal. This orbital is the 5s, which has two electrons according to the electronic configuration:

5s ²

And thus, a full 5s band, and a 5p conduction band (band theory) originate.

Regarding other metallic phases, there is not much bibliographic information, although it is certain that their crystals undergo transformations when they are subjected to high pressures.

Oxidation numbers

Strontium, like other metals, has a high tendency to lose its valence electrons; These are the two electrons of the 5s orbital. Thus, the Sr atoms are converted into the divalent Sr ²⁺ cations (M ²⁺, like the rest of the alkaline earth metals), isoelectronic to the noble gas krypton. Strontium is then said to have an oxidation number of +2.

When instead of losing two electrons it loses only one, the Sr ⁺ cation is formed; and therefore, its oxidation number is +1. Sr ⁺ is rare in compounds derived from strontium.

Properties

Appearance

Silvery white with metallic sheen, with a slight yellow tint.

Molar mass

87.62 g / mol.

Melting point

777 ° C.

Boiling point

1,377 ° C.

Density

-Ambient temperature: 2.64 g / cm ³

-Liquid state (melting point): 2.375 g / cm ³

Solubility

Soluble in alcohol and acids. It is not soluble in water, as it reacts strongly with it.

Heat of fusion

7.43 kJ / mol.

Heat of vaporization

141 kJ / mol.

Thermal molar capacity

26.4 J / (mol · K).

Electronegativity

0.95 on the Pauling scale.

Ionization energy

First level of ionization: 549.5 kJ / mol.

Second ionization level: 1,064.2 kJ / mol.

Third level of ionization: 4,138 kJ / mol.

Atomic radio

Empirical 215 pm.

Covalent radius

195 ± 10 pm.

Thermal expansion

22.5 µm / (m · K) at 25 ° C.

Thermal conductivity

35.4 W / (mK).

Electrical resistivity

132 nΩ · m at 20 ° C.

Hardness

1.5 on the Mohs scale.

Fire potential

Strontium, when finely divided, burns spontaneously in air. Additionally, it ignites when heated above the melting point, and can be an explosion hazard when exposed to the heat of a flame.

Storage

To avoid oxidation of the strontium, it is recommended to store it immersed in kerosene or naphtha. Strontium should be stored in a cool, well-ventilated place, away from organic and other easily oxidized materials.

Nomenclature

As oxidation number +1 is not that common, it is assumed that only +2 exists for simplification of nomenclature around strontium compounds. That is why in the stock nomenclature the (II) at the end of the names is ignored; and in traditional nomenclature, they always end with the suffix -ico.

For example, SrO is strontium oxide or tin oxide, according to stock and traditional nomenclatures, respectively.

Shapes

Due to its great reactivity, metallic strontium does not appear isolated in nature. However, it can be found in its elemental state protected from oxygen, by immersion in kerosene or in an atmosphere of inert gases (such as noble gases).

It is also found forming alloys with aluminum and magnesium, as well as an aggregate to an alloy of tin and lead. Strontium is found in ionic form (Sr ²⁺) dissolved in soil or sea water, etc.

Therefore, to speak of strontium is to refer to the Sr ²⁺ cations (and to a lesser degree, Sr ⁺).

It can also interact in ionic form with other elements to form salts or other chemical compounds; such as strontium chloride, carbonate, sulfate, sulfide, etc.

Strontium is present mainly in two minerals: celestite or celestine (SrSO ₄) and strontite (SrCO ₃). Celestite is the main source of strontium mining extraction.

Strontium has 4 natural isotopes, of which the one found in greater abundance is ⁸⁸ Mr. Likewise, there are numerous radioactive isotopes, artificially produced in nuclear reactors.

Biological role

There is no known biological role for strontium in vertebrates. Due to its similarity to calcium, it can replace it in bone tissues; that is, Sr ²⁺ displaces Ca ²⁺. But the ratio found in bone between strontium and calcium is between 1 / 1,000 and 1 / 2,000; that is, extremely low.

Therefore, strontium must not fulfill a natural biological function in the bones.

Strontium ranelate has been used in the treatment of osteoporosis, since it causes a hardening of the bones; but in any case, this is a therapeutic action.

One of the few examples of a biological function of strontium occurs in Acantharea, a radiolarian protozoan that has a skeleton with the presence of strontium.

Where to find and production

Celestite crystal, a mineralogical source of strontium. Source: Aram Dulyan (User: Aramgutang)

Strontium is found in approximately 0.034% of all igneous rocks. However, only two minerals: celestite or celestine, are found in deposits with significant strontium content.

Of the two important minerals strontium, only celestite is found in sufficient quantity in sedimentary deposits to allow the creation of facilities to extract strontium.

Strationite is more useful than celestite, since most of the strontium is produced in the form of strontium carbonate; but hardly any deposits have been found that allow sustainable mining.

The strontium content in seawater ranges between 82 and 90 µmol / L, a much lower concentration than that of calcium, between 9.6 and 11 mmol / L.

Almost all mining is based on celestite deposits, since strontianite veins are scarce and not very profitable for the extraction of strontium from them. Despite this, most strontium is produced in the form of strontium carbonate.

Pidgeon method

Celestite is incinerated in the presence of coal to transform strontium sulfate into strontium sulfide. In the second stage, the dark material containing strontium sulfide is dissolved in water and filtered.

Then, the strontium sulfide solution is treated with carbon dioxide, to produce precipitation of the strontium carbonate.

Strontium can be isolated by a variant of the Pidgeon method. The reaction of strontium oxide and aluminum occurs in a vacuum, where the strontium is converted to gas and transported through the production retort to the condensers, where it precipitates as a solid.

Electrolysis

Strontium can be obtained in the form of rods by the contact cathode electrolysis method. In this procedure, a cooled rod of iron, acting as a cathode, comes into contact with the surface of a molten mixture of potassium chloride and strontium chloride.

As the strontium solidifies on the cathode (iron rod), the rod rises.

Reactions

With chalcogens and halogens

Strontium is an active reducing metal and reacts with halogens, oxygen, and sulfur to produce halides, oxides, and sulfur, respectively. Strontium is a silvery metal, but it oxidizes to strontium oxide when exposed to air:

Sr (s) + 1 / 2O ₂ (g) => SrO (s)

The oxide forms a dark layer on the surface of the metal. While its reaction with chlorine and sulfur are as follows:

Sr (s) + Cl ₂ (g) => SrCl ₂ (s)

Sr (s) + S (l) => SrS (s)

Strontium reacts with molten sulfur.

With the air

It can combine with oxygen to form strontium peroxide; but it requires a high pressure of oxygen for its formation. It can also react with nitrogen to produce strontium nitride:

3Sr (s) + N ₂ (g) => Sr ₃ N ₂ (s)

However, the temperature must be above 380 ° C for the reaction to occur.

With the water

Strontium can react violently with water to form strontium hydroxide, Sr (OH) _2, and hydrogen gas. The reaction between strontium and water does not have the violence observed in the reaction between alkali metals and water, as well as that observed in the case of barium.

With acids and hydrogen

Strontium can react with sulfuric acid and nitric acid to form, respectively, strontium sulfate and nitrate. It also combines hot with hydrogen to form strontium hydride.

Strontium, like other heavy elements in the s block of the periodic table, has a wide range of coordination numbers; such as 2, 3, 4, 22 and 24, seen in compounds like SrCd ₁₁ and SrZn ₁₃, for example.

Applications

- Elemental Strontium

Alloys

It is used as a eutectic modifier to improve the strength and ductility of Al-Ag alloy. It is used as an inoculant in ductile iron foundry to control the formation of graphite. It is also added to tin and lead alloys to add toughness and ductility.

In addition, it is used as a deoxidizer for copper and bronze. Small amount of strontium is added to molten aluminum to optimize the metal's meltability, making it more suitable for making objects that are traditionally made of steel.

It is an alloying agent for aluminum or magnesium that is used in the casting of engine blocks and wheels. Strontium improves the handling and fluidity of the metal to which it is alloyed.

Isotopes

Despite its damaging action, ⁹⁰ Sr is used as a thermoelectric generator, using the heat energy from its radiation to produce long-lasting electricity, with application in space vehicles, remote research stations and navigation buoys.

The ⁸⁹ Sr has been used in the treatment of bone cancer, using the radioactive emission β type for the destruction of tumor cells.

The strontium atom has been used to establish a system for measuring time, which barely lags one second every 200 million years. Which makes it the most accurate watch.

- Compounds

Carbonate

Ferrites and magnets

Strontium carbonate (SrCO ₃) reacts with ferric oxide (Fe ₂ O ₃) at a temperature between 1,000 and 1,300 ºC, to form a strontium ferrite. This family of ferrites has a general formula SrFe _x O ₄.

Ceramic magnets are made of ferrites and are used in various applications. Among them: manufacture of speakers, motors for automobile windshield wipers and in toys for children.

Strontium carbonate is also used in the production of glass for television screens and display units.

Glasses

In addition to improving the property of glass for liquid crystal displays (LCD), it is also used in glazing ceramic ware, reinforcing its resistance to scratching and bubble formation during firing.

It is used in the production of glass usable in optics, glassware and lighting. It is also part of fiberglass and laboratory and pharmaceutical glasses, since it increases hardness and resistance to scratching, as well as its brightness.

Production of metals and salts

It is used to obtain high purity zinc, as it contributes to the elimination of lead impurity. It helps in the production of strontium chromate, a compound that is used as a corrosion inhibitor in printing paints.

Waste water and phosphorescent lamps

It is used in the treatment of wastewater for the removal of sulfate. In addition, it is used in the production of orthophosphoric acid, used in the manufacture of fluorescent lamps.

Pyrotechnics

Strontium carbonate, like other strontium salts, is used in fireworks to give it a crimson red color. A stain that is also used in strontium testing.

Hydroxide

It is used in the extraction of sugar from beets, since strontium hydroxide combines with sugar to produce a complex saccharide. The complex can be dissociated by the action of carbon dioxide, leaving the sugar free. It is also used in the stabilization of plastic.

Oxide

It is present in the glass used in the manufacture of a television picture tube, beginning this application in 1970. Color televisions, as well as other devices that contain cathode rays, are required to use strontium in the front plate to stop X-rays.

These televisions are no longer in use, because cathode tubes have been replaced by other devices, and therefore the use of strontium compounds is not required.

On the other hand, strontium oxide is used to improve the quality of ceramic glazes.

Chloride

Strontium chloride is used in some toothpastes for sensitive teeth and in making fireworks. Furthermore, it is used in a limited way for the removal of unwanted gases in vacuum vessels.

Ranelate

It is used in the treatment of osteoporosis, as it increases bone density and reduces the incidence of fractures. Applied topically, it inhibits sensory irritation. However, its use has decreased due to the evidence that it increases the incidence of cardiovascular diseases.

Aluminate

It is used as a dopant in the electronics industry. It is also frequently used to make certain toys glow in the dark, as it is a chemically and biologically inert compound.

References

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