The alkaline earth elements

Isotopes

The naturally abundant isotopes of the alkaline earth elemets are listed in [link] . All of the 25 isotopes of radium are radioactive, and while radium-223, radium-224, and radium-228 are found in nature as decay products of either uranium or thorium, they are only present in trace amounts.

Although beryllium-7 and beryllium-10 are found as trace isotopes, they are so rare beryllium is considered mononuclidic element (a chemical element which is found essentially as a single nuclide, of only one atomic mass). Calcium has four stable isotopes plus two more isotopes (calcium-46 and calcium-48) that have such long half-lives (2.8 x 10 ¹⁵ and 4 x 10 ¹⁹ years, respectively) that for all practical purposes they can be considered stable.

Measurement of the ⁸⁷ Sr/ ⁸⁶ Sr ratio allows for geological dating of minerals and rocks. Strontium-90 (half-life = 28.9 years) is a by-product of nuclear fission and found in nuclear fallout. For example, the 1986 Chernobyl nuclear accident contaminated released a large amount of strontium-90. Since strontium substitutes for calcium in bones it prevents excretion from the body and thus presents a significant health risk, however, strontium-89 is a short-lived artificial radioisotope that is used in the treatment of bone cancer.

Naturally occurring barium is a mix of seven stable isotopes ( [link] ), but there are a total twenty-two isotopes known, most of which are highly radioactive and have half-lives in the several millisecond to several day range. The only notable exception is barium-133 which has a half-life of 10.51 years.

Industrial production

The industrial production of beryllium is usually from the reaction of beryl (Be ₃ Al ₂ Si ₆ O ₁₈ ) with Na ₂ (SiF ₆ ) which yields the beryllium fluoride, Na ₂ (BeF ₄ ). Subsequent reactions with base give the hydroxide.

Reaction of the hydroxide with the ammonium salt of HF ₂ ^- , followed by thermolysis gives beryllium fluoride (BeF ₂ ). Finally, reduction of the fluoride with magnesium yields beryllium.

Although magnesium is an abundant metal in dozens of mineral the majority of commercial production comes from sea water, where it is present at about a level of 12% that of sodium. Calcium hydroxide is added to seawater to form magnesium hydroxide precipitate.

Subsequent reaction with hydrochloric acid yields concentrated magnesium chloride solution.

Electrolysis of the magnesium chloride produces magnesium. At the cathode, the Mg ²⁺ ion is reduced to magnesium metal, [link] , while at the anode chlorine gas is formed, [link] .

Strontium metal is produced in an analogous manner; however, it may also be prepared from strontium oxide by reduction with aluminum in vacuum at a temperature at which strontium distills off.

Reactivity and toxicity

The chemistry of the Group 2 elements is dominated by the +2 oxidation state and the noble gas configuration of the M ²⁺ cation.

Calcium, strontium, and barium react with water on contact to produce the hydroxide and hydrogen gas. Although the lighter alkaline earth metals do not react violently with water, they do burn in air.

Magnesium burns with a very bright white flame such that caution should be taken not to look at the flame directly. Magnesium is capable of reducing water, [link] , and as a result, water cannot be used to extinguish magnesium fires; the hydrogen gas produced will only intensify the fire. In addition, magnesium also reacts with carbon dioxide, [link] , precluding the use of carbon dioxide fire extinguishers. Class D dry chemical fire extinguisher or sand are used for magnesium fires.

Strontium and barium burn in air to produce both the oxide and the nitride, but since the metals do not react with nitrogen except at high temperatures they only form the oxide spontaneously at room temperature.

Beryllium is a class 1 carcinogen, i.e., it is carcinogenic to both animals and humans. Beryllium is harmful if inhaled; if the concentration in air is high enough (greater than 100 µg/m ³ ) an acute condition can result, called acute beryllium disease, which resembles pneumonia. Acute beryllium disease was reported as being associated with the manufacture of fluorescent lighting tubes (a practice that ceased in 1949).

The human body absorbs strontium as if it were calcium, and while stable isotopes have pose no significant health threat, the uptake of radioactive strontium-90 can lead to various bone disorders and diseases, including bone cancer. All water or acid soluble barium compounds are extremely poisonous. At low doses, barium acts as a muscle stimulant, while higher doses affect the nervous system. Radium is highly radioactive and its decay product, radon gas, is also radioactive.