Alkaline Earth Metals
Alkaline earth metals are the six elements forming Group IIa in the Periodic Table: beryllium (Be), magnesium (Mg), Calcium (Ca), Barium (Ba), Strontium (Sr), and Radium (Ra)✶. Their oxides are basic (alkaline), especially when combined with water. "Earth" is a historical term applied to nonmetallic substances that are insoluble in water and stable to heating, and also the properties of the oxides. Hence, the term "alkali earths" is often used to describe these elements.
✶ See Periodic Table in the For Your Reference section of Volume 1.
Each metal has the electron configuration of an inert (noble) gas plus two electrons in the next higher s orbital. Thus, Mg is 1 s 2 2 s 2 2 p 6 3 s 2 or alternatively (Ne)3 s 2 . The bonds of most compounds of alkali earths are ionic in nature because these outermost electrons are readily lost, forming stable divalent cations. Mg, however, can form compounds with both ionic and covalent bonds , whereas most compounds of Be are covalent. The heavier alkali earths are sometimes compared to Group IIb elements (zinc [Zn], cadmium [Cd], mercury [Hg]) that also have a filled s orbital (5 s 2 ), but the filled 4 d 10 orbitals and higher ionization energies of the latter make compounds of Group IIb elements markedly less ionic in character than those of alkali earths.
Mg and Ca are the eighth and sixth most abundant elements in Earth's crust at 2.5 and 3.6 percent, respectively. Be, Sr, and Ba comprise 0.001, 0.025, and 0.05 percent, respectively. Ra is radioactive, and since its longest-lived isotope 226 Ra has a half-life of 1,600 years, there is very little Ra in Earth's crust. It is nonetheless present because 226 Ra is continuously formed by the decay of uranium ( 238 U). Alkali earth elements are very reactive and strongly reducing in character; thus, none occurs in a free state in the environment. They readily react with oxygen, and the pure metals tarnish in air, forming a surface layer of the oxide. The metals are soluble in liquid ammonia, forming covalent compounds with the general formula M(NH 3 ) 6 . These solutions are strongly basic and frequently find application in industry.
Oxides of alkali earths were known in ancient times, calcium oxide being lime (from the Latin word calx ). Magnesium oxide or magnesia was also known, its name probably deriving from a district in Asia Minor. Oxides of the other alkali earths were identified in the eighteenth century. Barium oxide or baryta was found in the mineral called heavy spar and given the name barys (from the Greek, meaning "heavy"). Strontia or strontianite (strontium carbonate) was found in a lead mine at Strontian in Scotland. Beryllium oxide was extracted from the mineral beryl (from the Greek word bèryllos ). Be was originally called glucina (from the Greek glykys , meaning "sweet") because of its taste and is sometimes still referred to as glucinum in France.
The English chemist Sir Humphry Davy first isolated Mg, Ca, Sr, and Ba in 1808 by means of electrolysis. (Mg was originally called magnium since Davy had already applied the word "magnesium" to the element manganese.) Be was initially isolated from beryl by the French chemist Antoine Bussy and independently in Germany by Friedrich Wöhler in 1828. The discovery of Ra did not occur until 1898 when Marie and Pierre Curie purified it from barium using its radioactivity. They named it from the Latin word radius (meaning "ray") because the strength of its radioactivity was more than a million times that of uranium.
Because of their metallic properties and low mass, Be and Mg are used to form lightweight alloys for structural purposes. Ca sees less industrial use, although the phosphate is sometimes utilized in fertilizers. Sr and Ba have no significant industrial applications. Both Be and Ra are used in various devices, the former because it is quite transparent to x-rays and the latter because it is a ready source of both α - and γ -radiation. Mg and Ca are essential to all living systems for many reasons; the other alkali earths have no known biological roles.
Michael E. Maguire
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Winter, Mark (2003). WebElements Periodic Table, Scholar Edition. WebElementsLtd. Additional information available from http://www.webelements.com .