Overview
Alkaline earth metals constitute group 2 of the periodic table, comprising six elements: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These elements share similar electronic configurations, with two valence electrons in their outermost shell ([noble gas] ns²). The term "alkaline earth" originates from their oxides, known as "earths" in historical chemistry, which form strongly basic (alkaline) solutions when dissolved in water. All group 2 elements exhibit a +2 oxidation state in compounds, reflecting their tendency to lose two electrons to achieve a stable noble gas configuration. Radium, the heaviest and most radioactive of the group, is rarely encountered in bulk due to its short half-life and high reactivity.
Physical Properties
Alkaline earth metals are lustrous, malleable, and generally silvery-white in appearance, though they tarnish upon exposure to air. Their atomic radii increase down the group, correlating with a decrease in melting and boiling points. Beryllium, the lightest and hardest member, has a high melting point (1287 °C) due to strong metallic bonding, while radium, the heaviest, melts at 700 °C. Density values also rise down the group, with beryllium (1.85 g/cm³) being significantly less dense than barium (3.51 g/cm³) or radium. These metals are poor conductors of heat and electricity compared to transition metals but exhibit moderate thermal and electrical conductivity. Magnesium, in particular, is known for its low density and high strength-to-weight ratio, making it valuable in engineering applications.
Chemical Properties
Alkaline earth metals are highly reactive, though less so than alkali metals (group 1). Reactivity increases with atomic size, driven by decreasing ionization energy and increasing atomic radius. Beryllium is an exception, showing minimal reactivity due to a protective oxide layer and higher ionization energy. All group 2 elements react with oxygen to form oxides (e.g., MgO, CaO), though beryllium oxide has amphoteric character, dissolving in both acids and strong bases. Reactions with water vary: magnesium reacts slowly with cold water and vigorously with steam, while calcium, strontium, and barium react exothermically with cold water to produce hydrogen gas and metal hydroxides. Radium reacts violently with water, producing radium hydroxide and hydrogen.
These metals react with halogens to form metal halides (e.g., MgCl₂, CaF₂) and with acids to generate salts and hydrogen gas. For example, calcium reacts with hydrochloric acid to yield calcium chloride, water, and hydrogen. Their hydroxides (e.g., Ca(OH)₂, Ba(OH)₂) are strong bases, used in industrial processes like cement production. The solubility of group 2 compounds generally decreases with increasing ionic size, except for sulfates and hydroxides, which show increased solubility for heavier members.
Occurrence and Production
Alkaline earth metals are never found in elemental form in nature; they occur in various minerals and ores. Beryllium is present in beryl (Be₃Al₂Si₆O₁₈) and bertrandite (Be₄Si₂O₇(OH)₂). Magnesium is abundant in minerals like magnesite (MgCO₃), dolomite (CaMg(CO₃)₂), and seawater, from which it is extracted via electrolysis. Calcium is prevalent in limestone (CaCO₃), gypsum (CaSO₄·2H₂O), and fluorite (CaF₂). Strontium is mined from celestite (SrSO₄), while barium is derived from barite (BaSO₄). Radium, a decay product of uranium, is rare and typically isolated from uranium ores via fractional crystallization.
Production methods vary by element. Magnesium is produced through the Pidgeon process (thermal reduction of MgO with