Alkaline Earth Metals

FIELDS OF STUDY: Inorganic Chemistry; Geochemistry; Metallurgy

ABSTRACT

The basic properties and characteristics of the alkaline earth metal elements are presented. The alkaline earth metals constitute group 2 of the periodic table and are highly reactive, easily forming ions with a 2+ charge.

Characteristics of the Alkaline Earth Metals

The alkaline earth metals are the six elements in the second column, group 2, of the periodic table. Their atoms have two valence electrons in the outermost s orbital, an arrangement that accounts for the ease with which alkaline earth metals form cations with a 2+ charge, their relatively high reactivity, and their ability to form numerous salts. The alkaline earth metals typically form compounds in which they take on an oxidation state of +2, although the +1 state is also known to occur. Beryllium (Be) is the simplest member of group 2 and is the principal metal ion found in beryl minerals. The other alkaline earth metals are magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Radioisotopes of radium are known to occur naturally.

Atomic Structure of the Alkaline Earth Metals

The electronic structure of the alkaline earth metals is relatively simple. Their valence shells are composed of just the outermost s orbital, containing a single pair of electrons. Atoms seem to be the most stable when their electron shells are completely filled, with no extra electrons in higher orbitals. Accordingly, each alkaline earth metal is highly electropositive and easily donates its two valence electrons to form the corresponding cation, or positively charged ion (Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ra²⁺). This makes alkaline earth metal ions relatively comparable in size to alkali metal ions, but with twice the positive charge density. Because of their size and charge, calcium and magnesium are essential components of biochemical processes.

Alkaline Earth Metal Compounds

The alkaline earth metals commonly form either monatomic (single-atom) ions or electrically neutral coordination compounds in which a central alkaline earth metal atom is surrounded by neutral organic molecules or a cage-like neutral molecule. Alkaline earth metals most commonly bond to halogen anions (negatively charged ions), forming compounds called halides, or other monatomic or complex anions with a charge of 2−.

The alkaline earth oxides and hydroxides are important commercial minerals. Having two valence electrons enables alkaline earth metals to form long chain-like molecular structures in many of the minerals in which they occur. In theory, a corresponding anion balances each metal ion; structurally, however, each metal ion essentially shares its anions with the next metal ion in the chain. The anions thus act as bridges between the metal cations in the molecular structure of the mineral. This feature gives rise to a wide variety of mineral structures having the same chemical formula, some of which are classified as gemstones.

Occurrence of Alkaline Earth Metals

Alkaline earth metals are never found in their elemental form in nature, only as compounds or dissolved salts. They are normally found as ores and minerals in which they exist as various oxides, sulfides, sulfates, sulfites, silicates, carbonates, and other inorganic compounds. They are typically extracted by mining and smelting operations that release the materials as relatively pure elements.

Magnesium is particularly useful for its strength and is the lightest structural metal known. There are certain dangers associated with magnesium, however. The activation energy of its oxidation reaction is high, but the reaction liberates so much heat energy that, once started, a magnesium fire is extraordinarily difficult to extinguish. Attempting to extinguish a magnesium fire with water only makes it worse, as magnesium reacts with room-temperature water, and it also reacts with nitrogen in air. Because of this, heat operations involving magnesium, such as casting and welding, are carried out in an atmosphere of an inert gas that cannot react with the metal. Magnesium is also an important laboratory reagent since it can form reactive complexes with alkyl and aryl halides that are useful in Grignard reactions. In biochemical systems, the Mg²⁺ ion is a vital component of nerve and enzyme function.

PRINCIPAL TERMS

• halide: a binary compound consisting of a halogen element (fluorine, chlorine, bromine, iodine, or astatine) bonded to a non-halogen element or organic group; alternatively, an anion of a halogen element.
• hydroxide: an anion consisting of one oxygen atom and one hydrogen atom, represented as OH⁻; also, an ionic compound in which the OH⁻ ion is bonded to another element or group.
• oxidation state: a number that indicates the degree to which an atom or ion in a chemical compound has been oxidized or reduced.
• oxide: a compound formed by the reaction of any element with oxygen, such as carbon dioxide, carbon monoxide, iron oxide, or diphosphorus pentoxide.
• reactivity: the propensity of a chemical species to undergo a reaction under applied conditions.

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