Alkaline earth metal

The alkaline earth metals are a group of six metallic elements located in Group 2 of the periodic table. They include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and the highly radioactive radium (Ra). These elements share similar chemical and physical properties and play substantial roles in technology, industry and biological systems. All known members of the group occur naturally, although radium is found only in trace quantities as part of the decay products of uranium and thorium.

Electronic Structure and General Characteristics

Alkaline earth metals possess two electrons in their outermost s-orbital, giving them the electron configuration ns². This full valence subshell accounts for many of their characteristic features:

• They readily lose both valence electrons to form 2⁺ cations, achieving stable noble-gas configurations.
• They typically exhibit an oxidation state of +2 in their compounds.
• Their metallic character is pronounced, and they are silvery, relatively soft and moderately reactive.

Although helium shares a full outer shell, it remains classified among the noble gases; however, under extreme conditions it is predicted to show bonding similarities with beryllium, which has led to occasional theoretical speculation about its comparison with Group 2 behaviour.

Physical and Chemical Properties

Alkaline earth metals display characteristic trends down the group:

• Density, atomic radius and reactivity increase from beryllium to barium.
• Melting and boiling points are comparatively low, though higher than those of the alkali metals.
• They exhibit moderate electropositivity, enabling them to form a wide variety of ionic compounds.

Important reactivity observations include:

• With halogens, they form metal halides (e.g. CaCl₂). These are predominantly ionic, except for the halides of beryllium, which are covalent due to its high charge density.
• With oxygen, they form oxides (e.g. SrO); some metals also form peroxides and superoxides under particular conditions.
• With water, calcium, strontium and barium react vigorously, yielding metal hydroxides and hydrogen gas. Magnesium reacts slowly, while beryllium does not react unless under exceptionally high temperatures.
• Their first and second ionisation energies are relatively low, favouring the formation of M²⁺ ions.

The distinct behaviour of beryllium is notable. Owing to its high charge density and small ionic radius, it forms entirely covalent compounds, does not react readily with water and often shows chemical similarities to aluminium.

Compounds, Reactions and Applications

The Group 2 elements form a wide range of compounds significant to both industry and biological systems.
Key compound types include:

• Halides, such as MgCl₂ and BaBr₂, essential in metallurgy and materials science.
• Oxides and hydroxides, used in construction, ceramics and desulfurisation processes.
• Carbonates and sulfates, important for minerals such as limestone (calcium carbonate) and barite (barium sulfate).

Chemically, alkaline earth metals undergo:

• Salt formation with acids, yielding metal salts and hydrogen gas.
• Transmetalation reactions, particularly notable in magnesium chemistry.
• Redox behaviour, typically acting as reducing agents because of their tendency to lose electrons.

These reactions have extensive industrial relevance, for example in alloy production, pyrotechnics, catalysts and pharmaceuticals.

Atomic and Nuclear Characteristics

Group 2 elements exhibit a variety of isotopic forms, with many possessing stable or long-lived nuclides.
Stable and observationally stable isotopesFive of the six elements—beryllium, magnesium, calcium, strontium and barium—have at least one stable or observationally stable isotope, giving a total of nineteen such nuclides. Examples include:

• Be-9
• Mg-24, Mg-25, Mg-26
• Ca-40, Ca-42, Ca-43, Ca-44, Ca-46
• Sr-84, Sr-86, Sr-87, Sr-88
• Ba-132, Ba-134, Ba-135, Ba-136, Ba-137, Ba-138

Certain of these isotopes are considered observationally stable because, although energetics predict that they should decay through pathways such as double beta decay, no such decay has been detected experimentally.
Long-lived primordial isotopes

• Calcium-48 and barium-130 have extraordinarily long half-lives, vastly exceeding the age of the universe, making them effectively stable for practical purposes.
• Radium, by contrast, has no stable isotopes, with Ra-226 (half-life 1600 years) being its longest-lived form.

Radioisotopes and their originsGroup 2 contains numerous radioisotopes, none surviving from the formation of the solar system except for the two extremely long-lived primordial species above. Others are generated through:

• Cosmogenic processes, such as Be-7, Be-10 and Ca-41, produced by cosmic ray interactions.
• Decay chains of uranium and thorium (producing various isotopes of radium).
• Nuclear fission, notably Sr-90 and Ba-140, which are significant fission products of uranium-235 and have important implications for environmental radioactivity.

Calcium-48 is particularly notable as the lightest nuclide known to undergo double beta decay, but at a rate so slow that its radioactivity is virtually undetectable without specialised equipment.

Biological and Environmental Considerations

Because of their chemical resemblance to calcium, many radioisotopes of the heavier alkaline earth metals become bone-seeking when introduced into organisms. They accumulate in bone tissue and can damage bone marrow through emitted radiation. This property, harmful in cases of environmental contamination, is used therapeutically in controlled medical treatments for certain bone cancers, where targeted accumulation assists in delivering radiation to tumour sites.
Elemental calcium, magnesium and to some extent strontium also play essential roles in biological systems:

• Calcium is central to bone mineralisation, signalling pathways and muscle function.
• Magnesium is a vital cofactor in hundreds of enzymatic reactions.
• Strontium compounds have been explored for their influence on bone density.

Significance in the Periodic Table

Alkaline earth metals occupy an important position in the periodic table as representative s-block elements. Their consistent valence configuration (ns²), predictable chemical patterns and relevance in natural and industrial processes make them central to understanding periodic trends. Compared with neighbouring groups, they possess a relatively large number of stable isotopes, reflecting the nuclear stability associated with even atomic and neutron numbers.
The continuing search for potential heavier group members, such as the predicted element 120 (unbinilium), remains an active research area in superheavy element chemistry, although no such element has yet been successfully synthesised.