Alkaline earth metal

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The alkaline earth metals are a group of chemical elements in the periodic table with very similar properties: they are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure[1] and readily lose their two outermost electrons to form cations with charge +2.[2] In the modern IUPAC nomenclature, the alkaline earth metals comprise the group 2 elements.[note 1]

The alkaline earth metals are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).[4] This group lies in the s-block of the periodic table as all alkaline earth metals have their outermost electron in an s-orbital.[1][5][6]

All the discovered alkaline earth metals occur in nature.[7] Experiments have been conducted to attempt the synthesis of unbinilium (Ubn), which is likely to be the next member of the group, but they have all met with failure. However, unbinilium may not be an alkaline earth metal due to relativistic effects, which are predicted to have a large influence on the chemical properties of superheavy elements.[8]

Chemical

Like other groups, the members of this family show patterns in its electronic configuration, especially the outermost shells, resulting in trends in chemical behavior:
Z Element No. of electrons/shell Electron configuration[note 2]
4 beryllium 2, 2 [He] 2s2
12 magnesium 2, 8, 2 [Ne] 3s2
20 calcium 2, 8, 8, 2 [Ar] 4s2
38 strontium 2, 8, 18, 8, 2 [Kr] 5s2
56 barium 2, 8, 18, 18, 8, 2 [Xe] 6s2
88 radium 2, 8, 18, 32, 18, 8, 2 [Rn] 7s2
120 unbinilium 2, 8, 18, 32, 32, 18, 8, 2 (predicted)[9]:1722 [Uuo] 8s2 (predicted)[9]:1722

Most of the chemistry has been observed only for the first five members of the group. The chemistry of radium is not well established due to its radioactivity,[1] and unbinilium has not yet been discovered; thus, the presentation of their properties here is limited.

The alkaline earth metals are all silver-colored, soft, and have relatively low densities, melting points, and boiling points. In chemical terms, all of the alkaline metals react with the halogens to form the alkaline earth metal halides, all of which ionic crystalline compounds (except for beryllium chloride, which is covalent). All the alkaline earth metals except beryllium also react with water to form strongly alkaline hydroxides and thus should be handled with great care. The heavier alkaline earth metals react more vigorously than the lighter ones.[1] The alkaline metals have the second-lowest first ionization energies in their respective periods of the periodic table[6] because of their somewhat low effective nuclear charges and the ability to attain a full outer shell configuration by losing just two electrons. The second ionization energy of all of the alkaline metals is also somewhat low.[1][6]

Beryllium is an exception: It does not react with water or steam, and its halides are covalent. If Beryllium did form compounds with an ionization state of −2, it would polarize electron clouds that are near it very strongly and would cause extensive orbital overlap, since beryllium has a high charge density. All compounds that include beryllium have a covalent bond. Even the compound beryllium fluoride, which is the most ionic beryllium compound, has a low melting point and a low electrical conductivity when melted.[citation needed]

The chemistry of unbinilium, the undiscovered seventh alkali metal, is predicted to be closer to that of calcium or strontium or rubidium[10] instead of barium or radium. This is unusual as periodic trends would predict unbinilium to be more reactive than barium and radium. This lowered reactivity is due to the energetic properties[clarification needed] of unbinilium’s valence electrons, increasing unbinilium’s ionisation energy and decreasing the metallic and ionic radii.[10]

All the alkaline earth metals have two electrons in their valence shell, so the energetically preferred state of achieving a filled electron shell is to lose two electrons to form doubly charged positive ions.