Beryllium - Chemical Properties

Chemical Properties

Beryllium's chemical behavior is largely a result of its small atomic and ionic radii. It thus has very high ionization potentials and strong polarization while bonded to other atoms, which is why all of its compounds are covalent. It is more chemically similar to aluminium than its close neighbors in the periodic table due to having a similar charge-to-radius ratio. An oxide layer forms around beryllium that prevents further reactions with air unless heated above 1000°C. Once ignited, beryllium burns brilliantly forming a mixture of beryllium oxide and beryllium nitride. Beryllium dissolves readily in non-oxidizing acids, such as HCl and diluted H₂SO₄, but not in nitric acid or water as this forms the oxide. This behavior is similar to that of aluminium metal. Beryllium also dissolves in alkali solutions.

The beryllium atom has the electronic configuration 2s2. The two valence electrons give beryllium a +2 oxidation state and the thus the ability to form two covalent bonds; the only evidence of lower valence of beryllium is in the solubility of the metal in BeCl₂. Due to the octet rule, atoms tend to seek a valence of 8 in order to resemble a noble gas. Beryllium tries to achieve a coordination number of 4 because its two covalent bonds fill half of this octet. A coordination of 4 allows beryllium compounds, such as the fluoride or chloride, to form polymers.

This characteristic is employed in analytical techniques using EDTA as a ligand. EDTA preferentially forms octahedral complexes – thus absorbing other cations such as Al3+ which might interfere – for example, in the solvent extraction of a complex formed between Be2+ and acetylacetone. Beryllium(II) readily forms complexes with strong donating ligands such as phosphine oxides and arsine oxides. There have been extensive studies of these complexes which show the stability of the O-Be bond.

Solutions of beryllium salts, e.g. beryllium sulfate and beryllium nitrate, are acidic because of hydrolysis of the 2+ ion.

2+ + H₂O + + H₃O+

Other products of hydrolysis include the trimeric ion 3+. Beryllium hydroxide, Be(OH)₂, is insoluble even in acidic solutions with pH less than 6, that is at biological pH. It is amphoteric and dissolves in strongly alkaline solutions.

Beryllium forms binary compounds with many non-metals. Anhydrous halides are known for F, Cl, Br and I. BeF₂ has a silica-like structure with corner-shared BeF₄ tetrahedra. BeCl₂ and BeBr₂ have chain structures with edge-shared tetrahedra. All beryllium halides have a linear monomeric molecular structure in the gas phase.

Beryllium difluoride, BeF₂, is different than the other difluorides. In general, beryllium has a tendency to bond covalently, much more so than the other alkaline earths and its fluoride is partially covalent (although still more ionic than its other halides). BeF₂ has many similarities to SiO₂ (quartz) a mostly covalently bonded network solid. BeF₂ has tetrahedrally coordinated metal and forms glasses (is difficult to crystallize). When crystalline, beryllium fluoride has the same room temperature crystal structure as quartz and shares many higher temperatures structures also. Beryllium difluoride is very soluble in water, unlike the other alkaline earths. (Although they are strongly ionic, they do not dissolve because of the especially strong lattice energy of the fluorite structure.) However, BeF₂ has much lower electrical conductivity when in solution or when molten than would be expected if it were fully ionic.

Beryllium oxide, BeO, is a white refractory solid, which has the wurtzite crystal structure and a thermal conductivity as high as in some metals. BeO is amphoteric. Salts of beryllium can be produced by treating Be(OH)₂ with acid. Beryllium sulfide, selenide and telluride are known, all having the zincblende structure.

Beryllium nitride, Be₃N₂ is a high-melting-point compound which is readily hydrolyzed. Beryllium azide, BeN₆ is known and beryllium phosphide, Be₃P₂ has a similar structure to Be₃N₂. Basic beryllium nitrate and basic beryllium acetate have similar tetrahedral structures with four beryllium atoms coordinated to a central oxide ion. A number of beryllium borides are known, such as Be₅B, Be₄B, Be₂B, BeB₂, BeB₆ and BeB₁₂. Beryllium carbide, Be₂C, is a refractory brick-red compound that reacts with water to give methane. No beryllium silicide has been identified.