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The elements in group 13 are also capable of forming stable compounds with the halogens, usually with the formula MX 3 (where M is a boron-group element and X is a halogen.) [14] Fluorine, the first halogen, is able to form stable compounds with every element that has been tested (except neon and helium), [15] and the boron group is no exception.
Boron atoms lie in hexagonal aromatic graphite-like layers, with a charge of −1 on each boron atom. Magnesium(II) ions lie between layers. Binary metal-boron compounds, the metal borides, contain only boron and a metal. They are metallic, very hard, with high melting points. TiB 2, ZrB 2, and HfB 2 have melting points above 3000 °C. [70]
Amorphous powder boron and polycrystalline β-rhombohedral boron are the most common forms. The latter allotrope is a very hard [n 1] grey material, about ten percent lighter than aluminium and with a melting point (2080 °C) several hundred degrees higher than that of steel. [6]
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Crystalline boron is a very hard, black material with a high melting point and exists in many polymorphs: Two rhombohedral forms, α-boron and β-boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atom tetragonal boron are the most common. Boron has a density of 2.34 −3. [17]
The Gmelin rare earths handbook lists 1522 °C and 1550 °C as two melting points given in the literature, the most recent reference [Handbook on the chemistry and physics of rare earths, vol.12 (1989)] is given with 1529 °C.
The boron rich borides contain 3-dimensional frameworks of boron atoms that can include boron polyhedra. The metal rich borides contain single boron atoms, B 2 units, boron chains or boron sheets/layers. Examples of the different types of borides are: isolated boron atoms, example Mn 4 B; B 2 units, example V 3 B; chains of boron atoms, example FeB