Chemistry · Ch 5 — Alkali and Alkaline Earth Metals
Distinctive Behaviour of Beryllium
Distinctive Behaviour of Beryllium
Just as lithium heads group 1 with distinctive behaviour, beryllium heads group 2 with anomalous behaviour of its own. This traces mainly to beryllium's small size, high electronegativity, high ionisation energy and high polarising power compared to the rest of the group (Figure 5.8).
Beryllium versus the rest of group 2 (Table 5.11). Beryllium forms covalent compounds, while the rest of the family forms ionic ones; it has a high melting and boiling point compared to the low values of the others. It does not react with water even at elevated temperature (the others do), and it does not combine directly with either hydrogen or the halogens (the others do; and where beryllium's halides are covalent, the other metals' halides are electrovalent/ionic). Beryllium's hydroxide and oxide are amphoteric, whereas the rest of the family's are simply basic. Because of a protective oxide film, beryllium is not readily attacked by acids, unlike the rest of the group. Its carbide, Be₂C, evolves methane on hydrolysis, where the carbides of the other metals evolve acetylene instead. And beryllium's salts undergo extensive hydrolysis, more so than the (still hydrolysed, but less extensively) salts of the rest of the family. …
What this figure shows. A three-box flow diagram listing the three causes of beryllium's anomalous behaviour: (1) its small size and high polarising power, (2) its relatively high electronegativity and ionisation enthalpy compared to other members of the group, and (3) the absence of vacant d-orbitals in its va …
| Beryllium | Other elements of the family |
|---|---|
| Forms covalent compounds | Form ionic compounds |
| High melting and boiling point | Low melting and boiling point |
| Does not react with water even at elevated temperature | React with water |
| Does not combine directly with hydrogen | Combine directly with hydrogen |
| Does not combine directly with halogens; halides are covalent | Combine directly with halogens; halides are electrovalent |
| Hydroxide and oxide of beryllium are amphoteric in nature | Basic in nature |
| Not readily attacked by acids, because of a protective oxide film | Readily attacked by acids |
| S.No. | Property |
|---|---|
| 1 | BeCl2 forms a dimeric structure like AlCl3, with chloride bridges (and also a polymeric chain structure); both are soluble in organic solvents and are strong Lewis acids |
| 2 | Be(OH)2 dissolves in excess alkali to give the beryllate ion [Be(OH)4]²⁻ and hydrogen, just as Al(OH)3 gives the aluminate ion [Al(OH)4]⁻ |
| 3 | Beryllium and aluminium ions have a strong tendency to form complexes, e.g. BeF4²⁻, AlF6³⁻ |
| 4 | Both beryllium and aluminium hydroxides are amphoteric in nature |