The hydrides are classified into three types:
- Ionic or saline or salt like hydrides
- Covalent or molecular hydrides
- Metallic or non-stoichiometric hydrides
Ionic or Saline Hydrides
Hydrides formed between hydrogen and electropositive element of group I and II belonging to s-block. These are known as stoichiometric compounds.
Properties of saline or ionic hydrides:
- The hydrides of lighter elements like Li, Be, Mg etc. have significant covalent character.
- Ionic hydrides are white colourless solid (crystalline) having high m.p. and b.p. easily decomposed by water, CO2 or SO2.
- Ionic hydrides are non-volatile, non-conducting in solid state.
- They conduct electricity in molten state and liberate hydrogen at anode.
Molecular or Covalent Hydrides
These are formed by elements of p-block having higher electronegativity than hydrogen.
- Electron deficient hydrides: These are the hydrides which do not have sufficient number of electrons needed to form normal covalent bond, e.g., BH3, AIH3, etc.
- Electron precise hydrides: These are the hydrides which have exact number of electrons needed to form normal covalent bond, e.g., hydrides of group 14 (CH4, SiH4, etc.)
- Electron rich hydrides: These are the hydrides which have greater number of electrons than required to form normal covalent bond, e.g., hydrides of group 15, 16, 17, (NH3, PH3, H2S, etc). The excess electrons in these hydrides are present as lone pairs of electrons.
Metallic or Interstitial Hydrides
The transition metals and rare earth metals combine with hydrogen to from interstitial hydrides. They exhibit metallic properties and are powerful reducing agents. They are non-stoicluometric hydrides and their composition varies with temperature and pressure for e.g., LaH2.76, TiH1.73.
Metals of group 7, 8 and 9 do not form hydrides and this region of the Periodic table is called hydride gap.