Hydrides : Hydrogen forms binary hydrides of the type MH_x or M_mH_n with

(a) All important group elements except noble gases and probably indium and thallium.

(b) All the lanthanoids and actinoids.

(c) The transition metals (Zr, Sc, Tc, Y, La, Ac, Hf and to a lesser extent Cr, V, Cu Nb, Ta, and Zn). In group 6 only Cr forms hydride (CrH).

The Hydrides can be classified into three basic categories.

(i) Saline or ionic hydrides : Most of the s-block metals form this type of hydrides. They are non-conducting, non-volatile, crystalline solids. Although, BeH2 and MgH2 posses covalent polymeric structure. These ionic hydrides posses rock-salt structure. The thermal stability of 1^st and 2^nd group hydrides are in the order;

LiH > NaH > KH > RbH > CsH

CaH₂ > SrH₂ > BaH₂

BeH₂, MgH₂ and LiH posses important covalent character.

Electrolysis of solution of saline hydride in molten alkali halide produces H₂ at anode. Saline hydrides react explosively with water.

The fire so produced cannot be extinguished by CO₂ as it gets reduced by the hot metal hydride. Only the sand is useful, as it is a solid in state.

Alkali metal hydrides are used for making LiAlH₄, NaBH₄ etc. Alkali metal hydrides are also used for the removal of last traces of water from organic compounds.

(ii) Metallic or interstitial hydrides : Elements of the groups 3, 4, 5 (which is d-block) and f-block elements form the metallic hydrides. In the group number 6, only Cr forms the hydride (CrH). The metals of group number 7, 8, 9 do not form hydrides. This area of periodic table from group 7 to group 9 is known as hydride gap. Examples of the hydrides of group 3 to 5 are, ScH₂, YH₂, LaH₂, LaH₃, TiH₂, ZrH₂, HfH₂, VH, VH₂, NbH, NbH₂, TaH

The f-block metals form hydrides of limiting compositions of MH₂ and MH₃. All these hydrides are non-stoichiometric with variable composition e.g.,

.

Almost all these hydrides are good conductors of electricity in solid state.

The metallic hydrides can be taken in use to store hydrogen especially in cars working on fuel cells.

(iii) Molecular or covalent hydrides : Hydrogen form molecular compounds with p-block elements (B, C, N, O, F; Si, P, S, Cl; Ga, Ge, As, Sb, Br; In, Sn, Sb, Te, I; Tl, Pb, At). general examples of such hydrides are CH4, NH3, H2O, HF etc. The stability of these hydrides decreases down the group. For instance, NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃. In a period the stability increases with increasing electronegativity. For example, CH₄ < NH₃ < H₂O < HF. Molecular hydrides are classified as electron precise electron rich, and electron deficient hydrides.

(a) Electron rich molecular hydrides : These hydrides have one or more lone pairs of electrons around the central more electronegative element. The example is given as

(b) Electron precise molecular hydrides : Elements of group 14 form such hydrides. The bond length increases on moving down the group. A general example of electron precise molecular hydrides is CH₄.

(c) Electron deficient molecular hydrides : These hydrides have lesser number of electrons than that required for writing the conventional Lewis structure. A general example of such molecular hydride is diborane, B₂H₆.

(d) Systematic names of molecular hydrides : The systematic names of these hydrides are obtained from the name of the element and the suffix -ane. For example,

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