Hydrides :

Hydrogen forms binary hydrides of the type MH_x or M_mH_n with

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

(b) All lanthanoids and actinoids.

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

Hydrides are classified into three main categories.

(i) Saline or ionic hydrides :Most of the s-block metals form this type of hydrides. These are non-volatile, non-conducting crystalline solids. However, BeH₂ and MgH₂ have covalent polymeric structure. These ionic hydrides have rock-salt structure. 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 have significant covalent character.

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

NaH(s) + H₂O(aq) → NaOH(aq) + H₂(g)

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

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 groups 3, 4, 5 (d-block) and f-block elements form metallic hydrides. In group 6, only Cr forms hydride (CrH). Metals of group 7, 8, 9 do not form hydrides. This region of periodic table from group 7 to group 9 is known as hydride gap. Examples of hydrides of group 3 to 5 are, ScH₂, YH₂, YH₃, LaH₂, LaH₃, TiH₂, ZrH₂, HfH₂, VH, VH₂, NbH, NbH₂, TaH

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

ZrH_x (1.30 < x < 1.75), TiH_x (1.8 < x < 2.0)

Most of these hydrides are good conductors of electricity in solid state.

Metallic hydrides can be used 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). common examples of such hydrides are CH₃, NH₃, H₂O, HF etc. The stability of these hydrides decreases down the group. For example, 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 rich, electron precise 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. For example

(b) Electron precise molecular hydrides : Elements of group 14 form such hydrides. The bond length increases on going down the group. A common 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 common 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,