There are four main types of hydrides: saline (ionic), metallic, covalent, and dimeric. Saline hydrides contain hydrogen as a negatively charged ion that reacts violently with water to produce hydrogen gas. Metallic hydrides consist of hydrogen ions and metal atoms in an electron sea. Covalent hydrides involve shared electron pairs between hydrogen and nonmetals, forming volatile liquids and gases. Hydrides have many applications including use as reducing agents, bases, drying agents, and hydrogen storage in batteries and fuel cells.

1. HYDRIDES

2. HYDRIDES
A hydride is any class of chemical compound in which hydrogen is combined with
another element.
Three basic types of hydrides—saline (ionic), metallic, and covalent—may be
distinguished on the basis of type of chemical bond involved.
A fourth type of hydride, dimeric(polymeric) hydride, may also be identified on the
basis of structure.
Aluminum and, possibly, copper and beryllium hydrides are non-conductors that exist
in solid, liquid, or gaseous forms.
All are thermally unstable, and some explode on contact with air or moisture.

5. SALINE (IONIC) HYDRIDES
 Saline, or ionic, hydrides are defined by the presence of hydrogen as a
negatively charged ion, H−. The saline hydrides are generally considered those
of the alkali metals and the alkaline-earth metals (with the possible exception
of beryllium hydride, BeH2, and magnesium hydride, MgH2).
 These metals enter into a direct reaction with hydrogen at elevated
temperatures (300–700 °C [570–1,300 °F]) to produce hydrides of the general
formulas MH and MH2.

7.  The hydride ion in the saline hydrides is a strong base, and these hydrides react
instantly and quantitatively with the hydrogen ion (H+) from water to produce
hydrogen gas and the hydroxide ion in solution.
H− + H2O → H2 + OH−
 Because saline hydrides react vigorously with water, giving off large volumes of
gaseous hydrogen, this property renders them useful as light, portable sources of
hydrogen.

8. METALLIC HYDRIDES
 The transition metals and inner transition metals form a large variety of compounds
with hydrogen.
 Metallic hydrides are formed by heating hydrogen gas with the metals or their alloys.
 Metallic alloy like hydrides possess some of the characteristics of metals, such
as lustre and strong electrical conductivity. They tend to have variable physical
properties, with some being more brittle and others being harder than the metals
from which they are made.
 Metallic hydrides essentially consist of protons (positive hydrogen ions, H+) and metal
atoms in an electron sea.

10. COVALENT HYDRIDES
 Covalent hydrides are primarily compounds of hydrogen and non-metals, in
which the bonds are evidently electron pairs shared by atoms of comparable
electronegativities.
 For example, most non-metal hydrides are volatile compounds, held together in
the condensed state by relatively weak van der Waals intermolecular
interactions.

12.  Covalent hydrides are liquids or gases that have low melting and boiling points,
except in those cases (such as water) where their properties are modified by
hydrogen bonding.
For example, NH3, H2O, and HF are held together in the liquid state
primarily by hydrogen bonding.
 The hydrogen compounds formed with the elements that follow oxygen—H2S,
H2Se—are all volatile, toxic gases with repulsive odours. They are easily prepared
by adding dilute acid to the corresponding metal sulfide, selenide.

14.  Each of the halogens forms a binary compound with hydrogen, HX. At ambient
temperature and pressure, these compounds are gases, with hydrogen
fluoride having the highest boiling point owing to intermolecular hydrogen
bonding.

15. APPLICATIONS
 Hydrides such as sodium borohydride, lithium aluminium hydride, are commonly
used as reducing agents in chemical synthesis.
 Hydrides such as sodium hydride and potassium hydride are used as
strong bases in organic synthesis. The hydride reacts with the weak acid releasing
H2.
 Hydrides such as calcium hydride are used as drying agents, to remove trace water
from organic solvents. The hydride reacts with water forming hydrogen
and hydroxide salt.

16.  Hydrides are important in storage battery technologies such as nickel-metal
hydride battery.
 Various metal hydrides have been examined for use as a means of hydrogen
storage for fuel cell-powered electric cars and other purposed aspects of
a hydrogen economy.

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