When two or more atoms of the same element or atoms of
different elements having similar electro negativities
react, the transfer of electrons does not occur. In
these instances, the atoms achieve inert gas (noble gas)
structure by sharing of electrons. Thus, the atoms
complete their outer most shell by means of sharing of
unpaired electrons, and a covalent bond is formed. In
covalent bond each atom has to contribute equal number
of unpaired electrons. The shared pair of electrons
which links the atoms in a molecule is known as covalent
In covalent bond the shared electron pair is commonly
expressed by single short line (—) For example, the
halogens (chlorine atoms) posses an electronic
configuration in which there are seven electrons in
their outer most shell, and lacking only one electron in
order to attain the structure of an inert gas. This
structure may be attained by the halogens molecule
(chlorine molecule) when both atoms share one electron
for Bond formation
5.5.1 Single, Double and Triple Covalent Bonds:
Single Covalent Bond:
In single covalent bond only one pair of electrons
is shared by the bonded atoms, in which each atom
has to share one electron. This type of bond is
represented by single short line (—). For example,
in the formation of (H2) and hydrogen
chloride (HCl) molecules, only one pair of electrons
Double Covalent Bond:
In double covalent bond only two pairs of electrons
are shared by the bonded atoms, and each atom has to
share two unpaired electrons. This type of bond is
represented by two short lines (=) as shown in the
molecules of oxygen (O2) and carbon
Triple Covalent Bond:
In triple covalent bond only three pairs of
electrons are shared between the bonded atoms, and
each atom has to share three unpaired electrons.
This type of bond is denoted by three short lines
(=) as shown in the molecules of nitrogen (N2)
and ethyne (C2H2).
5.5.2 Characteristics of Covalent Compounds:
Compounds with covalent bonds are usually made up of
discrete units (molecules) with a weak inter
In the solid state, there are weak Vander wall
forces between the molecules. Hence covalent
compounds are often gases, liquids or soft solids
with low melting points. In few cases, three
dimensional covalent structures are formed rather
than discrete units, hence diamond and silica (SiO2)
are covalent but are very hard and. have high
melting points. Usually covalent compounds have low
melting and boiling points.
They are insulators because they do not conduct
Covalent compounds are usually insoluble in polar
solvents like water, but soluble in organic solvents
like benzene, ether, carbon tetra chloride etc.
5.5.3 Electro negativity: (E. N.)
If the covalent bond is formed between two like atoms,
that molecule is called non-polar-because the electron
pair is shared equally between the two atoms, as in case
of (H-H), (Cl-Cl), (0=0) and (N==N) molecules. However,
if the covalent bond is formed between the two
dissimilar atoms as in hydrogen chloride (H-Cl)
molecule, the attraction for electron pair, would not be
equal, one atom will attract more than the other. Hence
the electron pair will be displaced from the central
position and reaches near to the chlorine atom.
This power of an atom to attract the shared pair of
electrons towards, itself, is known as electro
Consequently, the chlorine atom being more
electronegative tends to be partially negative and
hydrogen atom would be partially positive.
These values are based upon an arbitrary scale, in which
fluorine is given an arbitrary standard value of electro
negativity as 4.0. It is the most electronegative
element. The electro negativity values of other elements
are compared with that of fluorine. Note that the non
metals have higher electro negativity values than the
metals. Fluorine (F) has the highest electro negativity
(4.0) and the metal cesium (Cs) has the lowest electro
negativity (0.7) value.
5.4 Ionic Character in a Covalent Bond:
If covalent bond is formed between two like atoms e.g.
in case (H-H), the hydrogen atoms are identical, and
hence the shared pair of electrons is not disturbed from
the centre. This molecule is called non-polar because it
is electrically neutral as well as symmetrical.
If covalent bond is formed between dissimilar atoms e.g.
in case of H-Cl molecule, the shared pair of electrons
move closer to one of the both atoms. In H-Cl molecule
chlorine is more electronegative, hence the shared pair
of electrons, is drawn nearer to chlorine than hydrogen
atom. This results in partial positive charge on H atom
and partial negative charge on CI atom.
H+8 ------------ Cl-8
The covalent bond between H and CI in H-Cl is partially
ionic or polar covalent bond, because of positive and
negative charges or poles.
Covalent bonds are partially ionic, if they exist
between two dissimilar atoms and heir ionic character
depends upon the difference in electro negativities of
Non-Polar Covalent Bond:
According to the scale of Linus Pauling, if the
difference in the electro negativities of bonded
atoms is zero, then the bond is pure covalent bond
or non-polar bond. The molecule containing like
atoms or the atoms of same electro negativities form
a pure covalent bonds or non-polar bonds.
Polar Covalent Bond:
If the difference in the electro negativities of
bonded atoms is up to 1.7 that bond is called polar
covalent ¥or partially ionic in character. The
elements of different electro negativities always
form polar covalent bond.
If the difference in the electro negativities of
bonded atoms is more than 1.7 then that bond is
purely ionic or electrovalent. The bond between
sodium and chlorine in the common salt (NaCl) is"
clearly ionic, because the difference in the electro
negativities is 2.1 i.e. more than 1.7.
Polar covalent bond (HCl) and Ionic bond (NaCl)