The Nature of the Co-Valent Bond
Covalent bonds are formed as a result of the sharing of one or more pairs of bonding electrons. Each atom donates half of the electrons to be shared.
This sharing of electrons is as a result of the electronegativity(electron attracting ability) of the two bonded atoms are either equal or the difference is no greater than 1.7.
If the electronegativity difference is greater than 1.7 then the higher electronegative atom has an electron attracting ability large enough to force the transfer of electrons from the less electronegative atom. This would be an ionic bond.
As long as the electronegativity difference is no greater than 1.7 the atoms can only share the bonding electrons.
Single Co-valent Bond
A single co-valent bond would be the sharing of two electrons between the two bonded atoms. Examples are:
Double Co-valent Bond
A double co-valent bond is two pairs of electrons being shared. Examples are:
Triple Co-valent Bond
A triple co-valent bond is the sharing of three pairs of electrons. Examples are:
triple bond between two Nitrogen atoms
Triple bond between two carbon atoms
Triple bond between a Carbon and a Nitrogen
The Polarity of the Co-valent Bond
Two atoms with the same electronegativity will share the bonding electron pairs equally. As a result the bonding electrons will be evenly distributed between the bonded atoms. There will be no accumulation of bonding electrons on any one atom and the bond dipole moment will be zero.
Such a co-valent bond will be called a "non-polar" bond.
The bond between two Hydrogens as in H2 or two Oxygens as in O2 or two Nitrogens like N2 will all be non-polar bonds.
On the other hand, if the two bonded atoms have a different electronegativity then the bonding pairs of electrons will be shared unequally. The atom with the higher electronegativity will attract the bonding electrons closer to itself. As a result the electron distribution will be unequal and a bond dipole moment will be formed.
For example the single bond between a Hydrogen and a Chlorine as in H-Cl will have the bonding pair closer to the higher electronegative atom (Chlorine). As a result the Chlorine end will be partially negative since the electrons are closer to the Chlorine. The Hydrogen end will be partially positive since the bonding pair is farther from the Hydrogen.
This two pole condition is called a dipole and it generates a dipole moment that is a vector force directed toward the higher electronegative atom in the bond. Such a bond is referred to as a "polar bond".
The greater the difference in the electronegativity between the two bonded atoms the more polar the bond.