If a molecular substances is polar (has polar bonds and an overall uneven distribution of electron density) permanent dipole-dipole interactions exist between molecules
The slightly negative part of one molecule is attracted to the slightly positive part of another molecule.
The attractions are permanent and are stronger than temporary, induced dipole-dipole forces
The greater the polarity of a bond or molecule, the stronger the permanent dipole-dipole interactions
Permanent Dipole-Dipole Forces
The theory behind permanent dipole-dipole interactions is very similar to temporary, induced dipole-dipole interactions. There is an unequal distribution of electrons within a molecule, leading to a slightly positive and a slightly negative region of charge. These different regions of charge can then induce a similar charge on a neighbouring molecule, and interactions arise.
In permanent dipole-dipole interactions, the dipole is not temporary. It arises because the electrons within a covalent bond are not evenly shared between the bonding atoms. This results in one atom experiencing a greater share of electrons and becomes slightly negative. The other atom becomes slightly positive, and such a bond is described as polar.
The ability of an atom to attract electrons towards itself in a covalent bond is called its electronegativity. The greater the electronegativity of a covalently bonded atom, the ‘greedier’ it is and the more likely it is to pull electrons towards itself.
The important thing to remember though is that electronegativity is relative. It’s the difference in electronegativity between two atoms that leads to a bond becoming polar.
If you have an atom which is not very electronegative (hydrogen, for example) and an atom this is very electronegative (fluorine, for example) then there will be a very polar bond. Because the fluorine is much more electronegative than hydrogen it will pull the electrons within the bond towards itself.
But, if you have two atoms which are similar in terms of electronegativity (hydrogen and carbon, for example) there will not be a polar bond, as neither atom is significantly better at pulling the electrons towards itself than the other.
There is nearly always an uneven distribution of electrons within a covalent bond if the bonding atoms are not the same, but the difference is often so small that at A-level we do not consider the bond to be polar.
So, if we have a polar bond within a molecule then the charges of the polar bond will align themselves with the opposite charges on the polar bond of a neighbouring molecule.
Opposite charges attract and this electrostatic attraction allows forces to arise between the polar bonds. These forces are known as permanent, dipole-dipole interactions.
The more polar a molecule or bond is, the stronger the dipole-dipole forces are, as the forces of attraction are greater.