Electronegativity is a measure of an atoms ability to attract the shared pair of electrons in a covalent bond. It's a really important concept with huge implications for how molecules interact with each other.

Intermolecular forces

Intermolecular forces are forces of attraction between compounds that are molecules. They are responsible for the way most substances interact with each other, particularly in living organisms.

There are three types:

1) Permanent dipole-dipole interactions are those in which a permanent dipole in a molecule is attracted to another molecule in which there is a permanent dipole of the opposite charge.

2) Van der Waals forces are forces of attraction between molecules which are also known as induced dipole-dipole interactions. A non-polar bond exists when the electronegativity difference between the two atoms in the bond is less than 0.4. The electron density between the atoms fluctuates causing an instantaneous temporary dipole in the molecule, which then induces a dipole in a neighbouring molecule. The opposite dipoles attract which is the Van er Waals force. As  the number of electrons increase so do strength in VdW.

3)Hydrogen bonding occurs when there is a strong permanent dipole-dipole interaction between an electron-deficient hydrogen atom on one molecule and a lone pair of electrons on a highly electronegative atom (only Nitrogen, Oxygen or Fluorine)  in a different molecule.  

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