What is the difference between intramolecular and intermolecular forces for covalently bonded molecules?
Intramolecular forces are the forces that hold atoms together or arrange atoms in specific position relative to other atoms within one molecular entity. For example, water, whose chemical notation is H2O, the hydrogen atoms are held to the oxygen atom by covalent bond, which is a type of intramolecular force. Covalent bonding rises from balanced electrostatic forces of attraction and repulsion, and is responsible for holding hydrogen and oxygen atoms together in specific distance. Also, the water molecule is in a bent shape, with an angle of approximately 109.5 degrees. This is because the oxygen atom contains two lone pairs of electrons, which pushes the O-H bonds closer together by electrostatic forces of repulsion, and this is the basis of VSEPR theory (Valence Shell Electron Pair Repulsion). This also counts as a type of intramolecular forces.Intermolecular forces are the forces that exist between two or among many molecular entities. For example, due to difference in electronegativity between hydrogen and oxygen atom, a polar bond is formed within water molecule. A partial difference in charge is formed (oxygen with a greater electronegativity is partially negatively charged, and hydrogen with a smaller electronegativity loses the electron density around the nucleus to the oxygen atom, and become partially positively charged) and this creates a weak (yet significant!) electrostatic force of attraction AMONG water molecules (not WITHIN A water molecule). Water molecule is a polar molecule due to these partial differential charges set up, however, this is not a necessity for intermolecular forces to exist. Even non-polar molecules, like hydrogen gas, oxygen gas, and methane gas, can also have intermolecular forces that are weaker than polar-polar interaction, due to momentary distortion of electron density. This is what we call van der Waals force.
