Bonds BETWEEN molecules
Lowest boiling point = lowest intermolecular forces (requires less energy to separates "bonds" of liquid molecules into gas molecules)
Not as strong as intramolecular (bonds keeping molecule together)
- Uneven sharing of electrons in covalent compounds
- VSEPR structure showing asymmetry or a central atom with lone pairs of electrons often is a polar molecule or a high difference in electronegativity.
A dipole is created by equal but opposite charges that are separated by a short distance. The direction of a dipole is from the dipole’s positive end toward its negative end.
The negative region in one polar molecule attracts the positive region in another molecule. The forces of attraction between polar molecules are known as dipole-dipole forces.
Induced dipole- A polar molecule can also induce a dipole in a nonpolar molecule by temporarily attracting its electrons. This short- range force is weaker than the dipole-dipole force.
One especially strong type of dipole-dipole force is known as hydrogen bonding. Hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom (N, O, F) is attracted to an unshared pair of electrons on the highly electronegative atom of another molecule.
Hydrogen bonding occurs because the covalent bonds H–F, H–O, and H–N are highly polar. This gives the hydrogen atom a partial positive charge that is almost half the charge of a proton. Also, because a hydrogen atom has a small atomic radius, it can get close to unshared pairs of electrons on other molecules.
London Dispersion Forces
Electrons are always in motion, moving around an atom or in shared regions between atoms. Therefore, at any moment, the electron distribution in a molecule may be uneven. This temporary dipole can induce a dipole in another molecule, causing a brief attraction between the two molecules.
The intermolecular attractions resulting from the motion of electrons and temporary dipoles are called London dispersion forces. This is the only type of intermolecular force between nonpolar molecules and noble gas atoms. The strength of these forces increases as atomic mass increases.