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Dispersion forces:
- Non-polar molecules/atoms have small, temporary, fluctuating dipoles due to random movement of electrons.
- A temporary dipole induces the dipole on other molecules/atoms.
- The larger the molecule – greater number of electrons – the larger temporary dipoles and so greater attractive forces between molecules, which are van der Waal's forces.
When drawing diagrams of dispersion forces, you have to write what type of dipole it is.
e.g. it can be temporary or induced.
If you don't write the type of dipole, its assumed to be a permanent dipole.
Dipole-dipole attraction:
- Some molecules are polar.
- Consider:
- Symmetry
- Difference in electronegativity of elements.
- e.g. HCl contains hydrogen and chlorine, which have significant differences in electronegativity. Also. There is no symmetry that would cancel the dipole. Thus, HCl is a polar molecule.
- Molecules with permanent dipoles have stronger forces between molecules than molecules with temporary dipoles, assuming the molecules have a relative similar size
Hydrogen Bonding:
- Hydrogen bonding is the electrostatic force of attraction between a hydrogen atom that is covalently bonded to an NOF atom, and the lone pair of an NOF atom bonded to a hydrogen atom.
- This occurs due to the extreme difference in electronegativity, leading to a greater polarity.
- A hydrogen bond is a strong dipole-dipole attraction between an electron deficient hydrogen atom attached to an electronegative atom (N, O, F), and the lone pairs of electrons on a highly electronegative atom on a different molecule.
- When drawing a diagram, always label the bond as hydrogen bonding!
NOTE: If you're ever drawing a diagram of intermolecular forces, label the force.
