London Forces
These intermolecular forces of attraction occur
between NON-POLAR molecules.
They result from temporary dipoles which occur in
molecules (due to electrons always moving around
the nucleus) inducing dipoles in neighbouring
molecules. A WEAK, TEMPORARY
ATTRACTION occurs between the molecules.
Dipole-dipole
These intermolecular forces occur between
POLAR molecules (i.e. ones with a
permanent dipole). However they exclude
ones in which Hydrogen bonds occur.
They result from the attraction between the
oppositely charged ends of molecules.
Examples include:
HCl, HBr, HI H2S
Examples of substances in which London forces
occur are:
1. All noble gases.
- He, Ne, Ar, Kr, Xe
2. All diatomic elements
- Cl2, F2, Br2, I2
- O2, N2, H2
3. All tetrahedral molecules of the type:
- CH4, CCl4, SiF4
4. CO2
Hydrogen Bonds
These intermolecular forces occur between
molecules where hydrogen is bonded to
either N, O or F. There must also be a lone
pair on the N, O and F.
These result because the very electronegative N, O, or F strip the hydrogen of its
electron. This results in a “naked proton”
which is able to bury itself in a lone pair.
The attraction is strong resulting in higher
that expected b.p and m.p and vapour
pressure and viscosity.
Examples include:
-
These are found in crystals or liquids of
ionic compounds where the oppositely
charged ions attract one another. They are
usually strong.
The compounds in which they occur are
usually metal-non-metal compounds
Examples include:
- NaCl, KF, CaO
Covalent Bonds
In some substances the whole structure is
held together by covalent bonds.
Examples include:
Carbon – diamonds/graphite
Sand – silica.
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Ion-dipole attractions
These are attractions between ions and a
polar molecules.
They occur when an ionic substance is
dissolved in a polar solvent. The polar
solvent molecules surround each of the ions
and holds them suspended in the solution.
NH3, H2O, HF,
Examples include:
-
All alcohols e.g. CH3OH
-
All organic acids e.g. CH3COOH
Ionic bonds
-
NaCl in water
HCl in water