1.2.7 Electronegativity and Intermolecular Forces

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Electronegativity and Intermolecular Forces
There's a gradual transition from ionic to covalent bonding. Very few compounds come even
close to being purely ionic. Only bonds between atoms of a single element, like diatomic gases
such as hydrogen (H2) and oxygen (O2), can be purely covalent. So really, most compounds
come somewhere in between the two extremes- meaning they've often got ionic and covalent
properties e.g. covalent hydrogen chloride gas molecules dissolve to form hydrochloric acid,
which is an ionic solution: HCl(g) H+(aq) + Cl-(aq)
Covalent bonds may be polarised by differences in electronegativity. The ability to attract
the bonding electrons in a covalent bond is called electronegativity. Fluorine is the most
electronegative element. Oxygen, nitrogen and chlorine are also strongly electronegative.
In a covalent bond between atoms of different electronegativities, the bonding electrons are
pulled towards the more electronegative atom. This makes the bond pola. The covalent bonds
in diatomic gases are non polar because the atoms have equal electronegativities and so the
electrons are equally attracted to both nuclei.
Some elements, like carbon and hydrogen, have pretty similar electronegativities, so
bonds between them are essentially non polar. In a polar bond, the difference in the atoms'
electronegativities cause a dipole. A dipole is a difference in charge between the two
atoms caused by a shift in electron density in the bond. The greater the difference in
electronegativity the more polar the bond.
Polar molecules mean intermolecular attraction. The + and - charges on polar molecules
cause weak electrostatic forces of attraction between molecules. These are called permanent
dipole-dipole interactions.
Intermolecular forces are very weak. There are many types of intermolecular force but they are
all very weak compared to the bonds within the molecule:
1) Permanent dipole-dipole interactions
2) Hydrogen bonding
3) Temporary dipole-induced dipole or van der Waals forces
Hydrogen bonding is the strongest intermolecular force. Hydrogen bonding can only happen
when hydrogen is covalently bonded to fluorine, nitrogen or oxygen. Hydrogen has a high
charge density because it's so small and fluorine, nitrogen and oxygen are very electronegative.
The bond is so polarised that the hydrogen of one molecule forms a weak bond with the
fluorine, nitrogen or oxygen of another molecule.
Molecules which have hydrogen bonding are usually organic, containing -OH or -NH groups.
Hydrogen bonding has a huge effect on the properties of substances. They are soluble in water
and have higher boiling and freezing points than non-polar molecules of a similar size.
In ice, molecules of H2O are held together in a lattice by hydrogen bonds. And because
hydrogen bonds are relatively long, ice is less dense than liquid water.


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