Polarisation of bonds
ways of representing bonds.
size affect polarising power?
does different size and charge mean different polarising power?
what conditions favour fully ionic bonds?
Now try the quick quiz
Polarity is the distortion of the electron cloud of
one atom by another.
The standard example is often hydrogen chloride (HCl)
This distortion is said to be a dipole.
There are several methods of representing this shift
in the position of electrons.
covalent bond, representing a pair of shared electrons.
representation of a double bond, normally C=C or C=O. This shows the
presence of a s and a p
of a dative bond, donation of electrons to B by A.
A polar bond, B is more electronegative, so
bonding pair closer to B. Also shown using the d+
Yes, and so does electronegativity. The greater the
electronegativity, the greater the polarising power.
So for hydrogen halogen compounds:
Bond polarity has a huge hand in determining
The size mismatch of the anions (-ve) and cations (+ve)
is of huge importance also.
two ions are similar in size, then they exist quite happily
there is a size mismatch, then is it quite likely that covalent
bonding will occur.
melts at 801°C,
strong attraction between particles in solid lattice structure (Ionic
sublimes (goes from solid to gas not via the liquid phase) at 180°C,
so there are no strong attractions present (Covalent bonding likely).
In essence yes. Al3+ has a high charge
density (3+) and its very small. This gives it a high polarising power.
the cation is small and highly charged, it has a large polarising
the anion is large and has a relatively low charge, then it is said to
have a large polarisability.
If the above is the case, and the anion is being
polarised by the cation, there will be a degree of covalent character to
So the bonding in AlCl3 is virtually
Small highly charged cation + large easily polarised
anion = covalent character.
There are some ionic compounds that do not exist at
all. Aluminium carbonate is one such example.
aluminium 3+ cation is so small and highly polarising that is
completely distorts the large CO32- ion into
leaves instead of Al2(CO32-)3,
carbon dioxide is driven off, leaving aluminium oxide.
mismatch in size.
Now try the quick