4.1 When solid, simple molecules form a
simple molecular lattice - a regular
structure where the molecules are held
close together by weak intermolecular
forces. The covalent bonds are very strong.
4.1.1 When heat energy is given to a molecule, the
intermolecular forces break down NOT THE
184.108.40.206 Weak intermolecular forces require less energy to
4.2 Simple molecules: They all have strong covalent
bonding between atoms which aren't broke n by
melting or boiling. They also have weak IMFs
whether they are London forces or permanent
dipole-dipoles. As they are weak, melting and boiling
points are low.
4.2.1 Solubility: Non-polar molecules are soluble in non- polar solvents
e.g.F2,CL2.IMFs between the molecules in the simple molecular lattice break
and new IMFs form between the molecules and the solvent molecules.
Non-polar molecules are insoluble in polar solvents (water) as IMFs between
the H20 molecules are too strong to be broken by non-polar molecules.
220.127.116.11 H20 is a polar solvent so many ionic compound can
dissolve in it e.g. NaCl.
18.104.22.168.1 The H20 molecules are attracted to the
Na+ and Cl- ions. The ionic lattice breaks
down as it dissolves. When the ions are in
solutions, the ions are surrounded by
water molecules. The ions then become
4.3 Polar Molecules
4.3.1 They are soluble in polar solvents as
the polar solute molecules and the
polar solvent molecules attract each
22.214.171.124 A permanent dipole-dipole interaction is
formed between the Cl and H doe to polar
4.4 They don't
as there are no
4.5 Intermolecular Forces (IMFs)
4.5.1 Covalent and ionic bonds are very strong
but when heated, it is the intermoleculr
forces that break.
4.5.2 Intermolecular forces are weak attractive forces vetween
dipoles of different molecules.
126.96.36.199 They effect the physical
properties. Covalent bonds
determine the chemical
4.5.3 There are 3 types of intermolecular forces
188.8.131.52 Induced dipole-dipole interaction: These exist between all molecules and in inert gas atoms whether they are polar or non-polar. They
can also be called london forces or dispersion forces. They occur due to an instantaneous dipole which induces neighbouring dipoles
causing them to attract eachother. They are only temporary and disappear or reappear in a different direction.
184.108.40.206.1 The strength of these forces depend on the circulating electrons.
220.127.116.11.1.1 The more electrons in a molecule: The larger the instantaneous and induced
dipoles, the greater the induced dipole-dipole interactions, the stronger the
attractive forces between the molecules, the more energy needed to break the
IMFs, the higher the MP and BP.
18.104.22.168 Permanent dipole-dipole interactions: Molecules with polar bonds that have
permanent dipoles which give rise rise to permanent dipole-dipole interactions
between the molecules.
22.214.171.124.1 These are the bonds that break and
require the extra energy to break.
126.96.36.199 Hydrogen Bonding
188.8.131.52.1 This is a special type of permanent dipole-dipole
interactions between molecules containing: a very
electronegative atom with a lone pair of electrons, either
O,N or F. Bonded to a H atom so either a O-H, N-H, F-H bond.
The bond forms between a lone pair of eectrons on one
molecule and the H atom on a different one.
184.108.40.206.1.1 It is therefore the strongest
type of intermolecular
220.127.116.11.2 It is shown with a dotted line:
18.104.22.168.2.1 The bonding gives water anomalous properties e.g. ice is less dense than
liquid water as the H bonds hold the H20 molecules apart in an open
lattice and in ice there are 4 H bonds in a molecule.
22.214.171.124.2.1.1 This means ice floats on water. On melting some H
bonds break and the strcuture collapses.
126.96.36.199.2.1.2 Water has quite high MP+BP. If the only IMf is london forces, the
BP would be -75 without the H bonds meaning there would be no
life on earth. The H bonds water does have are however strong and
require lots of energy to break them so the actual BP is much
higher at 100.
188.8.131.52.2.1.3 It also has a high viscosity (thick) which is the ability of molevules to move
past each other. It also has a high surface tension.
184.108.40.206 Simple Covalent Bonding
220.127.116.11.1 They are made up of simple molecules. They are particles with a
definate number of atoms with a definate molecular fomrmula e.g.
4.5.4 Strongest to weakest: Induced
dipole-dipole interaction, permanent
dipole-dipole interaction, hydrogen
bonding, single covalent bonfing.
5.1 The bonded pair is evenly
shared when the shared pair
electrons are attracted to the
nuclei of the same elemnt.
5.1.1 The bonded pair will be shared unevenly when:
- The nuclear charges are different -The atoms
are different sizes - The shared pair are closer
to one nucleus that another.
18.104.22.168 The result will be a polar covalent bond.
22.214.171.124 The attraction of a bonded
atom for the pair of electrons
in a covalent bond is called
126.96.36.199.1 The higher the attraction the
higher the value of
electronegativity (0-4), there are
188.8.131.52.1.1 In the periodic table, (across to the right form the left and
upwards) the nuclear charge increases (no. of protons). The
atomic radius decreases (as electrons are pulled closer in by
increases in the number of protons.)
184.108.40.206.1.2 Non- metals are most electronegative e.g. Cl
220.127.116.11.1.2.1 Group 1 metals are least electronegative e.g. K
5.2 The difference in electronegativity
of a compound shows if its ionic,
covalent or polar.
5.2.1 The greater the electronegativity
difference between the bonded atoms,
the more ionic the bond will be.
18.104.22.168.1 Covalent - The electrons
are equally shared.
22.214.171.124.2 Polar covalent - The electrons are
126.96.36.199.3 Ionic - The electrons are transferred.
188.8.131.52.4 Polar bonds bonded electron
pair are unequally shared as
the atoms have different
electronegativities so the bond
184.108.40.206.4.1 e.g. HCl - H has a small amount of
positive charge and Cl has a small bit of
negative which is shown with a delta
220.127.116.11.4.1.1 The charge separation is called a dipole. This
dipole doesn't change so is called a
permanent dipole. This is now a polar
18.104.22.168.22.214.171.124 The direction of the dipole is positive
126.96.36.199.188.8.131.52.1 e.g. H20. It has 2 permanent dipoles and as
H20 is a symmetrical, non-linear molecule, the
overall one is in the middle. this makes water a
very polar molecule.
184.108.40.206.220.127.116.11 Polar molecules with one permanent dipole. For
molecules with more than 2 atoms, there may be polar
bonds. The shape fo the molecule determines if the
dipoles can add together to produce a larger dipole over
the whole molecule o0r if they cancel eachtoher outif they
are in opposite directions.
18.104.22.168.22.214.171.124 CO2 has 2 permanent dipoles but the shape is linear so
they cancel each other out sot theres no overall dipole
meaning the molecule is non-polar.
126.96.36.199 Non- polar bonds are
where shared electrons
are equally shared
between bonded atoms
(covalent) so have little
or no electronegativity.
188.8.131.52.1 Non- polar bonds produce non-