1. 3 bond pairs and 1 lone electron pair
Shape slap
3 bond pairs and 1 lone electron pair
Bond angle 90o
6 pairs of electrons
PCl5
:PH3
AlCl3
pyramidal
methane
octahedral
SF6
Bond angle 107o
SO2
water
CO32-
BH3
Bond angles 120o
tetrahedral
Trigonal bipyramidal

2. To explain trends in the boiling points of noble gases and alkanes
Tb/K
2He 4
10Ne 27
18Ar 87
36Kr
120
54Xe
165
86Rn
211

3. Boiling point against atomic number for the noble gases
Tb/K
Atomic number

4. Questions What is the trend in boiling points for the noble gases?
Explain the trend in terms of the relative strength of the intermolecular (well in this case interatomic) forces
Given that gravity is a negligible intermolecular force why is it surprising there are any forces between these elements?

5. Modelling a helium atom
What does this tell you about a helium atom?
Draw a helium atom
List three things that your drawing doesn’t show very well

6. Explaining the attractions
In the middle of a piece of paper draw the nucleus of a helium atom
Use two ‘bonds’ to represent the electrons
Move the electrons around randomly until told to stop
STOP!
Put the ‘electrons’ on the sheet and mark (i) their positions and (ii) the midway point between them – this is the atom’s centre of negative charge at this moment.
Where is the centre of positive charge?
Repeat the movement…
…STOP!

7. Explaining the attractions
What do you notice about the centre of positive and negative charges in your atom?
Line up your atoms and repeat the movements…
…STOP!
Look at the surrounding atoms
Where are the centres of positive and negative charge?
What effect would they have on your electrons?
What effect would they have on your atom?
Repeat the movements once more…
Can you explain the trend in boiling points down group 0?

8. London Forces Some definitions Dipole – separation of charge δ+ δ-
δ δ-
Temporary dipole – a separation of charge caused by the random movements of negative electrons around the positive nucleus
Induced dipole – a separation of charge caused by a nearby dipole
London forces (often called van der Waals’ forces): weak attractions between all molecules caused by attractions between temporary induced dipoles
A task
Use these terms to explain (i) why there are forces between helium atoms and (ii) why these forces increase with atomic number

9. London Forces and alkanes
Tb/K
CH4
109
C2H6
185
C3H8
231
C4H10
273
C5H12
309
C6H14
342
C7H16
372
C8H18
399
C9H20
424
C10H22
447
2-methylpentane
333
2,3-dimethylbutane
331

10. Boiling points of the alkanes
Plot a graph of boiling point against number of carbons for the first ten unbranched alkanes
Add a trendline
Describe the trend in boiling points
Explain the trend in terms of London Forces
Add points for the two named compounds
Make models of the three isomers of C6H14 and use them to suggest reasons for the difference in boiling points.
Use skeletal formulae in your written explanation

11. Spot the mistake!
Chemistry - a volatile history(50 minutes in)