1. Key learning
shapes of molecules and an explanation of their polar or non-polar character with reference to the electronegativities of their atoms and electron-pair repulsion theory

2. Intramolecular and Intermolecular Attractions
Intermolecular attractions are attractions between one molecule and a neighbouring molecule.
Intramolecular attractions are the forces of attraction which hold an individual molecule together for example the covalent bonds.

3. Shapes Of Molecules
VSEPR stands for valence-shell electron pair repulsion. It refers to a theory in which both bonding and non-bonding pairs of electrons in the valence shell of the atoms of a molecule repel other electrons pairs.
This determines the shape of the molecule because the electron pairs move as far from each other as possible in three dimensions.
For example:
2 bonding pairs (can be single, double or triple bond) of electrons will take up positions 180 apart, taking on a linear shape.
3 bonding pairs of electrons will be 120 apart, taking on a trigonal planar shape.
4 bonding pairs of electrons will be 109.5 apart, taking on a tetrahedral shape.
2 bonding pairs of electrons and 2 lone pairs of electrons,the bond angle closes slightly more to 104.5° due to the repulsion of the two lone pairs, this shape is described as bent linear or V-shaped.
In some circumstances there might be 4 bonding pairs of electrons (a double bond is needed) and 1 lone pair of electrons, this time the bond angle be 117  again this shape is described as V-shaped.
3 bonding pairs of electrons and 1 lone pair of electrons, gives a bond angle of 107° which is described as trigonal pyramidal shape.
Greatest repulsion: lone pair - lone pair
lone pair - bond pair
Least repulsion: bond pair - bond pair

4. VSEPR Theory
The shape of a molecule is determined by the number of groups of electrons around the central atom.
The 'groups' might be a non-bonding or bonding pair of electrons, a double pair of bonding electrons etc.
The electron 'groupings' repel to make the angles between them as wide as possible.

5. 3D shapes in 2D form H
represents a bond in the plane of the paper C
represents a bond in a direction behind the plane of the paper H H H
represents a bond in a direction in front of the plane of the paper

6. two groups of electrons around central atom
Usually two bonding pairs of electrons.
LINEAR shape
bond angle 180
BeCl2 and CO2 are examples
Draw a dot cross diagram to be sure
How is CO2 slightly different?

7. three groups of electrons around a central atom
three bonding pairs
TRIGONAL PLANAR shape
bond angle exactly 120
AlCl3 and BF3 are examples
Draw a dot cross diagram to be sure

8. H H C C H H Planar Molecules
Ethene is a good example of a planar molecule.
remember – it’s the number of groups of electrons that determine the shape not the number of electron pairs. H H C C H H
To predict the shape of a molecule you need to:
Count the number of bonding pairs of electrons
Count the number of non-bonding (lone) pairs of electrons

9. four groups of electrons around the central atom
four bonding pairs
TETRAHEDRAL shape
bond angle exactly 109.5
CH4 is an example
Draw a dot cross diagram to be sure

10. four groups of electrons around the central atom
three bonding pairs & one lone pair
TRIGONAL PYRAMIDAL shape
bond angle approx. 107 (lone pair has an ‘extra’ repulsive push)
NH3 is an example
Draw a dot cross diagram to be sure
Why is ammonia not tetrahedral?

11. four groups of electrons around the central atom
two bonding pairs & two lone pairs
V-SHAPED or BENT shape
bond angle approx (lone pairs have an ‘extra’ repulsive push)
H2O is an example
Draw a dot cross diagram to be sure

12. Effect of lone pairs on bond angles
Boardworks AS Chemistry
Structure and Shape
Effect of lone pairs on bond angles
Teacher notes
It may be worth pointing out to students that the dotted line represents a bond extending behind the plane of the screen, and the wedge-shaped bond represents a bond extending in front of the plane of the screen.

13. What is electronegativity?
Boardworks AS Chemistry
Bonding and Intermolecular Forces
shapes of molecules and an explanation of their polar or non-polar character with reference to the electronegativities of their atoms and electron-pair repulsion theory
What is electronegativity?
In a covalent bond between two different elements, the electron density is not shared equally.
This is because different elements have differing abilities to attract the bonding electron pair. This ability is called an element’s electronegativity.
Teacher notes
Elements in group 8 do not commonly form bonds so electronegativity values have not been measured.
Electronegativity was first proposed in 1932 by the American chemist Linus Pauling ( ). It is after him that the scale used here for measuring electronegativity is named. While the Pauling scale is the most common, there are other measures of electronegativity in use.
Electronegativity values for some common elements. Values given here are measured on the Pauling scale.

14. Electronegativity and atomic radius
Boardworks AS Chemistry
Bonding and Intermolecular Forces
Electronegativity and atomic radius
The electronegativity of an element depends on a combination of two factors:
1. Atomic radius
As radius of an atom increases, the bonding pair of electrons become further from the nucleus. They are therefore less attracted to the positive charge of the nucleus, resulting in a lower electronegativity.
Teacher notes
See the ‘Trends in Period 3’ presentation for more information about atomic radius.
Note that the inner electron levels have not been included in the diagram.
higher electronegativity
lower electronegativity

15. Electronegativity, protons and shielding
Boardworks AS Chemistry
Bonding and Intermolecular Forces
Electronegativity, protons and shielding
2. The number of unshielded protons
The greater the number of protons in a nucleus, the greater the attraction to the electrons in the covalent bond, resulting in higher electronegativity.
However, full energy levels of electrons shield the electrons in the bond from the increased attraction of the greater nuclear charge, thus reducing electronegativity.
greater nuclear charge increases electronegativity…
Teacher notes
See the ‘Trends in Period 3’ presentation for more information about shielding.
…but extra shell of electrons increases shielding.

16. Electronegativity trends: across a period
Boardworks AS Chemistry
Bonding and Intermolecular Forces
Electronegativity trends: across a period
Electronegativity increases across a period because:
1. The atomic radius decreases.
Teacher notes
Fluorine has, with a Pauling electronegativity value of 4.0, the highest electronegativity of all the elements.
2. The charge on the nucleus increases without significant extra shielding. New electrons do not contribute much to shielding because they are added to the same principal energy level across the period.

17. Electronegativity trends: down a group
Boardworks AS Chemistry
Bonding and Intermolecular Forces
Electronegativity trends: down a group
Electronegativity decreases down a group because:
1. The atomic radius increases.
2. Although the charge on the nucleus increases, shielding also increases significantly. This is because electrons added down the group fill new principal energy levels.

18. Electronegativity and the Covalent Bond
shapes of molecules and an explanation of their polar or non-polar character with reference to the electronegativities of their atoms and electron-pair repulsion theory
Electronegativity and the Covalent Bond
Remember ELECTRONEGATIVITY?
INCREASES across periods (left to right) DECREASES down groups (top to bottom)
Electrons are shared in these compounds because there is a medium to small difference in electronegativity of the atoms associated with a bond.
The attraction of these shared electrons to the nucleus of each of the atoms forms the covalent bond. This attraction acts as the ‘glue’ that holds the atoms together.

19. Electronegativity
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.
What happens if two atoms of equal electronegativity bond together?
Consider a bond between two atoms, A and B. Each atom may be forming other bonds as well as the one shown - but these are irrelevant to the argument.
If the atoms are equally electronegative, both have the same tendency to attract the bonding pair of electrons, and so it will be found on average half way between the two atoms. To get a bond like this, A and B would usually have to be the same atom. You will find this sort of bond in, for example, H2 or Cl2 molecules.

20. Polar Bonds What happens if B is slightly more electronegative than A?
B will attract the electron pair rather more than A does.
That means that the B end of the bond has more than its fair share of electron density and so becomes slightly negative. At the same time, the A end (rather short of electrons) becomes slightly positive.
This is described as a polar bond. A polar bond is a covalent bond in which there is a separation of charge between one end and the other - in other words in which one end is slightly positive and the other slightly negative.

21. Boardworks AS Chemistry Bonding and Intermolecular Forces
Polar bonds

22. Effect of electronegativity on polarization
Boardworks AS Chemistry
Bonding and Intermolecular Forces
Effect of electronegativity on polarization
The greater the electronegativity difference between the two atoms in a bond the greater the polarization of the bond.
This can be illustrated by looking at the hydrogen halides:
Pauling elecronegativities
Element H F Cl Br I
2.2
4.0
3.2
3.0
2.7
Molecule
Electronegativity difference between atoms
H–F
H–Cl
H–Br
H–I
1.8
1.0
0.8
0.5
decreasing polarization

23. Boardworks AS Chemistry Bonding and Intermolecular Forces
Ionic or covalent?
Rather than saying that ionic and covalent are two distinct types of bonding, it is more accurate to say that they are at the two extremes of a scale.
Less polar bonds have more covalent character.
More polar bonds have more ionic character. The more electronegative atom attracts the electrons in the bond enough to ionize the other atom.
increasing polarization

24. Boardworks AS Chemistry Bonding and Intermolecular Forces
Non-polar bonds
If the electronegativity of both atoms in a covalent bond is identical, the electrons in the bond will be equally attracted to both of them.
cloud of electron density
This results in a symmetrical distribution of electron density around the two atoms.
Bonding in elements (for example O2 or Cl2) is always non-polar because the electronegativity of the atoms in each molecule is the same.
both atoms are equally good at attracting the electron density

25. Boardworks AS Chemistry Bonding and Intermolecular Forces
Polar molecules
Molecules containing polar bonds are not always polar.
Non-polar molecules
Polar molecules
If the polar bonds are arranged symmetrically, the partial charges cancel out and the molecule is non-polar.
If the polar bonds are arranged asymmetrically, the partial charges do not cancel out and the molecule is polar.

27. Identifying polar molecules
Boardworks AS Chemistry
Bonding and Intermolecular Forces
Identifying polar molecules