1. Unit 2.2 - Intermediate bonding and polarity
Electronegativity
• Electronegativity is a very important word and concept for us as chemists
• You need to know and understand what it means:
Electonegativity is a measure of the attractive force of a specific atom in a molecule for a
pair of electrons in a covalent bond.
Example:
Iodine (I 2 ) Iodine fluoride (IF)
Measuring the electronegativity of an atom
• Electronegativity is measured on the Pauling scale
• The scale runs from 0 to 4 and has no units
• The higher the number the ______________ the
electronegativity and the _______ strongly the electrons in a
bond are attracted to that atom
Visualisation of electronegativites
of the elements
Trends in electronegativity

2. • The most electronegative element is…
NB You need to
• The least electronegative element is… know these trends!
• Non-metals have ______________ electronegativities than metals
• Electronegativity ______________ across a period
• In the main block (not including the TMs) electronegativity _______________ down a group
Intermediate bonding
• Ionic bonding is…
• Covalent bonding is…
Covalent and ionic bonding are the two extremes of bonding. In practice bonds generally occur as
something between the two or intermediate:
Ionic bond Polar ionic bond Covalent bond
How ionic/covalent is a bond? (Bond character)
Deciding how much ionic and covalent character a specific bond has is easily done using this table:
Difference in Percentage Percentage
electronegativity ionic covalent
character character
Covalen

3. 0.1 0.5 99.5
0.2 1 99
0.3 2 98
0.4 4 96
0.5 6 94
0.6 9 91
0.7 12 88
0.8 15 85
0.9 19 81
1.0 22 78
1.1 26 74
1.2 30 70
1.3 34 66
1.4 39 61
1.5 43 37
1.6 47 33
1.7 51 49
1.8 55 45
1.9 59 41
2.0 63 37
2.1 67 33
2.2 70 30
2.3 74 36
2.4 76 24
2.5 79 21
2.6 82 18
2.7 84 16
2.8 86 14
2.9 88 12
3.0 89 11
3.1 91 9
3.2 92 8
Ionic
Questions on bond character
1. Calculate the percentage of ionic and covalent bonding in the following compounds and add
them to the diagram on the previous page:
a. Lithium fluoride (LiF)
b. Methane (CH4)
c. Calcium bromide (CaBr2)
2. Which of the following compounds has the most covalent character?
HF HCl HBr HI
Polar covalent bonds
• Covalent bonding involves the ________________ of electrons between two or more atoms
• The electrons are not always shared equally:

4. Hydrogen (H 2 ) Hydrogen chloride (HCl)
H- H H- Cl
• Because chlorine is ________ electronegative than hydrogen it _____________ electrons
more strongly
• The electrons in the bond are pulled more closely towards ________________ distorting the
bond
• This results in chlorine having a partial _______________ charge (-0.17)
• Hydrogen also has a partial charge, but it is _________________ (+0.17)
• We indicate partial charges using the lower case Greek letter delta (δ+ or δ-)
δ+
Polarity of bonds and molecules
We have already seen that, depending on the difference in _______________
between the two atoms involved, the pair of ____________ in a covalent bond
can be shared unequally . This leads to molecules, which have an uneven
distribution of charge . We call such molecules polar molecules .
Spotting polar molecules: δ-
N- N H- F
Nitrogen (N2) Hydrogen fluoride (HF)
Methane (CH4) Water (H2O)
Benzene (C6H6) Methanol (CH3OH)

5. Finding polar molecules experimentally
You should now undertake the core practical (2.1), which tests several
liquids to see if they are deflected by an electric field.
1. Make a table below with two columns ‘polar’ and ‘non-polar’
2. Draw in displayed formulae for the compounds tested in the
appropriate column
jet of water being
3. Add partial charges (δ+ and δ-) to the polar molecules pulled to one side
+ + + + + + + +
Measuring polarity - Dipoles and dipole moments
• Polar molecules have a ______________ ‘end’ and a ________________ ‘end’
• Because of these two ‘poles’ we call them dipoles
• The polarity (how polar it is) of a molecule is measured by its dipole moment
• Dipole moment is given by the amount of charge separation multiplied by the distance
between the centres of charge

6. • Dipole moment is measured in Debye (D)
Slightly trickier examples of polar and non- polar molecules
Polar Non- polar
Cyclohexene Cyclohexane
Trichloromethane Tetra chloromethane
Boron trifluoride
Quick checklist - Is it polar?
Draw out the molecule
Are there any electronegative atoms? Look out for O, OH, N, Cl
and F
Add the partial charges
Is one end positive and the other negative? Watch out for
symmetrical molecules, they can’t be polar!