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 (I2) 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

Trends in electronegativity

Visualisation of electronegativites of the elements

The most electronegative element is…

The least electronegative element is…

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:

NB You need to know these trends!

Difference in electronegativity

Percentage ionic character

Percentage covalent character

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

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)

Ionic

Covalent

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:

Hydrogen (H2) 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 (+ or -)

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.

+

-

jet of water being pulled to one side

+ + + + + + + +

Spotting polar molecules:

N-N H-FNitrogen (N2) Hydrogen fluoride (HF)

Methane (CH4) Water (H2O)

Benzene (C6H6) Methanol (CH3OH)

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

3. Add partial charges (+ and -) to the polar molecules

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

Dipole moment is measured in Debye (D)

Slightly trickier examples of polar and non-polar molecules

Polar Non-polar

Cyclohexene Cyclohexane

Trichloromethane Tetrachloromethane

Boron trifluoride

Quick checklist - Is it polar?Draw out the moleculeAre there any electronegative atoms? Look out for O, OH, N, Cl and FAdd the partial chargesIs one end positive and the other negative? Watch out for symmetrical molecules, they can’t be polar!