1. Starter: Spot the errors

2. Answers

3. What is electronegativity?

4. Electronegativity
A measure of the ability of an atom in a covalent bond to attract the electron bonding pair.

5. Pauling Scale A scale for measuring electronegativity.
What do you notice about the electronegativity across the periods?
What do you notice about the electronegativity down the groups?
Which element has the highest electronegativity?

6. Pauling Scale Values increase across periods
Values decrease down groups.
Fluorine has the highest value.

7. Why do we have this trend?
Electronegativity increases across a period as the nuclear charge on the atoms increases but the shielding stays the same, so the electrons are more strongly attracted to the atom.
Electronegativity decreases down a group as the number of shells increases, so shielding increases and the electrons are less strongly attracted to the atom.

8. Values
An atom which has a high electronegativity is said to be electronegative; an atom which does not have a high electronegativity is said to be electropositive.
Electronegativities are relative; electronegativity has no units and is measured on a scale from 0.7 to 4.0
Noble gases cannot be ascribed an electronegativity since they do not form bonds

9. Electronegativity and bond polarity

10. Bonds between identical atoms
In a covalent bond, electrons are shared.
When the 2 atoms are identical, the electrons are shared equally.
H-H
Cl-Cl

11. Bonds between different atoms
What do you think happens when different atoms form bonds?

12. Bonds between different atoms
Different atoms have different electronegativities.
One will pull the electrons closer to its end
it will have a slight negative charge δ-.
The other will have a slight positive charge δ+ . δ+ δ-
H-Cl

13. Bonds between different atoms
This leads to a charge difference called a dipole.
Dipoles can be permanent.
The greater the difference in electronegativity the greater the polarity of the bond. δ+ δ-
H-Cl

15. Differences in electronegativity
If the difference is greater what type of character do you think will form? δ+ δ-
H-Cl

16. Differences in electronegativity
Large difference between the electronegativities of the bonding atoms, the greater the ionic character of the bond.
The greater the similarity between the electronegativities of the bonding atoms, the greater the covalent character. δ+ δ-
H-Cl

17. Differences in electronegativity
Predict the polarity in the following bonds; where applicable, draw in the δ+ and δ−
a) S—Cl
b) S—O
c) N—O
d) C—O
e) F—Cl
f) C—Cl
Element
Electronegativity Cl
3.16 O
3.44 N
3.04 F
3.98 C
2.55 S
2.58

18. POLAR and NON-POLAR molecules.
For molecules that are symmetrical , the dipoles of any bonds within the molecule can cancel out. Tetrachloromethane is a non-polar molecule despite having bonds with polar bonds.
Each C-Cl bond is polar but as CCl4 is a symmetrical molecule and the dipoles act in different directions they cancel each other out.

19. Which of the following molecules are polar?
CH3Cl
H2S
C2H5OH
NH3

20. Van der Waals forces
Weak intermolecular forces made up of dipole-dipole and induced dipole-induced dipole forces of attraction.

21. What causes van der Waals’ forces?
The movement of electrons in shells unbalances the distribution of charge within the electron shells. At any moment there will be an instantaneous dipole across the molecules. The instantaneous dipole induces a dipole in neighbouring molecules which in turn induce further dipoles on their neighbouring molecules.
Induced dipole δδ
We use 1 δ symbol to represent permanent dipoles and 2 δδ to represent fluctuating ones.
Random dipole δδ

22. Strengths of different types of bond.
Bond type
Relative strength
Ionic and covalent bonds
1000
Hydrogen bonds 50
Dipole-dipole forces 10
Van der Waals’ forces 1

23. A bit of biology!
A gecko’s toes are covered with microscopic hairs that create van der Waal’s forces. It is the combined attraction from the great number of hairs that allows the gecko to stick to virtually any surface , even polished glass.

24. Boiling points and van der Waal’s
Noble gas
Boiling point (˚C)
Number of electrons He
-269 2 Ne
-246 10 Ar
-186 18 Kr
-153 36 Xe
-108 54 Rn
-62 86
Electrons increase
Van der Waals’ forces increase
Boiling point increases

25. Intramolecular bonding effects chemical properties e. g. reactivity
Intramolecular bonding effects chemical properties e.g. reactivity. Intermolecular bonding effects physical properties e.g. boiling points.

26. Plenary:
The boiling point of the Group 7 elements are shown below. Each element exists as diatomic molecules.
F ˚C
Cl ˚C
Br ˚C
I ˚C
Explain this trend, in terms of intermolecular forces.