1. Is this an outlier?
R2 =

2. Inter- and intramolecular forces
Watch the animation
Notice what happens to the water molecules when the water evaporates
Have the bonds between the atoms been broken?
Why have the molecules split up – what forces have been overcome?
Describe condensation and evaporation in terms of intramolecular (bonds between atoms) and intermolecular (forces between molecules) forces.
Explain why the gases in air have different boiling points

3. Covalent bonding Learning objective:
Understand how non-metals form chemical bonds with each other
Understand the difference between intermolecular forces and intramolecular forces (chemical bonds)
28/01/2018
Covalent bonding

4. Attractive forces between molecules are weak.
Small molecules are gases or liquids at room temperature.

5. The bonds inside molecules are strong.
water molecule
water molecule
The bonds inside molecules are strong.
Water molecules do not fall apart when the liquid boils and turns to steam.

6. How are the atoms in an H2 molecule held together?
To picture this you need to look at the structure of a hydrogen atom.
The positively charged nucleus and the negatively charged electron attract each other.

7. When two hydrogen atoms approach each other . . .
. . . the electron of one atom and the nucleus of the other atom start to attract each other.

8. When the atoms are close, the attractions are so strong that a molecule of hydrogen (H2) is formed.
The atoms are held together by the attractions between the two nuclei and the shared pair of electrons. This is a single covalent bond.

9. H—H
hydrogen, H2
O O
oxygen, O2
N N
nitrogen, N2
Chemists draw a line between element symbols to show a covalent bond.
Each line represents a shared pair of electrons
A pair of lines represents a ‘double bond’
Covalent bonds are strong
Forces between simple covalently bonded molecules are weak
O H H
water, H2O
O C O
carbon dioxide, CO2 H
H—C—H
methane, CH4
H H      
H—C—C—O—H
ethanol, C2H5OH

10. Molecules have a definite shape.
ethene
water
methane
methanol
Molecules have a definite shape.

11. H O N C

12. Chemists have different ways of describing molecules.
molecular formula
ball-and-stick model
displayed formula with lines for covalent bonds
space-filling model
Chemists have different ways of describing molecules.

13. How do non-metal atoms form covalent bonds?
Teacher notes
This illustration contains representations of non-metals forming covalent bonds.
It can be used as an introduction to the topic.
There are several discussion points relating to the topic:
The common room is only for non-metals – no metals allowed Covalent bonding only occurs between non-metals.
Ipod sharing Indicating the presence of covalent bonds. The ipods represent shared electrons. All the elements sharing ipods are happy indicating they are stable.
Ipod and drink sharing between oxygen atoms Indicating the presence of a double covalent bond.
Unstable non-metal atom not bonded to anything else Non-metals need to bond to become stable (apart from the noble gases).

14. Covalent bonding in hydrogen
Many non-metal elements, such as hydrogen, exist as simple diatomic molecules that contain covalent bonds.
How is a covalent bond formed in hydrogen? H H H H
Each hydrogen atom needs one more electron in its outer shell and so each atom shares its single unpaired electron.
This shared pair of electrons forms a covalent bond and so creates a diatomic molecule of hydrogen.
Some molecules contain double or triple covalent bonds. How are these are formed?

15. What are the types of covalent bonds?
Teacher notes
This three-stage animation shows how covalent bonds form between atoms of different elements. While showing the animation, the difference between single, double and triple bonds could be pointed out.

16. Can compounds contain covalent bonds?
Covalent bonding can also occur between atoms of different non-metals to create molecules of covalent compounds. These covalent bonds can be single, double or triple.
How is a covalent bond formed in hydrogen chloride (HCl, also represented as H–Cl)? Cl Cl H H
Teacher notes
It should be pointed out that the inner electron shells of chlorine have not been included in the diagrams above because they are not involved with the bonding. By just showing the shells that are involved in bonding, it is clearer to see what is happening.
Hydrogen and chlorine both need one more electron to fill outer shells. By sharing one electron each, they both have a stable outer shell and a covalent bond is formed.

17. Covalent bonding in water
Compounds can contain more than one covalent bond.
Oxygen (2.6) needs 2 more electrons, but hydrogen [1] only needs 1 more. How can these three elements be joined by covalent bonding? H O
The oxygen atom shares 1 electron with 1 hydrogen atom, and a second electron with another hydrogen atom.
What is the name of the molecule that is formed?
H2O (or H–O–H) is water.

18. How is the ratio of atoms calculated?
To calculate the ratio of atoms in a stable covalent compound:
1. Work out how many electrons are needed by each
non-metal element to complete its outer electron shell.
2. Work out the ratio of atoms that will provide enough
shared electrons to fill all the outer shells.
For example, how many nitrogen and hydrogen atoms bond together in an ammonia molecule?
element N H
electron configuration
(2.5)
(1)
electrons needed 3 1
ratio of atoms 1 3

19. Covalent bonding in ammonia
How do nitrogen and hydrogen atoms form covalent bonds in a molecule of ammonia?
element N H N
electron configuration
(2.5)
(1) H H
electrons needed 3 1
ratio of atoms H 1 3
NH3 or H N H H

20. Covalent bonding in methane
How do carbon and hydrogen atoms form covalent bonds in a molecule of methane? H
element C H
electron configuration
(2.4)
(1) C H H
electrons needed 4 1
ratio of atoms 1 4 H
CH4 or H C H H

21. Covalent bonding in carbon dioxide
How do carbon and oxygen atoms form covalent bonds in a molecule of carbon dioxide?
element C O
electron configuration O O
(2.4)
(2.6) C
electrons needed 4 2
ratio of atoms 1 2
double bonds
CO2 or O C O
A double bond is when two pairs of electrons are shared. In carbon dioxide there are two double bonds
– one between each oxygen atom and the carbon atom.

22. What are simple covalent structures?
Covalent molecules that contain only a few atoms are called simple covalent structures.
Most substances that contain simple covalent molecules have low melting and boiling points and are therefore liquids or gases at room temperature, e.g. water, oxygen, carbon dioxide, chlorine and hydrogen. Why?
The covalent bonds within these molecules are strong but the bonds between molecules are weak and easy to break.
weak bonds between molecules
strong bonds within molecules

23. What is the structure of a molecular solid?
A few substances that contain simple covalent molecules are solid at room temperature. These are molecular solids.
Iodine is a molecular solid at room temperature.
Two iodine atoms form a single covalent bond to become an iodine molecule.
The solid is formed because millions of iodine molecules are held together by weak forces of attraction to create a 3D molecular lattice.
Photo credit:
Dr John Mileham
Image of iodine crystals.
weak forces of attraction
What properties would you expect molecular solids to have with this type of structure?

24. What are the properties of molecular solids?
The properties of a molecular solid, such as iodine, are:
low melting and boiling points;
usually soft and brittle – they shatter when hit.
cannot conduct electricity.
Why do molecular solids have these properties?
The weak forces of attraction between the molecules can be broken by a small amount of energy. This means that the molecular solids are soft and brittle and melt and boil at low temperatures.
Photo credit: Dr John Mileham
Image of iodine crystals.
Molecular solids are also unable to conduct electricity because there are no free electrons or ions to carry a charge.

25. Covalent bonds – true or false?
Teacher notes
This true-or-false activity could be used as a plenary or revision exercise on covalent bonds, or at the start of the lesson to gauge students’ existing knowledge of the subject matter. Coloured traffic light cards (red = false, yellow = don’t know, green = true) could be used to make this a whole-class exercise.