1. Learning objective:
highlight some of the special properties of water
explain why the sea is salty
describe what happens to the ions as salts crystallize from solution
28/04/2018

2. 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).

3. 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?

4. 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.

5. 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.

6. 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.

7. 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

8. 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

9. 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

10. 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.

11. 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

12. 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?

13. 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.

14. 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.

15. Crystallisation
You are going to evaporate a solution with two dissolved ionic salts:
chrome alum, KCr(SO4)2.12H2O, which is coloured
magnesium sulphate, MgSO4.7H2O, which is colourless
These colours have been chosen to make your observations easier.

16. To do
Put on eye protection.
1 Pour approximately 40 cm3 of the solution of salts into an evaporating dish.
2 Half fill a 250 cm3 glass beaker with water and place this on a tripod and gauze.
3 Place the evaporating dish on top of the beaker.
4 Light the Bunsen and bring the water in the beaker to the boil. Then adjust the flame so that it gently simmers. The steam produced will heat the evaporating basin.
5 Keep heating until the volume of the salt mixture has fallen to about half its original volume. Turn off the Bunsen and allow the apparatus to cool.
6 When the evaporating basin is cool enough to handle, place it on a heat proof mat to cool further.
a Observe the colour of any crystals that form.
b Leave the dish to dry out completely and observe the crystals again.

17. To answer
7 Which of the two dissolved salts started to crystallize out first? How could you tell?
8 When no more salt can dissolve we say a solution is saturated. Once a solution is saturated, removing more water will make crystals appear. Which of the two salts in your mixture produced a saturated solution first?
9 Two factors could affect which salt produced a saturated solution first. One factor is how much of each salt was present. What might the other factor be? (Remember – not all salts are equally soluble.)

18. Timetable
Week 1: Research information for the extraction of each metal
Week 2: Draft information section of report
Week 3: Peer review in lesson. Develop comparisons
Week 4: Complete task

19. Peer review C
Showed good knowledge and understanding of extraction methods and how this relates to the reactivity series.
Explained simply why aluminium is cheaper than gold, with reference to the extraction method and abundance.
Used word equations of any reactions where appropriate.
Used simple chemical symbols.
Discussed a range of ethical issues to do with digging for gold.
Used correct units where appropriate.
Used most key words correctly.
Written your ideas in all your own words, using proper sentences.
Used at least two sources of information to find out about extraction of metals. A
Showed detailed knowledge and understanding of extraction methods and how this relates to the reactivity series.
Used detailed labelled diagrams to explain metal extraction methods.
Compared why some metals are cheaper than others with reference to the extraction method and abundance.
Used balanced symbol equations of any equations where appropriate.
Discussed in detail the ethical issues to do with metal extraction around the world.

20. Water and salts in the hydrosphere
Complete ‘Salts and evaporite minerals’ sheet

21. weathering of rocks followed by dissolving of soluble salts
condensation
Sun
rain
evaporation
rivers
solutions of soluble salts
and sediment end up in the sea

22. What is so special about water and water molecules?
Liquid water consists of H2O molecules that are close together but moving randomly.

23. Water is a liquid at room temperature.
Water is a good solvent even for salts.

24. When NaCl is added to water, the ions are pulled off the structure and they become free to move in the water. +
The ions can move independently, so a solution of NaCl in water conducts electricity.

25. Salt harvesters in Thailand raking salt into piles

26. Machinery at work inside a salt mine

27. The IONS are forming a GIANT IONIC lattice
Crystals of sodium chloride
A sodium chloride crystal is made up of millions of Na+ and Cl– ions arranged in a regular pattern.
The regular pattern is responsible for the cubic range of all sodium chloride crystals.
The IONS are forming a GIANT IONIC lattice
This is called IONIC BONDING

28. Oppositely charged ions attract each other.
Each ion will attract oppositely charged ions from all directions.
Each of these ions will in turn attract more ions.
A giant ionic structure made up of millions of ions is built up and forms a sodium chloride crystal.
Ionic compounds are giant lattices – they do not form molecules
Saying a ‘molecule of sodium chloride’ is wrong