1. STRUCTURES
Module C2

2. Covalent Bonding Describe, simply, how atoms form covalent bonds (P)
List the properties of simple covalent compounds and giant covalent structures (P)
Construct dot and cross diagrams for single covalent bonds (hydrogen, chlorine, fluorine, water) (P)
Construct dot and cross diagrams for double covalent bonds (oxygen, carbon dioxide) (M)
Explain the properties of simple covalent compounds (M)
Construct dot and cross diagrams of complex covalent compounds including triple bonds (D)
Explain the properties of diamond and graphite (D)

3. eg carbon dioxide, water
PURE substances have different STRUCTURES depending on the type of BONDING they have
IONIC
eg sodium chloride (salt)
COVALENT
METALLIC
eg copper
SIMPLE MOLECULAR
eg carbon dioxide, water
GIANT MOLECULAR
eg diamond, graphite
The structure of a substance decides what its PHYSICAL PROPERTIES will be.

4. SIMPLE MOLECULAR SUBSTANCES
These are substances like carbon dioxide CO2, water H2O and methane CH4 which are always made of simple molecules whether they are SOLIDS, LIQUIDS OR GASES
H atom
O atom
Whole thing
= H2O molecule

5. MOLECULES ONLY WEAKLY ATTRACT EACH OTHER
VERY STRONG bonds BETWEEN ATOMS
(so molecule is very hard to break up)
WEAK bonds BETWEEN MOLECULES
(so molecules are easy to separate)

6. SOLID
LIQUID
GAS
Simple molecular substances can only be a liquid or a solid when the temperature is LOW enough for the WEAK BONDS to pull the molecules together

7. This means simple molecular substances have LOW melting and boiling points
Compound
Mpt (°C)
Bpt (°C)
State at room temp
Water H2O
100
Liquid
Butane C4H10
-138
-0.5
Gas
Methane CH4
-182
-164
Carbon dioxide CO2 -
-78
Oxygen O2
-218
-183
Hydrogen H2
-259
-252
Liquid oxygen boiling at -183°C
Solid oxygen at -240°C

8. As the bonds between the molecules are weak, simple molecular substances are weak and soft when solid.
As the molecules are NOT CHARGED simple molecular substances DON’T CONDUCT ELECTRICITY when solids, liquids or gases.

9. Every C atom joined to 4 others
GIANT MOLECULAR SUBSTANCES
In these materials strong covalent bonds join atoms together with other atoms of the same type to make giant structures, rather than little groups.
Every C atom joined to 4 others
DIAMOND
Carbon atom
Only STRONG bonds
(this is only part of the structure - the same pattern carries on in every direction)

10. Every Si atom joined to 4 O atoms
SILICA (Silicon dioxide SiO2) has a similar structure to diamond
Every Si atom joined to 4 O atoms
Silica is the main substance in ROCKS. Pure silica is called QUARTZ

11. Because all the atoms in Giant Structures are joined by STRONG BONDS they:
Have HIGH melting / boiling points
Are usually HARD and STRONG
Because all the atoms in Giant Structures are UNCHARGED, they will not conduct electricity.

12. GRAPHITE – a special case
Common form of carbon found in soot, charcoal, pencil leads etc
Carbon atoms each joined to 3 others with STRONG bonds to make hexagonal sheets of atoms
WEAK BONDS
STRONG BONDS
The sheets of atoms are joined to other sheets by WEAK bonds

13. As the bonds between the layers of atoms are weak, they can easily slide over each other
As the C atoms are only bonded to 3 others, the extra electrons form clouds of ‘free electrons’ between the layers

14. GRAPHITE - Properties
The STRONG BONDS between the ATOMS mean it has HIGH MELTING and BOILING POINTS
The WEAK BONDS between the LAYERS mean it is SOFT and SLIPPERY as the layers SLIDE over each other easily (used in pencils and as a solid lubricant)
The FREE ELECTRONS between the layers mean that graphite CONDUCTS ELECTRICITY (used as sliding contacts in electric motors)

15. IONIC
Ionic substances are compounds of metals and non-metals (eg sodium chloride, copper oxide, magnesium sulphide etc)
They are made of IONS: atoms which have lost or gained electrons giving them a positive or negative CHARGE
Positive sodium ion Na+
Negative chloride ion Cl-
The + ions and – ions STRONGLY ATTRACT each other to make a regular crystal structure

16. Because of the very STRONG BONDS between the IONS, ionic compounds have HIGH MELTING & BOILING POINTS
Strong ionic bond
Sodium chloride melts at over 800°C

17. Ionic compound Melting point (°C)
Iron chloride
677
Potassium chloride
770
Sodium chloride
801
Copper oxide
1446
Calcium oxide
2707

18. As ionic compounds are made of CHARGED IONS, they can CONDUCT ELECTRICITY but ONLY if the ions can MOVE.
If it is MOLTEN the ions can move
If it is DISSOLVED the ions can move - + + -
MELT + -
DISSOLVE
H2O
800°C
20°C

19. + - - - - - - - MOLTEN IONIC COMPOUND + + + + + + +
MOLTEN ionic compounds CONDUCT ELECTRICITY

20. When salt is put in water, H2O molecules pull the ions apart to make a solution. This lets the ions move around.
H20 molecule
Ions free to move around

21. DISSOLVED ionic compounds also CONDUCT ELECTRICITY
PURE WATER
SOLID SALT
SALT SOLUTION
DISSOLVED ionic compounds also CONDUCT ELECTRICITY

22. METALS
In a metal the atoms are held together by strong bonds in regular structures.
This means most metals have high melting and boiling points and are hard and strong

23. In a metal the atoms LOSE SEVERAL OF THEIR OUTER ELECTRONS which drift around between the metal ions as FREE ELECTRONS.
As they have LOST a few electrons, the atoms become POSITIVE IONS
Free (“delocalised”) electrons

24. The large number of free electrons makes all metals are GOOD CONDUCTORS of electricity AND heat.
The regular structure means the layers of atoms can fairly easily slide over each other without breaking the bonds (though not as easily as graphite) and so metals are MALLEABLE (bend rather than snap)

25. SUMMARY - Descriptions
METALLIC
Strong malleable solids
Don’t dissolve
eg copper
IONS ONLY
IONIC
Crystals
Dissolve in water
eg sodium chloride (salt)
IONS +
FREE ELECTRONS
SIMPLE MOLECULAR
Usually Gases
eg CO2, H2O
ATOMS joined into GIANT MOLECULES
GIANT MOLECULAR
Hard strong solids
Don’t dissolve
eg diamond
(graphite – special case)
MOLECULES ONLY

26. SUMMARY - PROPERTIES Structure Property Reason Ionic HIGH mpt/bpt
CONDUCT: Solid NO
Molten YES Dissolved YES
Strong bonds between IONS
Ions can’t move
Ions can move to carry current
Covalent –
Simple molecular
LOW mpt/ bpt (often gas at room temp). Soft when solid
CONDUCT: Never
Bonds between MOLECULES very weak.
Molecules aren’t charged
Covalent – giant molecular
HIGH mpt/bpt. Hard & strong
Strong bonds between all ATOMS
No free charges/electrons
Covalent - graphite
Soft & slippery
CONDUCT: YES (fairly well)
Strong bonds between ATOMS
Weak bonds between LAYERS
Free electrons between layers
Metallic
Malleable
CONDUCT: YES (very well)
Regular structure, layers slide
Free electrons between ions