GCSE Chemistry for Unit 2 Additional.

1. GCSE CHEMISTRY
UNIT 2A
AQA

2. Atoms and Isotopes
ISOTOPES
Isotopes are different atomic forms of the same element, which have the
SAME number of PROTONS AND ELECTRONS, but a DIFFERENT number of
NEUTRONS.
Number of Neutrons = Atomic Mass – Atomic
Number
Can you see that they are all still carbon?
This is because it is the electrons which
determine whether or not something is a
different element.

3. Ionic Bonding
IONS
Electrically charged particles which are formed when an atom loses or
gains electrons.
 Metals form positively charged (+) ions.
 Non-metals form negatively charged (-) ions.
 Hydrogen forms a positive ion (H+).
 Atoms will want to either lose 1 or 2 electrons to gain a full outer shell OR
gain 1 or 2 electrons.
In the exam, you may need to be able to show the electronic structure of
some ions.
For example, this lithium atom has
one electron in its outer shell.
Therefore, it will lose this electron to
When this electron is
lost, the positive lithium
ion is formed.
You put charge
here (+ or -)
Draw these brackets
around the ion, to
distinguish it from the atom.
YOU MAY WANT TO WATCH THIS VIDEO
FIRST!
I will come to covalent bonding later!

4. Ionic Bonding
IONIC COMPOUNDS
When metals react with non-metals, electrons are transferred from the metal atoms
(which become +) to the non-metal atoms (which become -), forming ions. The result is
called an ionic compound.
There is a strong electrostatic force of attraction between the oppositely charged ions,
forming an ionic bond. Remember the rule; OPPOSITES ATTRACT!
Sodium Chloride (NaCl) is formed when sodium
and chlorine react together. It contains oppositely
charged ions held together by strong
electrostatic forces of attraction – the ionic
bonds. These ionic bonds then form into what is
called a giant ionic lattice (pictured right).
Ionic compounds all:
• Have high melting and boiling points due to strong attraction between ions. It
takes lots of energy to break this attraction.
• Have free electrons when melted, so can therefore carry electric current when
melted.
• Dissolve easily in water so the ions separate out, meaning it can then also carry
electric current when dissolved in water.

5. Ionic Bonding
FORMULAE
The formula of ionic compounds can be worked out if the ions within it
are known.
 First, check the charge on the ions see whether the charges
cancel each other out. If they do, for example with potassium (K+)
and chloride (Cl-), you only need to write one lot of each ion, KCl. It
is the same if ions have 2+ and 2- charges.
 However, if the ions have different numbers next to the charges,
you will need to change the number of ions within the
compound so that the charges cancel each other out.
 For example, the compound formed from calcium (Ca2+) and iodine
(I-) will contain two I- ions to every one Ca2+ ion, so that it is
neutral.
 The finished formula is CaI2.

6. Covalent Bonding
 Covalent bonding is when two non-metal atoms share electrons.
 They only share electrons in their outer shell.
 This completes both atom’s outer shells.
 Each covalent bond provides one extra shared electron for each atom.
 So, covalent bond is one shared pair of electrons.
 Covalent bonds are strong, so need a lot of energy to break them.
 Check page 43 and 44 in CGP Additional Science Revision for more info.

7. Covalent Bonding
SIMPLE MOLECULAR SUBSTANCES
 Made up of molecules where the atoms are joined with strong covalent
bonds but the molecules are only held together by weak intermolecular
forces.
 They do not conduct electricity.
 They have low melting and boiling points because the forces are broken
easily. It is the forces between the molecules that get broken when a
substance melts or boils NOT the bonds between atoms.

8. Giant Covalent Structures
 All atoms bonded by strong covalent bonds.
 High melting and boiling points.
 Most don’t conduct electricity (graphite is the exception.)

9. Metallic Structures
METALS
 Metals are malleable. This is because they are made up of
layers of atoms which can slide over each other.
 Metals make up a giant metallic structure. This is because the
positive metal ions and negative electrons form strong metallic
bonds between them, because OPPOSITES ATTRACT!
 Metals have free electrons which can carry electric current and
heat throughout the structure.

10. Alloys
 Alloys contain more than one metal.
 Alloys are harder than pure metals as the alloy contains atoms of
different sizes from the various different metals.
 These different sizes of atoms distort the regular arrangement of atoms in
metals.
 This makes it more difficult for the layers to slide over each other as they
do in pure metals.
• Shape memory alloys can return to their original shape after being bent
or twisted.
• Nitinol is a shape memory alloy made from nickel and titanium. It is used
in dental braces.

11. Nanoparticles
 Nanoparticles are MUCH smaller than ordinary-sized particles.
 They have a large surface area compared to their volume so they can react
quickly.
 This means that they can be very effective as catalysts.
 They often have different properties to what the ordinary-sized particles would
have in the same substance.
 Could be used to make stronger and lighter building materials.
 Nanoparticles could be used in medicine, as they are more easily absorbed by the
body.
 Nanoparticles include fullerenes. These are molecules of carbon, which are
shaped like hollow balls (buckyballs) or closed tubes.

12. Polymers

13. Relative Atomic/Formula Mass
 The symbol for relative atomic mass is Ar. You will ALWAYS be given this
value in the exam.
 The symbol for relative formula mass is Mr.
WORKING OUT Mr
To work out the relative formula mass of a compound, all you need to do is
add the relative atomic masses of the elements within the substance,
together.
For example:

14. Moles

15. Calculations
PERCENTAGE MASS

16. Calculations
EMPIRICAL FORMULA

17. Calculations
CALCULATING MASSES IN REACTIONS
WATCH VIDEO!

18. Calculations
PERCENTAGE YIELD
The percentage yield compares what you calculate you should get
(predicted yield), with what you actually got (actual yield).
Percentage yield = (actual yield ÷ predicted yield) x 100.
Yields are always less than 100% either because:
 The reaction is reversible. This means that the reactions will never be
complete because the reaction is always going both ways.
 You lose liquid or solid through filtration, as some is nearly always left
behind in apparatus.
 There could be a weighing error. When weighing out the products to be
used in the reaction, there has been an error. This means the actual yield
will differ from the predicted yield.

19. Paper Chromatography
 Used to analyse coloured substances, such as artificial food
colouring.
 Paper chromatography works because some of the colours are
better at dissolving in the liquid than they are at bonding with the
paper, so they will travel further up the paper.

20. Instrumental methods of
analysis
 Fast
 Accurate (reliable).
 Sensitive (can detect a small amount of a substance in a small amount of
sample).
GAS CHROMATOGRAPHY