AQA Additional Science C2 Notes

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Unit 1: Bonding
Unit 2: Structures and Properties
Unit 3: Chemical Calculations
Unit 4: Rates of Reaction
Unit 5: Energy and Reactions
Unit 6: Electrolysis
Unit 7: Acids, Alkalis and Salts
[This is Unit 2 Chemistry, Additional Chemistry. This section comes after Core
Chemistry in an AQA Course (Unit 1)]
C2-1 : Bonding
Structure of the Atom
In an atom you will find three sub-atomic particles: protons, neutrons and electrons.
We can find both the protons and neutrons in the nucleus of an atom, and the
electrons around the nucleus in a number of energy levels (or "shells").
Protons are Positive and have a charge of +1
Neutrons are Neutral and have zero charge (0)
Electrons are therefore negative, charge of -1
We call the number of protons in the nucleus an atom's atomic number (or proton
number). The number of protons in an atom is equal to the number of electrons
(unless it is an ion), so generally, the proton number gives both the number of
electrons and protons.
This is an extract from a period table of the elements. It shows
Oxygen (O). The bottom number, in this case 8, is the atomic
number. The top number, in this case 16, is its atomic mass. The
mass number is the value you get when you sum the number of
protons and the number of neutrons (i.e. atomic number + neutron
count). This means we can use these two values to work out the
number of electrons, protons and neutrons in an atom of oxygen. Because the
atomic number is 8, we know that there are 8 electrons and protons. With a mass
number of 16, we know that there are [16 - 8] neutrons - hence there are also 8

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Electronic Configuration
The electrons in atoms are arranged in shells, but each shell can only hold a certain
number of electrons:
The first shell can hold only 2 electrons
The second shell can hold up to 8 electrons
The third shell can also hold 8 electrons
Sodium (Na) has a relative atomic mass of 11.
This means that the first shell will be complete,
holding 2 electrons. The second will also be
complete, holding 8 electrons.…read more

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The diagram shows the formation of sodium chlorine (NaCl). The lone electron on
the outer shell of the sodium atom is lost to complete the outer shell of the chlorine
atom. This results in the sodium atom now becoming an ion (Na+) and the chlorine
atom now becoming a negative ion (Cl-). This is ionic bonding in completion.…read more

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Examples of
giant covalent structures are diamond and silicon dioxide (silica).
Metallic Bonding
Metals can bond using metallic bonding, which tends to produce more giant
structures also. You can consider metals as a lattice of metal atoms (or positively
charged ions), arranged in layers. The outer electrons here can easily move around
the structure, and it is said to have a "sea" of free electrons.…read more

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Remember OILRIG:
Oxidation Is Loss of electrons;
Reduction Is Gain of electrons
An ionic solid will not conduct electricity because the ions are in a fixed
A molten ionic compound will conduct electricity because ions are free to
An ionic compound in solution will also conduct electricity
Covalent Molecules
The forces that hold together covalent bonds are equally as strong in covalent
compounds as in ionic compounds. However, the bonds between each different
molecule in a covalent compound are very weak.…read more

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The atoms in the giant structure of diamond (left) are held together by extremely
strong covalent bonds. It has some special properties which other types of structure
do not possess. It is very hard, has high melting/boiling points and is very chemically
Another type of giant is a fullerene, which you would
find when carbon behaves in a way as to form large
"cages" of carbon atoms between the bonds.…read more

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Because atoms lose
electrons when becoming ions, it tends not to affect its relative atomic mass,
because the weighting of electrons in the whole atom is so tiny. This means that a
magnesium atom has a relative atomic mass of 24, and a magnesium ion (Mg+) has a
relative atomic mass of 24 also. The relative atomic mass is not affected.
We can use the relative atomic mass of various elements to calculate the relative
formula mass of chemical compounds.…read more

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Empirical Formulae
If you are given the percentage formula of an element in a compound, we can work
backwards and find the ration between the atoms in the compound. We call this
ratio the empirical formula, which is simply a compound stated in its simplest form
of ratio between the atoms. For example, carbon dioxide has the empirical formula
CO2 - which states there are 2 oxygen atoms for every carbon atom.…read more

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÷ 24) x 40g = 8.33g of MgO
We can also do it by calculating the proportion of the amounts from the equation:
5 x (80 ÷ 48)g = 8.33g of MgO
We use the term yield to compare how much is actually made in a chemical reaction
with the maximum amount possible. There is a difference because it is not possible
to collect the amount calculated in an equation. Also, reactions may not reach
completion - losing some product.…read more

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The Haber Process
We use a special process to make ammonia, called The Haber Process. This
ammonia can be used to make fertilisers and other commercial chemicals. There are
two reactants in the process:
- nitrogen from the air, and,
- hydrogen (usually obtained from natural gas)
These reactants are purified and mixed in their correct proportions (see equation
below) and are then passed over an iron catalyst at temperatures of around 450ºC
and a pressure of approximately 200 atmospheres.…read more


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