Chemistry C2 Complete Revision

Revision notes for AQA Chemistry C2

Copyright Daniel Holloway, GCSE Revision 101 - http://gcserevision101.wordpress.com/chemistry/

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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 neutrons.
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. The
third shell will remain incomplete as it will only have 1 electron [2 + 8 + 1 = 11]. This
arrangement is said to be (2,8,1)

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There are a number of ways in which atoms can bond:
transferring electrons, called ionic bonding
sharing electrons, called covalent bonding
Ionic Bonding
With ionic bonding, atoms involved either gain or lose electrons to complete their outer
shell (achieving a noble gas structure), and in the process become ions. For example, using
the sodium atom above (2,8,1) ­ we know that the sodium atom in ionic bonding would lose
one electron so that its outer shell is complete (2,8).…read more

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Covalent Bonding
The other way in which atoms may bond is via covalent bonding, which works by sharing
electrons, rather than transferring them around. Ionic bonding happens between a metal
and a non-metal, but covalent bonding occurs when two non-metals bond. Take hydrogren
gas, for example.…read more

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The outer electrons here can easily move around the structure, and it is said to have
a "sea" of free electrons. It is the strong electrostatic attraction between the positively
charged ions and the negative sea of electrons flowing around that hold the structure
together. This sea of electrons are called delocalised electrons.
C2-2 : Structures and Properties
Ionic Compounds
Already we know ionic compounds consist of giant structures of ions arranged in lattices.…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 position
A molten ionic compound will conduct electricity because ions are free to move
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 unreactive.
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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Relative Atomic Masses
Because the masses of atoms are so tiny, we cannot work with them in chemical
calculations, so we use their relative masses instead of their real masses. We call these
relative atomic masses. For example, a standard carbon atom has six protons and six
neutrons ­ we say it has a relative atomic mass (RAM) of 12 units. We use these units of
measurement to compare atoms with each other.…read more

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All we need to do is to (4 x 1) / 12 = 0.33
0.33 x 100 = 33%
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.…read more

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In this case, 5g = (5 ÷ 24) moles of magnesium and so it will produce:
(5 ÷ 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.…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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sheeplover123

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