C2.3 Atomic Structure, Analysis And Quantitative Chemistry
1 C2.3.1 Atomic Structure
1.1 Atoms are represented with
their chemical symbol, mass
number and atomic number.
1.1.1 The relative masses of protons, neutrons and electrons are:
proton - 1, neutron - 1 and electron - very small.
184.108.40.206 The total number of protons
and neutrons in an atom is
called its mass number.
220.127.116.11.1 Atoms of the same element can
have different numbers of neutrons;
these atoms are called isotopes of
18.104.22.168.1.1 The relative atomic mass of an
element (Ar) compares the mass
of atoms of the element with the
12C isotope. It is an average value
for the isotopes of the element.
22.214.171.124.1.1.1 The relative formula mass (Mr) of a
compound is the sum of the relative
atomic masses of the atoms in the
numbers shown in the formula.
126.96.36.199.188.8.131.52 The relative formula mass of a substance, in
grams, is known as one mole of that substance.
2 C2.3.2 Analysing Substances
2.1 Elements and compounds can be detected and
identified using instrumental methods.
Instrumental methods are accurate, sensitive
and rapid and are particularly useful when the
amount of a sample is very small.
2.1.1 Chemical analysis can be used to
identify additives in foods. Artificial
colours can be detected and identified by
184.108.40.206 Gas chromatography linked to mass
spectroscopy (GC-MS) is an example of an
instrumental method: gas chromatography allows
the separation of a mixture of compounds; the
time taken for a substance to travel through the
column can be used to help identify the
substance; the output from the gas
chromatography column can be linked to a mass
spectrometer, which can be used to identify the
substances leaving the end of the column; the
mass spectrometer can also give the relative
molecular mass of each substance, given by a
molecular ion peak.
3 C2.3.3 Quantitative Chemistry
3.1 The percentage of an element in a compound can be calculated from the relative
mass of the element in the formula and the relative formula mass of the compound.
3.1.1 The empirical formula of a compound can be
calculated from the masses or percentages of
the elements in a compound.
220.127.116.11 The masses of reactants and products can be calculated from balanced symbol equations.
18.104.22.168.1 Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated
amount of a product because: the reaction may not go to completion because it is reversible, some of the product
may be lost when it is separated from the reaction mixture and some of the reactants may react in ways different
from the expected reaction.
22.214.171.124.1.1 The amount of a product obtained is known as the yield.
When compared with the maximum theoretical amount as a
percentage, it is called the percentage yield.
126.96.36.199.1.1.1 In some chemical reactions, the products of the reaction can
react to produce the original reactants. Such reactions are
called reversible reactions.