THE MASS OF ATOMS
The sub-atomic particles
Atoms are made of three particles.
proton
neutron
electron
relative mass
1
1
1/1836
relative charge
+1
0
-1
The term relative mass means that protons and neutrons have the same mass
as one another and the mass of an electron is 1836 times smaller. These
figures are not an actual mass in grams. Similarly the relative charge means
that protons and electrons have equal and opposite charges and not a charge
of 1 coulomb.
The nucleus
The nucleus is at the centre of the atom and contains the protons and
neutrons. Protons and neutrons are collectively known as nucleons. Virtually
all the mass of the atom is concentrated in the nucleus, because the electrons
weigh so little.
Working out the numbers of protons and neutrons
No of protons = ATOMIC NUMBER of the atom
The atomic number is also given the more descriptive name of proton
number.
No of protons + no of neutrons = MASS NUMBER of the atom
The mass number is also called the nucleon number.
This information can be given simply in the form:
How many protons and neutrons has this atom got?
The atomic number counts the number of protons (9); the mass number
counts protons + neutrons (19). If there are 9 protons, there must be 10
neutrons for the total to add up to 19.
TOPIC 10.2.3: HOW MUCH? 1
The Periodic Table is arranged in terms of atomic number (proton number)
and therefore the number of protons defines what element you are talking
about. So if an atom has 8 protons (atomic number = 8), it must be oxygen. If
an atom has 12 protons (atomic number = 12), it must be magnesium.
Similarly, every chlorine atom (atomic number = 17) has 17 protons; every
uranium atom (atomic number = 92) has 92 protons.
Isotopes
Atoms of the same element must have the same number of protons, however,
the number of neutrons in an atom can vary within small limits. For example,
there are three kinds of carbon atom 12C, 13C and 14C.
carbon-12
carbon-13
carbon-14
protons
6
6
6
neutrons
6
7
8
mass number
12
13
14
These different atoms of carbon are called isotopes. The fact that they have
varying numbers of neutrons makes no difference whatsoever to the chemical
reactions of the carbon since the chemistry of an atom is determined by the
arrangement of its electrons.
Isotopes are atoms of the same element which have different mass numbers.
The electrons
Working out the number of electrons
Since we need 1836 electrons to have the same mass as 1 proton or neutron,
and even the heaviest element only has just over 100 electrons, we can forget
about any contribution from electrons towards the mass of the overall atom.
Atoms are electrically neutral, and the positive protons are balanced by the
negative electrons. It follows that in a neutral atom:
no of electrons = no of protons
So, if an oxygen atom (atomic number = 8) has 8 protons, it must also have 8
electrons; if a chlorine atom (atomic number = 17) has 17 protons, it must also
have 17 electrons.
TOPIC 10.2.3: HOW MUCH? 2
The arrangement of the electrons
The electrons are found at considerable distances from the nucleus in a series
of shells or energy levels. Each energy level can only hold a certain number of
electrons. The first level (nearest the nucleus) will only hold 2 electrons, the
second holds 8, and the third also seems to be full when it has 8 electrons. At
GCSE you stop there because the pattern gets more complicated after that.
THE MASSES OF ATOMS AND MOLECULES
MASS NUMBER AND RELATIVE ATOMIC MASS
The mass number of an isotope is equal to the number of protons plus
neutrons in one atom and is, therefore, a whole number.
In round numbers, mass number and relative atomic mass are equal, but the
relative atomic mass is not an exact whole number, partly because protons
and neutrons do not have exactly the same mass and partly because of the
existence of isotopes.
RELATIVE ATOMIC MASS
Because of the incredibly small size of atoms, it is not possible to weigh them
directly. Instead the masses of atoms are related to an arbitrary standard; for
this purpose the mass of one atom of 12C is taken to be 12.0000. The mass of
any other atom compared to that of the carbon atom is called it’s RELATIVE
ATOMIC MASS and is given the symbol Ar.
The relative atomic mass of an element is defined as the average mass of one
atom of the element compared with one twelfth the mass of an atom of 12C.
N.B. Relative atomic mass is a ratio, so it is simply a number and has no
units.
The 12C atom contains 6 protons, 6 neutrons and 6 electrons. The hydrogen
atom contains 1 proton and 1 electron. Since the mass of an electron is
negligible compared to that of a proton or a neutron, the hydrogen atom has
only 1/12 the mass of a carbon atom; therefore the relative atomic mass of
hydrogen is 1.
Similarly, the relative mass of an oxygen atom, which contains 8 protons, 8
neutrons and 8 electrons, is 16.
TOPIC 10.2.3: HOW MUCH? 3
RELATIVE FORMULA MASS
We can use the Ar to calculate the relative formula mass (given the symbol
Mr) of compounds regardless of whether they exist as individual molecules or
as a giant ionic lattice. For CO2 the molecule contains a carbon atom and two
oxygen atoms and so 1 molecule of CO2 has a relative formula mass of
12+16+16=44. Similarly the formula of NaCl shows 1 sodium atom for every
chlorine atom and so the Mr of NaCl is 23+35.5=58.5.
The method for calculating Mr is straightforward even if the formula of a
substance in not.
Ammonium sulphate has the formula (NH4)2SO4
Mr = (2x14)+(8x1)+32+(4x16) = 132
MOLES
Atoms and molecules are such minute particles that they are difficult to count.
Nevertheless, one of the most important things chemists need to know is the
NUMBER of particles they are dealing with.
If 1g of hydrogen is weighed out, it will contain a certain number of atoms (let
this no. be x). Therefore 1g of any other element will contain fewer atoms,
because each atom is heavier.
If
then
1g hydrogen contains x atoms
1g carbon contains x/12 atoms
1g oxygen contains x/16 atoms
etc.
Since chemical reactions take place between whole numbers of particles
(atoms, ions or molecules), it is more useful to know the number of particles
present than the mass. Thus, continuing the above example,
If
then
1g hydrogen contains x atoms
12g carbon contains x atoms
16g oxygen contains x atoms
etc.
These numbers correspond to the relative atomic masses of these elements
expressed in grammes. Therefore, if elements are measured out in the same
proportions as their relative atomic masses, we are dealing with the same
number of atoms, i.e. numbers of atoms can be controlled by weighing.
The number of atoms contained in 1g hydrogen, 12g carbon, 16g oxygen etc.
has been determined very accurately and has a value of 6.02x1023
TOPIC 10.2.3: HOW MUCH? 4
THE PERCENTAGE COMPOSITION OF COMPOUNDS
A pure chemical compound has a fixed chemical composition and is
represented by a fixed formula. This formula conveys information about:
a) the elements present in the substance
b) the mass of 1 mole of the substance (Mr expressed in g)
c) the number of moles of each element present in 1 mole of the
substance
The percentage composition of a compound is the % contribution by mass
which each component element makes to the substance.
CALCULATION OF % COMPOSITION
Example: Calculate the % composition by mass of ammonium nitrate.
1. Work out the formula of the compound. The formula of ammonium nitrate is
NH4NO3.
2. Calculate the relative molecular/formula mass of the compound.
Mr (NH4NO3) = 14 + (4x1) + 14 + (3x16) = 80
 80g NH4NO3 contains
28g nitrogen
4g hydrogen
48g oxygen
3. Express the mass of each element as a percentage of the molar mass.
%N = 28 x 100 = 35.0%
80
%H =
4 x 100 = 5.0%
80
%O = 48 x 100 = 60.0%
80
Answers are usually given to 1 d.p.
4. Check that the percentages add up to 100%.
N.B. Errors from rounding up or down may give answers between 99.8 and 100.2%,
which is satisfactory.
TOPIC 10.2.3: HOW MUCH? 5
CALCULATION OF EMPIRICAL FORMULAE
The EMPIRICAL FORMULA is the simplest WHOLE number ratio of the atoms
of each element present in a compound.
The MOLECULAR FORMULA is the actual number of atoms of each element
present in one molecule of the compound.
The two are related by the expression:
Molecular formula = Empirical formula x n
where n is a whole number.
For example:
The molecular formulae of methane and ethane are CH4 and C2H6
respectively.
The empirical formula of methane is CH4; in the above expression n = 1.
The empirical formula of ethane is CH3, which is the simplest whole number
ratio of carbon to hydrogen atoms. In the above expression n = 2.
CALCULATION OF AN EMPIRICAL FORMULA
Example: Calculate the empirical formula of a compound having the composition:
lead 8.32g; sulphur 1.28g; oxygen 2.56g
If its molecular mass is 303, what is its molecular formula?
PROCEDURE:
1) Convert the mass of each element into moles: this is done by dividing the mass
by the relative atomic mass.
Moles Pb 8.32 = 0.040 moles
207
Moles S = 1.28 = 0.040 moles
32
Moles O = 2.56 = 0.160 moles
16
This transformation counts the combining atoms; therefore, in the compound,
0.040 moles (i.e. 0.040 x 6 x 1023 atoms ) of Pb
0.040 moles (i.e. 0.040 x 6 x 1023 atoms) of S
0.160 moles (i.e. 0.160 x 6 x 1023 atoms) of O
have combined together.
TOPIC 10.2.3: HOW MUCH? 6
CALCULATING REACTING MASSES
A balanced equation tells us how many of each type of atom or molecule are
need to react with one another. It also tells us the number of moles of each
substance which react with one another. Consider the following.
When you bake a cake you follow a recipe.
e.g. Recipe for a cake
2 eggs
200g flour
50g butter

1 cake
If you only have 100g of flour you can scale down your ingredients and make
a smaller cake.
In chemistry we can calculate how much of substance will react with another
in a reaction. If we do not then one of the two reactants will be present in
excess and this is a waste of resources. Consider the following example.
How many grams of O2 will react exactly with 48g of Mg? (In this question we
are not interested in how much MgO is produced)
2Mg
+
O2

2MgO
This balanced equation tells us that 2 moles of magnesium will react with 1
mole of oxygen to produce 2 moles of MgO (a bit like a recipe)
1 mole of Mg has a mass of 24g.
So 12g of Mg is 12/24 = ½mole
(like the flour I am a little short for my
recipe)
The equation (recipe) tells me that 2 moles of Mg reacts with 1 mole of O 2. If I
keep the ratio 2:1 the same then ½moles of Mg will only need ¼moles of O2
to react.
I know that 1 mole of O2 has a mass of 32 (16+16).
So ¼ of a mole of O2 has a mass of 8g.
Therefore I need 8g of O2 to react exactly with my 24g of Mg.
Answer is 8g.
TOPIC 10.2.3: HOW MUCH? 7
How efficient are reactions?
We can gain an understanding of the efficiency of a reaction by considering
the percentage yield or by looking at the atom economy of a reaction.
Percentage yield
The amount of product obtained, usually measured in grams or kilograms, is
known as the yield. Often, when we carry out a reaction, the starting materials
do not fully react to form products. We say the reaction does not go to
completion. This results in many reactions producing a lower yield than
expected. This can occur because;
 the reactants were not pure (and so we didn’t have as much as we thought
we did),
the reactants were not mixed properly (reactions won’t take place if the
reactants are not in contact with each other)
 the reaction is reversible.
some of the product is lost during separation from the reaction mixture
It is useful, therefore, to know the percentage yield of a reaction. This
compares the amount of product that the reaction really produces with the
maximum amount it could possibly produce if it went to completion.
Percentage yield = amount of product produced by the reaction x 100
maximum amount of product possible
Eg It was calculated that a reaction could produce 24g of iron. When it was
carried out, the mass of iron produced was only 15.6g. Calculate a percentage
yield for this reaction.
Percentage yield =
15.6
24
x
100
=
65%
Chemical companies want to make money and ideally want 100% yield so
nothing is wasted and the maximum amount of product can be made and
sold. Chemical plants are designed to be as efficient as possible. This is
better for the environment too as less energy and resources are wasted and
there is less pollution.
TOPIC 10.2.3: HOW MUCH? 8
REVERSIBLE REACTIONS
Many chemical reactions do not go to completion, because the products of the
reaction can react to produce the original reactants. Reactions like these are
called reversible reactions and are represented:
A + B
C + D
For example, if ammonium chloride (a white solid) is heated, it splits up into
the colourless gases, ammonia and hydrogen chloride.
ammonium chloride
ammonia + hydrogen chloride
If one mole of A is allowed to react with one mole of B under a given set of
conditions, the concentrations of A and B start to fall as they are used up, and
the concentrations of C and D increase as they are formed. Eventually an
equilibrium is reached.
At equilibrium, the concentrations of A, B, C and D
stay constant and no longer change.
At equilibrium, the rate of the forward reaction is equal to
the rate of the backward reaction.
Individual molecules carry on reacting and pass continuously from one side of
the reaction to the other, but since reactants and products are consumed as
fast as they are formed, the total numbers of molecules of A, B, C and D stay
constant.
The relative amounts of A, B, C and D which exist at equilibrium depend upon
the particular set of conditions (e.g. temperature & pressure) used for the
reaction.
TOPIC 10.2.3: HOW MUCH? 9
ANALYSING SUBSTANCES
Elements and compounds can be detected and identified using a variety of
instrumental methods. Some methods are suited to the identification of
elements; others are suited to the identification of compounds.
MASS SPECTROMETRY
Mass spectrometry can be used as a method for the detection and
identification of elements. The substance is bombarded with electrons to form
ions. The ions are passed through a magnetic field where they are deflected
according to their masses: the bigger the mass, the smaller the deflection.
INFRA-RED SPECTROSCOPY
Infra-red spectroscopy can be used as a method for the detection and
identification of compounds. When infra-red radiation is absorbed by
compounds, it makes the bonds in the compound vibrate. Different bonds
absorb infra-red radiation of different frequencies. The machine detects the
frequencies which have been absorbed, allowing the different bonds in the
compound to be identified.
ADVANTAGES of physical methods of detection:





small samples can be used
rapid
sensitive (small quantities can be detected)
accurate
easy to automate and computerise
DISADVANTAGES
of physical methods of detection:


machines must be calibrated regularly using standards of known purity

a pure sample is needed, since tiny traces of impurities would be
detected
USES of physical methods of detection:
these methods are used for routine analysis in the fields of
 medicine
 agriculture
 pollution monitoring & environmental health
 food industry
 chemical industry
 forensic science
 drug detection in horse racing and athletics
TOPIC 10.2.3: HOW MUCH? 10
As progress has been made in electronics and computing, instrumental
methods have become:
 increasingly miniaturised
 more automated
 more accurate
 more sensitive
 more reliable
 easier to use
PAPER CHROMATOGRAPHY
This technique can be used to identify additives in foods. It works because
some compounds in a mixture dissolve better than others in particular
solvents. Their solubility determines how far they travel up the paper.
Place small a spot of the
mixture to be analysed
(and any possible
component for
comparison purposes) on
the paper. Dip the paper
in the solvent.
Allow the solvent to
rise up the paper. Each
component dissolves in
the solvent. Those
which are more soluble
travel further up the
paper.
The finished chromatogram can then be compared to those of known
substances and the additives identified. For this we need to use the same
solvent at the same temperature.
TOPIC 10.2.3: HOW MUCH? 11