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Atomic Structure
Introducing atoms
Atoms – the building blocks
All substances are made from very tiny particles called atoms.
John Dalton had ideas about the
existence of atoms about 200 years
ago but only relatively recently have
special microscopes (called
electron microscopes) been
invented that can ‘see’ atoms.
The yellow blobs in this image are
individual gold atoms, as seen
through an electron microscope.
Elements – different types of atom
Elements are the simplest substances. There are about
100 different elements.
Each element is made up of just one particular type of atom,
which is different to the atoms in any other element.
Copper is an
element made up of
copper atoms only.
Carbon is an
element made up of
carbon atoms only.
How small is an atom?
Atoms are extremely small – they are about 0.00000001 cm
wide.
To make an atom the size of a football it would have to be
enlarged by about 3,000,000,000 times.
N
X3,000,000,000
If a football was enlarged
by the same amount it
would stretch from the
UK to the USA.
The Amazing Atomic Zoom
Inside an atom
Where are the electrons and nucleus found in an atom?
Atomic Atomic
number and
mass number
Structure
How heavy is an atom?
A single grain of sand contains millions of atoms of silicon
and oxygen.
O
Si
O
millions of these atoms
join to form each tiny
grain of sand
Each atom must therefore have an extremely small mass.
Relative atomic mass
Atoms are so small that their mass is not measured in grams
but in atomic mass units.
The atoms of each type of element have a relative atomic
mass (RAM).
The element carbon is the atom that the mass of all other
atoms is compared to. Carbon has a RAM of 12.
C
Relative atomic mass = 12
Relative atomic mass – examples
The lightest atom is hydrogen. It has 1⁄12 the mass of carbon
and so has a RAM of 1.
12 atoms x 1 = 1 atom x 12
H H
H H HH
H
H H HH H
HHH H
H
H H
HH H
H H
C
Magnesium is twice as heavy as carbon. It has a RAM of 24.
1 atom x 24 = 2 atoms x 12
Mg
Mg
CC
Even smaller particles
For some time people thought that atoms were the smallest
particles and could not be broken into anything smaller.
Scientists now know that atoms are actually made from even
smaller subatomic particles. There are three types:
proton
neutron
electron
Where are subatomic particles found?
Protons, neutrons and electrons are NOT evenly distributed in
an atom.
The protons and neutrons
exist in a dense core at the
centre of the atom. This is
called the nucleus.
The electrons are
spread out around the
edge of the atom. They
orbit the nucleus in
layers called shells.
The atom: check it out!
Draw a labelled diagram of the atom showing the nucleus
and labelling protons, neutrons and electrons.
nucleus
neutron
electron
proton
Properties of subatomic particles
There are two properties of subatomic particles that are
especially important:
1. Mass
2. Electrical charge
Particle
Mass
Charge
proton
1
+1
neutron
1
0
electron
almost 0
-1
The atoms of an element contain equal numbers of
protons and electrons and so have no overall charge.
How many protons?
The atoms of any particular element always contain the same
number of protons. For example:
 hydrogen atoms always contain 1 proton;
 carbon atoms always contain 6 protons;
 magnesium atoms always contain 12 protons,
The number of protons in an atom
is known as its atomic number
or proton number.
It is the smaller of the two numbers
shown in most periodic tables.
What’s the atomic number?
What are the atomic numbers of these elements?
sodium
iron
tin
fluorine
11
26
50
9
More about atomic number
Each element has a definite and fixed number of protons.
If the number of protons changes, then the atom becomes
a different element.
Changes in the number of particles in the nucleus
(protons or neutrons) is very rare. It only takes place in
nuclear processes such as:
 radioactive decay;
 nuclear bombs;
 nuclear reactors.
Mass number
Electrons have a mass of almost zero, which means that the
mass of each atom results almost entirely from the number of
protons and neutrons in the nucleus.
The sum of the protons and neutrons in
an atom’s nucleus is the mass number.
It is the larger of the two numbers shown
in most periodic tables.
Atom
Protons
hydrogen
1
lithium
3
aluminium
13
Neutrons Mass number
0
1
4
7
14
27
What’s the mass number?
Mass number = number of protons + number of neutrons
What is the mass number of these atoms?
Atom
Protons Neutrons
helium
2
2
copper
29
35
cobalt
27
32
iodine
53
74
germanium
32
41
Mass number
4
64
59
127
73
How many neutrons?
Number of neutrons = mass number - number of protons
= mass number - atomic number
How many neutrons are there in these atoms?
Atom
helium
Mass
number
Atomic
number
4
2
fluorine
19
9
strontium
88
38
zirconium
91
40
238
92
uranium
Number of
neutrons
2
10
50
51
146
Building a nucleus
Electron
Atomicconfiguration
Structure
How many electrons?
Atoms have no overall electrical charge and are neutral.
This means atoms must have an equal number of
protons and electrons.
The number of electrons is therefore the same as
the atomic number.
Atom
Protons Neutrons Electrons
helium
2
2
2
copper
29
35
29
iodine
53
74
53
Atomic number is defined as the number of protons rather
than the number of electrons because atoms can lose or
gain electrons but do not normally lose or gain protons.
Calculating the number of electrons
What are the missing numbers?
Atom
Protons Neutrons Electrons Atomic Mass
number number
boron
potassium
chromium
mercury
5
19
24
80
6
20
28
121
19
24
80
argon
18
22
18
5
5
19
11
39
24
80
52
201
18
40
How are electrons arranged?
Electrons are not evenly spread but exist in layers
called shells.
The arrangement of electrons in these shells is often called
the electron configuration.
1st shell
2nd shell
3rd shell
How many electrons per shell?
Each shell has a maximum number of electrons that it can
hold. Electrons will fill the shells nearest the nucleus first.
1st shell holds
a maximum of
2 electrons
2nd shell holds
a maximum of
8 electrons
3rd shell holds
a maximum of
8 electrons
Calculating electron configurations
Properties of the nucleus and electrons
Summary: the atom so far
The nucleus is:
 Dense – it contains nearly all the mass
of the atom in a tiny space.
 Made up of protons and neutrons.
 Positively charged because of the
protons.
Electrons are:
 Thinly spread around the outside
of the atom.
 Very small and light.
 Negatively charged.
 Found orbiting the nucleus in layers called shells.
 Able to be lost or gained in chemical reactions.
Isotopes
What is an isotope?
Elements consist of one type of atom, but sometimes these
atoms can be slightly different.
Although atoms of the same element always have the same
number of protons, they may have different numbers of
neutrons.
Atoms that differ in this way are called isotopes.
mass number
is different
atomic number
is the same
Properties of isotopes
The isotopes of an element are virtually identical in their
chemical reactions.
This is because they have the same number of protons and
the same number of electrons.
The uncharged neutrons make no difference to chemical
properties but do affect physical properties such as melting
point and density.
Natural samples of elements are
often a mixture of isotopes.
Isotopes of carbon
Most naturally-occurring carbon exists as carbon-12, about
1% is carbon-13 and a much smaller amount is carbon-14.
6 protons
6 protons
6 protons
6 neutrons
6 neutrons
6 neutrons
6 electrons
7 electrons
8 electrons
Isotopes of hydrogen
Hydrogen-1 makes up the vast majority of the naturallyoccurring element but two other isotopes exist.
hydrogen
deuterium
tritium
1 proton
1 proton
1 proton
0 neutrons
1 neutron
2 neutrons
1 electron
1 electron
1 electron
Isotopes of chlorine
About 75% of naturally-occurring chlorine is chlorine-35 and
25% is chlorine-37.
17 protons
17 protons
18 neutrons
20 neutrons
17 electrons
17 electrons
Isotopes of oxygen
Almost all of naturally-occurring oxygen is oxygen-16 but
about 0.2% is oxygen-18.
oxygen-16
8
8
8
What are the
particle
numbers in
each isotope?
protons
neutrons
electrons
oxygen-18
8
8
10
Isotopes and RAM
Many elements are a mixture of isotopes. The RAM given in
the periodic table takes account of this.
To calculate the RAM of a mixture of isotopes,
multiply the percentage of each isotope by its
atomic mass and add them together.
For example, chlorine exists as two isotopes:
chlorine-35 (75%) and chlorine-37 (25%).
RAM of chlorine = (75% x 35) + (25% x 37)
= (0.75 x 35) + (0.25 x 37)
= 26.25 + 9.25
= 35.5
Calculating RAM
Bromine contains 50.5% bromine-79 and 49.5% bromine-81.
What is the RAM of naturally-occurring bromine?
RAM of bromine = (50.5% x 79) + (49.5% x 81)
= (0.505 x 79) + (0.495 x 81)
= 39.895 + 40.095
= 79.99
= 80 (the RAM is usually rounded
to the nearest whole number)
Atomic structure word check
Atomic Structure
Summary
activities
Glossary (part 1)
atom – The smallest particle that can exist on its own.
atomic number – The number of protons in the nucleus
of an element, also known as the proton number.
electron – Negative particle that orbits the nucleus of an
atom.
element – Substance made up of only one type of atom.
isotopes – Different atoms of the same element. They have
the same number of protons and electrons, but a different
number of neutrons.
Glossary (part 2)
nucleus – The dense positive centre of an atom, made up
of protons and neutrons.
neutron – A neutral particle, with a mass of 1. It is found in
the nucleus of an atom.
mass number – The number of protons and neutrons in
an atom.
proton – A positive particle, with a mass of 1. It is found in
the nucleus of an atom.
relative atomic mass (RAM) – The mass of an
element compared to the mass of 1⁄12 of the mass of
carbon-12.