3 Atomic Structure
15
3.1
You need to be
familiar with the
terms in italics
The diameter of the
nucleus is
approximately 10-15 m
and an atom 10-10 m.
All matter consists of atoms.
An atom can be thought of as having a central nucleus
surrounded by a cloud of particles called electrons.
The nucleus contains a number of particles called
protons and neutrons, together termed nucleons.
Each nucleon is nearly 2000 times the mass of an
electron. This means the mass of an atom is
concentrated in its nucleus, around which the much
lighter electrons orbit. If a nucleus was scaled up to
the size of an orange, the electrons would be orbiting
¾ of a mile away!
3.2
see GRAHAM DT
(1996) Principles of
Radiological Physics
(3rd ed) Section 26.5
Atoms
Atomic number (symbol: Z)
The number of protons in the nucleus of an atom, its
atomic number, determines the type of element eg a
nucleus with just one proton is hydrogen, which has an
atomic number of one. A nucleus with 8 protons is
oxygen, atomic number 8. A nucleus with 29 protons
is copper, atomic number 29. The names of all the
different types of elements and their atomic numbers
can be found in the Periodic Table.
3.3
Mass number (symbol: A)
The mass number of a nucleus gives the total number
of nucleons in the nucleus ie the number of protons
plus the number of neutrons. If you subtract the
atomic number from the mass number you will end up
with the number of neutrons in the nucleus.
For example, a nucleus of carbon may have an atomic
number of 6 - so 6 protons - and a mass number of 12.
Subtracting 6 from 12 means there must be 6 neutrons
as well as 6 protons in the nucleus.
A nucleus of sodium has an atomic number of 11 and
a mass number of 23. So it has 11 protons and 12
neutrons.
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naturally or artificially. For example, a nucleus of
carbon (atomic number 6) may have a mass number of
11, 12, 13 or 14, with 5, 6, 7 or 8 neutrons. Each is
called an isotope of carbon.
Some isotopes are stable and some are radioactive,
depending on the ratio of protons to neutrons.
3.5
Chemical symbols
An element can be represented by its chemical symbol
with its atomic number as a subscript and its mass
number as a superscript. An element identified in this
way is known as a nuclide.
For example:
mass number
63
Cu
atomic number
chemical symbol
29
3.6
Check you are familiar with the
terminology:
i)
The nucleons in a nucleus of an atom are
called ? and ?
ii)
The number of protons in the nucleus of an
atom is known as ?
iii)
the total number of nucleons in the nucleus
of an atom is known as ?
iv)
To calculate the number of neutrons in a
nucleus, you subtract ? from ?
v)
Nuclei with the same atomic number but
different mass numbers are called ?
vi)
How many protons and neutrons are in the
following nuclei?
14
138
208
Check your answers in
section 3.11
Look up the symbols in
Graham (1996) if you
are not sure of the
names of the elements
N
7
Ba
56
Pb
82
3.4
Isotop
es
The number of
neutrons in a
nucleus can
vary, either
occurring
17
The maximum number
of electrons in a shell is
2n2, where n = number
of shell and
K=1
Electrons are the small, light particles orbiting the
nucleus of an atom. They are arranged in shells,
called K, L, M, N etc. Normally, the number of
electrons orbiting the nucleus equals the number of
protons in the nucleus.
The number of electrons in each shell follows certain
rules: for example, the maximum number in the K
shell is 2, the L shell 8 and the M shell 18. However,
the outermost shell (valence shell) may only hold 8
electrons. This shell determines the chemical, thermal,
optical and electrical properties of the atom. Copper
for example has an atomic number of 29. There are 2
electrons in the K shell, 8 in the L shell, 18 in the M
shell and 1 in the N shell. This single valence electron
easily leaves the atom and acts as a free electron.
Hence copper is a good conductor of heat and
electricity.
3.8
We can use the
shorthand +ve and
-ve
Electric charge
Protons and electrons carry an electric charge,
meaning they exert a force on each other similar to the
North and South poles of magnets. Protons are said
to carry a positive electric charge and electrons a
negative electric charge. The number of positivelycharged protons in the nucleus of an atom normally
balances the number of negatively-charged electrons
orbiting the nucleus.
WHAT HAPPENS WHEN YOU BRING THE NORTH POLES OF
TWO MAGNETS NEAR TO EACH OTHER?
WHAT HAPPENS WHEN YOU BRING A NORTH AND A SOUTH
POLE NEAR TO EACH OTHER?
In the same way as like magnetic poles repel each
other due to the magnetic force of repulsion, like
charges repel each other due to the electrical force of
repulsion. Unlike magnetic poles attract each other in
the same way as unlike charges attract each other.
3.7
Electro
ns
18
positive charges in an atom cancel each other out, so
the atom is electrically stable. If an electron is
removed from the atom, the positive protons will
outnumber the remaining negative electrons, leaving
the atom with a net positive charge. The ionised atom
is known as a positive ion.
When an electron is removed from a shell, a certain
amount of energy is used to overcome the attractive
force of the protons in the nucleus: this is called the
binding energy. The more protons in the nucleus, the
higher the binding energy required to remove an
electron. The closer the shell to the nucleus, the higher
the binding energy.
Z is the symbol for
atomic number. See
section 3.2
Table 3:
keV are
kiloelectronvolts: units
of energy. See section
1.3
shell
Add up the total
number of electrons to
check they balance the
number of protons in
the tungsten nucleus
K
L
M
N
O
P
kinetic energy is energy
of a moving object.
See section 1.1
Ionising Radiation
warning symbol
Binding energies for tungsten
(Z = 74)
binding energy
(keV)
70
11
2
0.6
0.07
0.02
no. of
electrons
2
8
18
32
12
2
Atoms may be ionised by charged particles such as
free electrons: kinetic energy carried by the free
electron is transferred to overcome a bound electron’s
binding energy. This is part of the process of x-ray
production in the x-ray tube.
Atoms may also be ionised by x-ray photons: the
energy of the x-ray is used to overcome the binding
energy of one of the electrons.
If atoms making up important structures in the body,
such as DNA, are ionised, chemical changes occur
which may cause permanent damage. This is why xrays can be dangerous.
3.9
Ionisat
ion
Normally the
negative and
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atomic particles
recognise chemical symbols and identify atomic and
mass numbers
define the term isotope
describe the basic properties of electric charges
explain what is meant by ionisation
3.11 Answers to Section 3.6
i)
ii)
iii)
iv)
v)
vi)
protons and neutrons
atomic number
mass number
subtract atomic number from mass number
isotopes
nitrogen (N) has 7 protons and 7 neutrons
barium (Ba) has 56 protons and 82 neutrons
lead (Pb) has 82 protons and 126 neutrons
3.10 Check
that you
can:
name the
principle
20