1a.
The Structure of Atoms
– Fundamental
particles
-
See also
history of atomic structure ideas,
atomic structure and isotopes
- You can read the above page first and then
concentrate on isotopes and symbol notation here.
- Atoms, protons, neutrons and
electrons are the smallest particles of
matter whose properties we study in Chemistry.
- From
experiments done in the late 19th and early 20th
century it was deduced that atoms were made up of three fundamental
sub–atomic particles (listed below).
- Earlier theories of atomic
structure, eg the 'plum pudding' model in which 'protons' and
'electrons' were scattered or arranged evenly across the atom, were
superseded by the model described in the picture below.
- It was the only model that could
explain the scattering of alpha particles by a small dense and
positive atomic centre.
- Later experiments showed that the outer bits
could be knocked off atoms and these had a very tiny mass and a
negative charge, in other words the electron!
- Since then, even protons and
neutrons have been shown to be made up of even more fundamental
particles e.g. quarks and whole families of other particles which
you do not have to worry about here!
- It should be emphasised right from
the start that radioactivity is due to energy changes in the
nucleus and the surrounding electrons are not usually involved.
- Below are listed all the particles
you are most likely to come across in pre–university science courses
e.g. GCSE& A Level chemistry and physics courses. They may be others
in A level physics courses.
IGCSE/GCSE physics students -
forget the quarks!
See also
history of
atomic structure models, atomic structure and isotopes
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1b.
A
Portrait of an Atom
– what is it like?
The diagram below gives some idea on the
structure of an atom, it also includes some important definitions and notation
used to describe atomic structure.
The atomic number (Z) is also known as
the proton number, equals the number of positive protons in nucleus.
The mass number (A) is also known as the nucleon number,
equals the sum of the protons and neutrons in the nucleus.
The neutron number (N) = mass number (A) – atomic number (Z),
equals number of neutrons in the nucleus.
Protons and neutrons are the 'nucleons' present in
the nucleus
and the negative electrons are held by the positive nucleus in
'orbits' called energy levels or shells.
However when studying radioactivity and
radioactive decay we are
NOT interested in the
electrons moving around the nucleus,
but we are interested in
electrons (and positrons)
produced by certain types of radioactive decay AND
the nuclide symbol notation e.g. like
73Li.
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1c. ISOTOPES and radioisotopes
Isotopes are atoms
of the same element with different numbers of neutrons
in the nucleus and therefore atoms of the same element with different masses
(different nucleon number or mass number).
- Isotopes are atoms of the same atomic number but
different mass numbers.
- Some elements have just a few isotopes but others may have
up to many different isotopes.
- Isotopes of an element only differ in the number of
neutrons i.e. they have the same number of protons, electrons and electronic
structure.
- Most elements have one or more stable isotopes, but
many other isotopes are unstable, disintegrate spontaneously (nuclear decay)
and are known as radioactive.
- This gives each isotope of a particular element a
different mass or nucleon number, but, being the same element they have
the same atomic number or proton number, but different mass number.
-
How to read and write symbols
used in isotope notation AND nuclear equations
-
To indicate the composition of
isotopes and write nuclear equations for radioactive decay, fusion or
fission, we need some kind of shorthand notation - referred to as
nuclide notation
-
To
indicate the composition of a specific atom-isotope we use the
following style of notation:
-
is
an isotope of sodium, consisting of 11 protons, 11 electrons, 12 neutrons, mass 23,
we know this because ...
-
The top left number is the nucleon number or
mass number (A = sum of protons + neutrons =
nucleons),
-
and the bottom left number is the atomic number or
proton number (Z = protons in nucleus = electrons)
-
The neutron number N = A
– Z i.e. mass/nucleon number – atomic/proton number, but is NOT shown
directly.
-
Therefore from the following
'full' atomic symbols, assuming we are dealing with electrically
neutral atoms, the number of sub-atomic particles for the following
atoms will be as follows ...
-
Cobalt
atom (isotope cobalt–59), mass 59, 27 protons, 27 electrons, 32 neutrons (59 – 27)
-
Californium
atom (isotope californium–246), mass 246, 98 protons, 98
electrons, 148 neutrons (246 – 98)
-
You need to understand this
notation in order to write correctly balanced
nuclear equations.
Applying this notation to the three isotopes of carbon, all found
naturally, two are stable and one is radioactive and disintegrates
slowly over thousands of years.
isotope |
nuclide symbol |
protons |
neutrons |
%
abundance |
carbon–12 |
126C |
6 |
6 |
~98.9%, stable |
carbon–13 |
136C |
6 |
7 |
~1.1%, stable |
carbon–14 |
146C |
6 |
8 |
trace, unstable radioactive |
Atomic structure and
its connection with radioactivity
See also history of
atomic structure ideas, atomic structure and isotopes
RADIOACTIVITY IS DUE TO
CHANGES IN ATOMIC STRUCTURE IN THE NUCLEUS OF AN ATOM
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APPENDIX not for IGCSE/GCSE physics students
Quarks - proton & neutron structure
PLEASE NOTE
-
Some sciences courses
require some basic knowledge of the 'quark structure' of protons and
neutrons i.e. the composition of the nuclear protons and neutrons in
terms of up–quarks and down–quarks and their relative electric charge.
So this is an extra section I've added under the heading 'Atomic
Structure'. Section 1c. also discusses beta+ and beta– radioactive decay in
the context of quark changes in the nucleus, so it is only appropriate
to study section 1c. when you have already studied the nuclear equations
for the two modes of beta radioactive decay in section 7.
-
Quarks are elementary
particles even more fundamental than the 'fundamental'
sub–atomic particles we know as the proton and neutron. There mass
must be smaller than a proton or a neutron.
-
I think of them as
sub–sub–atomic particles and the proton and neutron are the most
stable particles known and are formed when quarks combine.
-
The quarks have some
properties you would associate with any elementary fundamental
particle e.g. mass and electric charge (the latter you must
know about).
-
Some textbooks quote
the relative mass of up–quarks and down–quarks as 1/3
but this not strictly true, its more complicated than
that (BUT don't worry about it, as 3 x
1/3
= 1),
-
AND as described below, protons and neutrons
are each composed of three quarks and as a result both
their relative masses are 1.
-
The electric charge
carried by quarks.
-
Up–quarks are
positive with a relative electric charge of +2/3.
-
Down–quarks
are negative with an relative electric charge of –1/3.
-
By adding up the
electric charges of the quarks in protons and neutrons you get
their overall electric charge.
-
Note: You are used
to integer electric charges like +, +1, 2+, +2, –, –1, –2 or 2– in chemistry
(ions, protons, electrons etc.),
but I'm afraid in nuclear physics things are not always that
simple!
-
Quark composition of
a proton
-
Quark composition of
a neutron
-
See section 7. for
quarks and radioactive decay
-
-
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Atomic structure, radioactivity and
nuclear physics revision notes index
Atomic structure, history, definitions,
examples and explanations including isotopes gcse chemistry
notes
1. Atomic
structure and fundamental particle knowledge needed to understand radioactivity gcse physics
revision
2.
What
is Radioactivity? Why does it happen? Three types of atomic-nuclear-ionising radiation
gcse physics notes
3. Detection of
radioactivity, its measurement
and radiation dose units,
ionising
radiation sources
- radioactive materials, background radiation gcse physics revision
notes
4. Alpha, beta & gamma radiation - properties of 3 types of radioactive
nuclear emission & symbols
,dangers of radioactive emissions - health and safety issues and ionising radiation
gcse physics revision
5.
Uses of radioactive isotopes emitting alpha, beta (+/–) or gamma radiation in
industry and medicine gcse notes
6. The half-life of a radioisotope - how
long does material remain radioactive? implications!, uses of decay data and half-life values
-
archaeological radiocarbon dating, dating ancient rocks
gcse physics revision
7. What
actually happens to the nucleus in alpha and beta radioactive decay and why? nuclear
equations!, the
production of radioisotopes - artificial sources of radioactive-isotopes,
cyclotron gcse physics revision notes
8.
Nuclear
fusion reactions and the formation of 'heavy elements' by bombardment techniques
gcse physics notes
9. Nuclear Fission Reactions, nuclear power
as an energy resource gcse physics revision
notes

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RADIOACTIVITY
multiple choice QUIZZES and WORKSHEETS
Easier Foundation
Tier Radioactivity multiple choice QUIZ
Harder Higher
Tier Radioactivity multiple choice QUIZ
Worksheet QUIZ Question 1 on
RADIOACTIVITY - absorption of alpha, beta and gamma radiation
Worksheet QUIZ Question 2 on
RADIOACTIVITY - dangers & monitoring ionising radiation levels
Worksheet QUIZ Question 3 on
RADIOACTIVITY - revision of atomic structure
Worksheet
QUIZ Question 4 on RADIOACTIVITY -
what happens to atoms in radioactive decay?
Worksheet QUIZ Question 5 on
RADIOACTIVITY - uses of radioisotope and half-life data
ANSWERS to the WORD-FILL WORKSHEET QUIZZES
Crossword
puzzle on radioactivity
and
ANSWERS!
[SEARCH
BOX]
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