Topic 7: Atomic and nuclear physics
7.1 The atom
Atomic structure
7.1.1 Describe a model of the atom that features a
small nucleus surrounded by electrons
The atom has a positively charged small nucleus and
is surrounded by negatively charged electrons orbiting
it.
7.1.2 Outline the evidence that supports a nuclear model of the atom
The Rutherford (Geiger-Marsden experiment).
The experiment consisted of firing very small, fast moving alpha particles at a very
thin gold foil. Had the previous model (J. J. Thompson’s Plum Pudding Model) been
true, then the alpha particles would have stuck to the gold foil. Instead, some of the
alpha particles bounced back, and some even went through. The fact that certain
particles went through suggested that there was empty space within the atom; the fact
that certain particles bounced back indicated that they had a head-on collision with a
heavy, positively charged particle. Rutherford therefore concluded that the atom had a
small, positive nucleus, surrounded by negatively charged electrons.
*Evidence for the existence of the nucleus.
7.1.3 Outline one limitation of the simple model of the nuclear atom
The outer electrons, having centripetal acceleration, should radiate, lose energy and
spiral into the nucleus. What actually happens: electrons show quantum behavior that
allows them to exist in different energy levels. Radiation is therefore not emitted
except when the electrons change energy levels.
7.1.4 Outline evidence for the existence of atomic energy levels
Atomic spectra provide evidence that the energy in atoms is discrete (there are energy
levels). When white light passes through a certain material (e.g. hydrogen), various
wavelengths of light are emitted. These wavelengths correspond to the emission line
wavelengths; this is because the electrons absorb the photons and use the energy to
change energy levels.
Nuclear structure
7.1.5 Explain the terms nuclide, isotope and nucleon
Nuclide: a combination of protons and neutrons that form a nucleus.
Isotope: nuclei with the same number of protons but different numbers of neutrons.
Nucleon: the name given to the particles of the nucleus (proton, neutron).
7.1.6 Define nucleon number A, proton number Z and neutron number N
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Nucleon number A: the number of protons plus neutrons in the nucleus. This will be
different for different isotopes of the same element.
Proton number Z: the number of protons in the nucleus. This is always the same for a
given element.
Neutron number N: the number of neutrons in a nucleus.
7.1.7 Describe the interactions in a nucleus
The two forces within the nucleus:
1. The strong nuclear force (between nucleons, mostly attractive, short-range)
2. The weak electrical repulsive force (between protons, repulsive, long-range)
7.2 Radioactive decay
Radioactivity
7.2.1 Describe the phenomenon of natural radioactive decay
Natural radioactive decay happens when nuclei are unstable and try becoming more
stable by decaying (emitting either alpha or beta particle, or gamma rays).
7.2.2 Describe the properties of alpha () and beta () particles and gamma ()
radiation
Alpha
Beta
Gamma
Made of
A helium nucleus (2 A loose electron
Photons
protons, 3 neutrons)
Range in air
(approx) 5 cm
(approx) 30 cm
Electromagnetic radiation
(doesn’t emit particles,
simply loses energy)
Stopped by
Paper
Aluminum
Lead
7.2.3 Describe the ionizing properties of alpha () and beta () particles and
gamma () radiation
Ionization is the process of a photon of high frequency and therefore high energy
absorbs another photon and causes an electron to be ejected from an atom.
Alpha
Beta
Gamma
Penetration
Least
Most
Ionization
Most
Least
7.2.4 Outline the biological effects of ionizing radiation
Very high dose- affects the central nervous system, loss of coordination and death.
Medium dose- damages the stomach and intestines, sickness and diarrhea, potential
death.
Low dose- loss of hair, bleeding and diarrhea.
Long term- cancer or gene mutations in the case of pregnancy.
7.2.5 Explain why some nuclei are stable while others are unstable
If the nucleus contains too many protons compared to neutrons, the repulsive force
will overpower the attractive force and the nucleus will be unstable. A small stable
nucleus has approximately the same number of protons as those of neutrons. A larger
nucleus also has approximately the same number of protons as those of neutrons,
except the number of neutrons will be several higher since neutrons are the “glue”
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holding the nucleus together. The greater the difference of the amount of protons and
neutrons, the more unstable the nucleus will be.
Half-life
7.2.6 State that radioactive decay is a random and spontaneous process and that
the rate of decay decreases exponentially with time
Decay is random (it cannot be known which nucleus or when decay will happen) and
spontaneous (it cannot be known when it is happening or be prevented). Decay
decreases exponentially over time. The nature of the decay is independent of the
initial amount.
7.2.7 Define the term radioactive half-life
The time taken for half of the nuclei in a sample to decay.
7.2.8 Determine the half-life of a nuclide from a decay curve
The half-life of a nuclide can be determined by looking at the point at which half of
the nuclei remain and finding the time period associated with it, as it is the case with
the graphs above. The one on the left shows the half-life of a nuclide, and the one on
the right shows several half-lives.
7.2.9 Solve the radioactive decay problems involving integral numbers of halflives
The half-life of an isotope is 4.0 days. Calculate the fraction of the original activity of
the isotope 12 days after it has been prepared.
An isotope X has a half-life of 2.0 minutes. It decays into isotope Y that is stable.
Initially no quantity of isotope Y is present. After how much time will the ratio of Y
atoms to X atoms be equal to 15?
A radioactive isotope has a half-life of 5.0 minutes. A particular nucleus has not
decayed within a 5.0 min time interval. A correct statement about the next 5.0 min
interval is that this nucleus
A has a lower than 50% chance of decaying
B will certainly decay.
C has a 50% chance of decaying.
D has a better than 50% chance of decaying.
7.3 Nuclear reactions, fission and fusion
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Nuclear reactions
7.3.1 Describe and give an example of an artificial (induced) transmutation
Transmutation is when radioactive particles are emitted from the nucleus not because
the nucleus is unstable but because another nucleon is added to the original nucleus.
For example, the production of nitrogen from carbon is possible through the
artificially initiated reaction by the high-energy particles.
7.3.2 Construct and complete nuclear equations
This website has exercises on constructing and completing nuclear equations:
http://www.chemtopics.com/unit11/ppu11.pdf
7.3.3 Define the term unified atomic mass unit
Unit used for masses on atomic or molecular state.
1 u = 1.66x10-27 kg
The unified atomic mass unit is 1/12 of the mass of the neutral atom of the carbon
isotope carbon-16.
7.3.4 Apply the Einstein mass-energy equivalence relationship
E = mc2
If work is done, then there must be some sort of transfer of energy. The energy has
been converted to mass. The mass of the particles when they are apart is greater than
when they are together.
7.3.5 Define the concepts of mass defect, binding energy and binding energy per
nucleon
Mass defect: the difference between the mass of the whole nucleus and the mass of
the parts of a nucleus.
Binding energy: the amount of work required to pull apart the constituents of a
nucleus.
Binding energy per nucleon: the amount of work required to pull apart the
constituents of a nucleus per nucleon.
7.3.6 Draw and annotate a graph showing the variation with nucleon number of
the binding energy per nucleon
7.3.7 Solve problems involving
mass defect and binding energy
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This website has a thorough summary of radioactive decay and practice problems
(with solutions):
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch23/modes.php
Fission and fusion
This is an other website that has more information on fission and fusion, it is a good
resource to look through:
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch23/fission.php#fusion
7.3.8 Describe the processes of nuclear fission and nuclear fusion
Fission: the nuclear reaction of the nucleus of an atom splitting into two smaller
pieces.
Fusion: the joining up of two small nuclei to form a big one.
Energy is released in both processes.
7.3.9 Apply the graph in 7.3.6 to account for the energy release in the processes
of fission and fusion
The masses of the individual atoms add up to be more than the mass of the whole,
therefore there must be some source of mass/energy loss; therefore in fission, where
the nucleus of an atom splits, the energy released is that of the binding energy of the
nuclei (holding the atom together). In fusion, where two nuclei combine to form
another one, the energy released is that of the parts fusing together; the binding
energy as well. The reason fusion releases more energy is because the difference in
the binding energies of the elements in fusion is greater than those of fission.
7.3.10 State that nuclear fusion is the main source of the Sun’s energy
Nuclear fusion is the main source of the Sun’s energy.
7.3.11 Solve problems involving fission and fusion reactions
This is a web link to Giancoli problems on chapter 30. They have hints and cover
fission, fusion and some other topics related to nuclear physics. (It also has links to
other Giancoli chapters and practice problems) :
http://wps.prenhall.com/esm_giancoli_physicsppa_6/17/4362/1116732.cw/index.html
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