Study Guide for Atomic and Nuclear Physics Answers
1. Describe a model of the atom that features a small nucleus surrounded by electrons.
Rutherford Model: the atom consists of a small dense positive nucleus,
surrounded by electrons that orbit the nucleus (as planets orbit the sun) as result of
electrostatic attraction between the electrons and the nucleus.
2. Outline the evidence that supports the nuclear model of an atom.
Geiger and Marsden’s experiment:
i. Alpha particles bombarded at a sheet of gold foil mostly passed
through—atoms mostly consist of empty space.
ii. Particles that were deflected bounced straight back from the foil—the
huge deflection of the alpha particles must have been caused by
electrostatic repulsions between the positive alpha particles and a dense,
positive nucleus.
3. Outline one limitation of the simple model of the nuclear atom.
Did not explain why electrons surrounding the nucleus were not drawn into the
nucleus by strong electrostatic attractions to the protons of the nucleus.
Did not specify composition of nucleus.
How did protons in the nucleus stay closely bound when electrostatic forces
should have forced them apart?
4. Outline evidence for the existence of atomic energy levels.
All elements will emit light in characteristic
When an electron falls between two energy levels it will emit a photon equal in
energy to the difference in energy levels. The energy of a photon is dependent on
its frequency. Thus, the existence of discrete wavelengths in the spectrum is
evidence that energy levels are discrete.
5. Define emission spectrum, absorption spectrum, spectroscopy, and line spectrum.
The patterns of light from heated gases differ from those of sunlight, consisting
of a series of bright lights separated by dark gaps—line spectrum.
Emission spectrum: a spectrum of light emitted by an element; a series of bright
lines, with dark gaps between the lines where no light is emitted.
Absorption spectrum: a bright continuous spectrum covering the full range of
visible colors, with dark lines where the element absorbs light.
Spectroscopy: the study of emission and absorption spectra.
6. Explain the terms nuclide, isotope, and nucleon.
Nuclide: a species of atom characterized by the constitution of its nucleus and
hence by the number of protons, the number of neutrons, and the energy
content.
Nucleon: a proton or neutron.
Isotopes: different forms of the same element that contains the same amount of
protons but different amount of neutrons.
7. Define nucleon number A, proton number Z and neutron number N.
Nuclide: a nuclear isotope,
, where X is the chemical symbol of the element, A
is the mass number of the isotope, and Z is the atomic number of the element.
Nucleon: mass number of a nuclear isotope, is the total number of nucleons
(protons and neutrons) found in the nucleus.
8. Describe the interactions in a nucleus.
Gravitational forces are proportional to the mass of the objects and inversely
proportional to the distance between them; thus they are negligible within the
nucleus.
Repulsive electromagnetic forces between the protons would cause the nucleus
to disintegrate if it were the only force.
The strong nuclear force is an attractive force, which exists between all nucleons
to hold them together. It is effective only over a very short range.
The weak nuclear force exists only in the nucleus and is responsible for the
disintegration of a neutron into a proton and an electron in beta decay.
9. Describe the phenomenon of natural radioactive decay.
Radioactive decay: process in which unstable atomic nucleus loses energy by emitting
radiation in form of particles or EM waves, resulting transformation of parent nuclide
into daughter nuclide.
Unit: becquerel (Bq), transformation per second.
Alpha decay: atomic nucleus emits alpha particle, equivalent to Helium nucleus
.
E.g. . Governed by strong nuclear force.
Beta decay: atomic nucleus emits beta particle (electron or positron).
a. In β-− decay, the weak interaction converts a neutron (n0) into a proton (p+) while
emitting an electron (e-) and an anti-neutrino ( ):
.
b. In β+ decay, energy is used to convert a proton into a neutron, a positron (e +) and
a neutrino ( ): energy + p+ →n0 + e+ + . Requires energy thus cannot occur in
isolation.
Gamma radiation: electromagnetic waves (high-energy photons, ) are emitted
during radioactive decay.
10. What is half life?
half life — symbol t1 / 2 — the time for half of a substance to decay.
11. What is mean lifetime?
mean lifetime — symbol τ — the average lifetime of any given particle.
12. What is a decay constant?
decay constant — symbol λ — the inverse of the mean lifetime.
Some nuclei have a larger neutron-proton ratio and thus a relatively larger strong
nuclear force as opposed to its repelling electromagnetic force. Those nuclei are more
stable.
13. What happens to biological organisms exposed to ionizing radiation?
Particle
Alpha ( )
Beta ( )
Gamma ( )
Nature
Helium nucleus
Electron
Photon
Charge
+2e
-e
0
Mass
4u
1/1850 u
0
Penetrative power
A few cm of air
A few mm of metal
Many cm of lead
Ions per mm of air for
2 MeV particles
10000
100
1
Magnetic fields
Is affected
Is affected
Not affected


Prompt effects: effects, including radiation sickness and radiation burns, seen immediately
after large doses of radiation delivered over short periods of time.
Delayed effects: effects such as cataract formation and cancer induction that may appear
months or years after a radiation exposure
14. Determine the half-life of a nuclide from a decay curve.
Refer to your book for this one or
15. Solve radioactive decay problems involving integral numbers of half-lives.
226 Ra, a common isotope of radium, has a half-life of 1620 years. Knowing this, calculate the first order
88
of radium-226 and the fraction of a sample of this isotope remaining after 100 years.
Solution
The rate of radioactive decay is expressed by the relationship:
k = 0.693/t1/2
where k is the rate and t1/2 is the half-life.
Plugging in the half-life given in the problem:
k = 0.693/1620 years = 4.28 x 10-4/year
Radioactive decay is:
log10 X0/X = kt/2.30
where X0 is the quantity of radioactive substance at zero time (when the counting
process starts) and X is the quantity remaining after time t. k is the first order rate
constant, a characteristic of the isotope that is decaying. Plugging in the values:
log10 X0/X = (4.28 x 10-4/year)/2.30 x 100 years = 0.0186
Taking antilogs: X0/X = 1/1.044 = 0.958 = 95.8% of the isotope remains
There will not be any this complex on the quiz, but IB could put one like this on their test.
16. What is planck’s constant and what is it used to calculate?
Planck's constant = 6.626068 × 10-34 m2 kg / s (see worksheet for how it is used.
17. What is Einstein’s mass energy equivalence relationship?
18. What is binding energy?
Binding energy: the work required to completely separate the nucleons of a nucleus.
Mass defect: the difference between the total mass of all nucleons in the atom and the
mass of the atom itself. Equivalent to and lost due to binding energy. Binding energy per
nucleon: the work required to remove one nucleon from the nucleus; roughly the binding
energy divided by number of nucleons in nucleus.
19. Describe the processes of nuclear fission and nuclear fusion. Describe the sun’s main
source of energy.
Artificial transmutation is causing particles to decay by bombardment of
particles. E.g. Uranium atoms bombarded with neutrons to start fission reaction.
A unified atomic mass unit is One twelfth of a carbon-12 atom.
1u = 1.6605402 × 10-27 kg
Nuclear fission: process in which a heavy nucleus splits up into lighter nuclei.
Large amounts of energy produced, can be self-sustaining due to chain reactions.
Nuclear fusion: joining of two light nuclei into a heavier one.
High temperatures required to provide sufficient kinetic energy to approach each
other, overcoming electrostatic repulsion.
Nuclear fusion is the main source of the Sun’s energy.
20. Know what an electron volt is and how it is used.
1 electron volt = 1.60217646 × 10-19 joules
(See handouts)
21. What is the equivalent of Hz on the metric system?
(See handouts)
22. What are the parts of an atom? How do you determine an isotope? Remember all that
from chemistry. Proton, neutron, electron difference in number of neutrons (more
info in notes)
The rest of the quiz will be over people….the Curie’s, Dalton, Rutherford, Bohr, etc. that we
covered in class and over the problems of line spectrum and half life. You have sample
problems already and there are problems on the website to assist you. I will put the answers to
the above questions on the web tomorrow 10/16/10 around 8:00 PM.