Chapter 10
Nuclear Chemistry
Radioactivity
Radioactivity is the process in which an
unstable atomic nucleus emits charged particles
and energy.
Any atom containing an unstable nucleus is
called a radioactive isotope, or radioisotope
for short.
Radioactivity
During nuclear decay, atoms of one element
can change into atoms of a different element
altogether.
Nuclear radiation is charged particles and energy
that are emitted from the nuclei of radioisotopes.
Common types of nuclear radiation include
alpha particles, beta particles, and gamma
rays.
Alpha Decay
When a uranium-238 sample decays, it emits
alpha particles.
An alpha particle is a positively charged particle
made up of two protons and two neutrons – the
same as a helium nucleus.
Alpha Decay
Alpha particles are the least penetrating type of nuclear
radiation. They can be stopped by a sheet of paper of
clothing.
Alpha particles can cause skin damage similar to a burn,
but they are not a serious health hazard unless an alphaemitting substance is inhaled or eaten.
Radon gas is a dangerous natural source of alpha particles
because it can be inhaled.
Beta Decay
When thorium-234 decays, it releases negatively
charge radiation called beta particles.
A beta particle is an electron emitted by an
unstable nucleus.
Beta Decay
Beta particles are more penetrating than alpha
particles. They pass through paper, but are
stopped by a thin sheet of metal.
Beta particles can damage tissues in the body
more than alpha particles.
Gamma Decay
When thorium-234 decays, it also releases
radiation that has no charge and no mass.
A gamma ray is a penetrating ray of energy
emitted by an unstable nucleus. It has no charge
and no mass.
Gamma Decay
Gamma rays are much more penetrating than
either alpha particles or beta particles. It can take
several centimeters of lead or several meters of
concrete to stop gamma radiation.
Gamma rays can penetrate deeply into the human
body, potentially exposing all organs to ionization
damage.
Radioactivity
You are exposed to radiation every day.
Most of this is background radiation, or nuclear
radiation that occurs naturally in the environment.
Nuclear radiation can ionize atoms. When cells are
exposed to nuclear radiation, the bonds holding together
proteins and DNA molecules may break. As these
molecules change, the cells may no longer function
properly.
Radioactivity
Radioisotopes in air, water, rocks, plants, and
animals all contribute to background radiation.
Cosmic rays (streams of protons and alpha
particles) also contribute to background radiation.
Background radiation levels are generally low
enough to be safe.
Detecting Nuclear Radiation
A Geiger counter uses a gas-filled tube to measure
ionizing radiation. When nuclear radiation enters
the tube, it ionizes the atoms of the gas. The ions
produce an electric current, which can be
measured.
Detecting Nuclear Radiation
A film badge contains a piece of photographic film
wrapped in paper. The film is developed and
replaced with a new piece periodically. The
exposure on the film indicates the amount of
radiation exposure for the person wearing the
badge.
Rates of Nuclear Decay
A nuclear decay rate describes how fast nuclear
changes take place in a radioactive substance.
Unlike chemical reaction rates, which vary with
the conditions of a reaction, nuclear decay rates
are constant.
Every radioisotope decays at a specific rate that
can be expressed as a half-life.
Rates of Nuclear Decay
A half-life is the time required for one half of a
sample of a radioisotope to decay.
• After one half-life, half of the atoms in a
radioactive sample remain
1
4
• After two half-lives, of the atoms in the sample
remain.
1
8
• After three half-lives, only of the atoms in the
sample will remain.
Rates of Nuclear Decay
Rates of Nuclear Decay
In radiocarbon dating, the age of an object is
determined by comparing the object’s carbon-14
levels with carbon-14 levels in the atmosphere.
Carbon-14 undergoes beta-decay to form
nitrogen-14
Rates of Nuclear Decay
Carbon-14 can only be used to date objects that
are less than 50,000 years old.
For objects older than 50,000 years, scientists use
other radioisotopes.
Geologists use potassium-40, uranium-235, or
uranium-238 to date rock formations. The older
the rock, the lower the levels of the radioisotope
present.
Nuclear Forces
The strong nuclear force is the attractive force
that binds protons and neutrons together in the
nucleus.
Over very short distances, the strong nuclear force
is much greater than the electric forces among
protons.
Nuclear Forces
A. The strong nuclear forces easily overcome the
electric force between the protons.
B. The larger number of electric forces makes the
nucleus less stable.
Nuclear Forces Acting on a
Proton of a Small Nucleus
Nuclear Forces Acting on a
Proton of a Large Nucleus
Nuclear Forces
A nucleus becomes unstable, or radioactive, when
the strong nuclear force can no longer overcome
the repulsive electric forces among protons.
The strong nuclear force does not increase with
the size of the nucleus, but the electric forces do.
All nuclei with more than 83 protons are
radioactive.
Fission
Fission is the splitting of an atomic nucleus into
two smaller parts.
In nuclear fission, tremendous amounts
of energy can be produced from very
small amounts of mass.
Mass-Energy Equation
E = mc2
Fission
Neutron
Energy
Fission
In a chain reaction, neutrons released during
the splitting of an initial nucleus trigger a series
of nuclear fissions.
A critical mass is the smallest possible mass of
a fissionable material that can sustain a chain
reaction.
Nuclear power plants use controlled fission of
uranium-235 in a fission reactor.
Nuclear Energy from Fission
Unlike power plants that burn fossil fuel, nuclear power
plants do not emit air pollutants.
However, nuclear plants have their own safety issues.
• Workers in nuclear plants must wear protective clothing
to reduce their exposure to nuclear radiation
• The fission of uranium-235 produces many radioactive
isotopes with half-lives of hundreds or thousands of
years.
• Operators of the plan could lose control of the reactor.
Fusion
Fusion is a process in which the nuclei of two
atoms combine to form a larger nucleus.
Very high temperatures (similar to the sun) are
required to start the reaction. At temperatures
this high, matter exists as plasma.
Plasma is a state of matter in which atoms have
been stripped of their electrons.
Fusion
The sun is not actually burning.
The sun, and other stars, are powered by the
fusion of hydrogen into helium.
The light and heat given off by the sun result from
nuclear fusion, not combustion.
Fusion
Fusion may someday provide an efficient and
clean source of energy.
Scientists envision fusion reactors fueled by two
hydrogen isotopes, deuterium (hydrogen-2) and
tritium (hydrogen-3).
Fusion
Advantages of using fusion over fission:
• Workers are not in as much danger from radiation
• Hydrogen is used, which is easily obtained from water
• No harmful waste products are produced
Two main problems in designing a fusion reactor are the
high temperatures required to start the reaction and that
they must contain the plasma.