C.W. BAKER HIGH SCHOOL

 

Nuclear Chemistry Outline

Prepared by M. Foster

 

I                     Types of Nuclear Reactions

a         Radioactive Decay – the nuclei of some atoms are unstable, and may  spontaneously emit small particles leaving behind an atom of a new element.  Atoms that undergo radioactive decay are called radioisotopes.

1        Types of natural radioactive decay

·        Alpha decay – some unstable atoms will emit an alpha particle.  An alpha particle is a nucleus of a helium atom composed of two protons and two neutrons.  It can be represented by the Greek letter a or by 42He.  Alpha particles carry a charge of +2.

·        Beta decay – some unstable atoms will emit a beta particle.  A beta particle is essentially an electron (which is emitted from the nucleus).  It can be represented by the Greek letter B or by –10e.  Beta particles carry a charge of  -1.

·        Gamma radiation – in addition to alpha and beta particles, unstable nuclei may emit high energy photons of electromagnetic radiation during radioactive decay.  These photons are call gamma rays and can be represented by g.  Gamma rays have no charge and no detectable mass.

2        The decay mode for various radioisotopes can be identified by looking in Chart N of the Reference Tables.

3        The different types of radioactive decay particles can be identified by passing them through an electric field as shown below:

The Beta particles are attracted to the positive plate due to their negative charge, while the alpha particles are attracted toward the negative plate due to their positive charge.  Gamma rays are not attracted toward either plate because they have no charge.  Notice that the beta particles are deflected more than the alpha particles.  This is because the alpha particles have more mass and are harder to deflect than the much lighter beta particles.

4        The rate at which a sample of atoms undergoes radioactive decay cannot be changed. 

·        The time which it takes for one half of a sample of an atom to undergo radioactive decay is called the half-life.

 

b        Fusion – a process by which smaller atoms are combined together to form larger atoms.  This process is responsible for the energy given off by stars (including the Sun).

1        In stars, such as our Sun, hydrogen atoms combine together to produce helium atoms.

2        Hydrogen bombs (thermonuclear devices) are utilize the fusion process.

3        Scientists and engineers are trying to make controlled fusion reactors.  Hopefully, these attempts will eventually result in a new and abundant source

of energy.

 

c         Fission – a process by which large atoms split into two smaller atoms, producing energy and neutrons.

1        Two isotopes are utilized in fission reactions

·        U – 235  (Uranium –235)

·        Pu –239 (Plutonium- 239)

2        The neutrons that are produced in one fission reaction can strike other U-235 atoms and cause them to undergo fission.  This process is called a nuclear chain reaction.

·        The minimum amount of fissionable material required to sustain a chain reaction is called the critical mass.

 

II                  Mass-energy conversions in nuclear reactions

a        Unlike chemical reactions, the total mass of the reactants is not equal to the total mass of the reactants in a nuclear reaction.  The difference in mass is called the mass defect.

b        The mass which appears to be “lost” in the reaction is actually converted into energy.  The equation E = Dm x c2 can be used to find out how much energy is produced for a given quantity of mass.

c         The quantity of energy produced in chemical reactions is much greater than in chemical reactions. 

1        Since fusion reactions involve the greatest mass defect, they produce the most energy .

 

III               Writing Balanced Nuclear Reactions

a         Conservation Laws

1        Law of Conservation of Mass Number – the total of the mass numbers of the reactants must equal the total of the mass numbers of the products.

 

2        Law of Conservation of Atomic Number – the total of the atomic numbers of the reactants must equal the total of the atomic numbers of the reactants.

 

3        By utilizing the two laws it is possible to predict the products of nuclear reactions:  For example:  Predict the product of the beta decay of C-14.

·        First, we write the reactant atom (C-14) and show it giving off a beta particle (-10e)

·        Then, utilizing the laws of conservation of mass number and atomic number, determine the mass number and atomic number of the other atom.

·        Finally, determine the symbol of the atom by looking it up in the Periodic Table.

 

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IV                Applications involving nuclear reactions

1        Nuclear power plants – utilize the fission reaction to generate heat.  The heat is used to drive turbines/generators and create electricity.

·        The fissionable material is held within the reactor cores as pellets packed inside of metal tubes called fuel rods.

·        The rate that neutrons released from one fission are allowed to cause subsequent fissions is controlled by the control rods.  These control rods are made of boron or cadmium steel and absorb neutrons preventing them from striking other fissionable atoms and causing additional fissions.

·        The chance of neutrons causing atoms to undergo fission is increased by slowing the neutrons down.  Modern reactors utilize water or graphite to slow down the neutrons and increase the likelihood of fissions taking place.  These substances are called moderators.

¨      Water often time serves a dual purpose in a reactor, functioning both as a coolant and a moderator.

·        Advantages of nuclear power

¨      Relatively small amount of fuel provides a large amount of energy.

¨      While operating, release no pollution into the atmosphere.  Fossil fuel plants release greenhouse gases and cause acid rain.

4        Disadvantages of nuclear power

5        Products of fission are radioactive with long half-lives.  Disposal requires isolating waste products for thousands of years.

¨      Danger to the environment if radioactive material in core is released due to some form of accident within the plant.

 

2        Dating samples using radioactive half-lives -  by analyzing the amount of undecayed radioactive isotopes it is possible to determine the length of time that a sample has been undergoing radioactive decay, and thus the age of the sample.

·        Carbon-14 Dating – compares the amount of radioactive C-14 left in a sample with the amount of the isotope which was originally present in the sample.  Used primarily for dating organic material (things which were once alive).

·        U-238 – Pb-206 series -  U-238 undergoes a number of steps in eventually decaying into the stable isotope of Pb-206.  By comparing the relative amounts of the U-238 and the isotopes produced in the decay it is possible to determine the age of a sample.  Used primarily in determining the age of rocks.

·        K-40 – Ar-40 – another substance with a very long half-life that makes it ideal for determining the age of very old rocks.

 

3        Medical treatments

·        Iodine-131 – used in the treatment of thyroid disease.

·        Cobalt-60 – used to irradiate cancers.  Cancerous growths, due to their rapid rate of cell division, are more vulnerable to radiation than normal cells.