CHEM101 Study Guide

Unit 8: Nuclear Chemistry

8a. Distinguish different types of nuclear decay

  • Define the characteristics of an alpha particle.
  • Define the characteristics of a beta particle.

There are two main types of nuclear decay. An alpha particle is a type of nuclear decay that is equivalent to a helium nucleus: 42He. When this type of decay occurs, the atomic number of the product will be reduced by two. A beta particle is a high energy electron. During beta particle decay, the neutron decomposes into a beta particle and a hydrogen nucleus, 11H. When this type of decay occurs, the atomic number of the product is increased.

Review examples of alpha particle emission in Alpha-Decay. Review examples of beta particle emission in Beta-Decay.

 

8b. Balance nuclear equations

To balance a nuclear equation, we must account for how the emission of nuclear particles changes the nucleus of the atom. Alpha particle decay will reduce the mass number of the product by four and the atomic number by two. Beta particle decay will keep the mass number constant but will increase the atomic number by one.

Review examples of Alpha particle decay in Alpha-Decay. Review examples of Beta particle decay in Beta-Decay.

 

8c. Explain the process of radioactive dating

  • What is half-life?
  • How does radioactive dating work?

Radioactive dating is an important technique that takes advantage of the known half-lives and natural abundances of radioisotopes.

All radioactive isotopes decay in a predictable pattern. We define the term half-life as the time it takes for half of a sample of a radioactive isotope to decay to its daughter element. Half-lives are known and are constant for different isotopes. This decay occurs in a predictable pattern, as seen below.

A graph that depicts half-life in terms of the number of atoms of a specific isotope and time

We can write a rate equation for the rate of radioactive decay:

 \mathrm{k}=\frac{\ln|2|}{t_{1/2}}=\frac{0.693}{t_{1/2}}

( \mathrm{k} is called the rate constant, and  t_{1/2} is the half-life of the isotope)

From this equation, we can determine the ratio of the concentration of the isotope at a certain time, Ct, to the initial concentration of the isotope, C0, by the equation:

 \ln \frac{C_0}{C_t}=\mathrm{kt}

This allows us to determine how much of a radioactive isotope will remain after a certain amount of time has passed. Review a detailed description of the half-life rate equations in Half-Life.

The most common type of isotope dating is carbon dating, which is used for determining the age of archeological and other artifacts. In carbon dating, the age of carbon-containing material is determined by comparing the decay rate of that material with living material.

Carbon-14 decays by the following reaction:

 ^{14}_{6}\mathrm{C}\rightarrow ^{14}_{7}\mathrm{N} + ^{0}_{-1}e with a half-life of 5.73 x 103 years

Review an example of how carbon dating was used to determine the age of the dead sea scrolls in Carbon Dating.

 

8d. Describe the processes of nuclear fission and fusion

The two types of nuclear reactions are nuclear fission and nuclear fusion. In nuclear fission, a large nucleus is split by being “hit” by a high energy neutron. This creates two new atoms, which each continue to form new atoms and neutrons if there is sufficient energy. This is known as a chain reaction, which produces an immense amount of energy. Nuclear fission reactions were what was used in the atomic bombs.

Diagram of nuclear fission, a chain reaction

Nuclear fusion, by contrast, is the process of combining small nuclei to form a larger nucleus.

The simplest example of this is combining two deuterium (hydrogen isotope) atoms to form helium:

 ^{2}_{1}\mathrm{H}+^{2}_{1}\mathrm{H}\rightarrow ^{4}_{2}\mathrm{He}

This produces significantly more energy than nuclear fission. Nuclear fusion is what takes place in the sun and other stars because they have sufficient hydrogen reserves to sustain the reaction.

Review background on nuclear reactions in Transmutation of the Elements, and The Mass Defect.

 

8e. Explain how radioactive decay is used as a source of energy

Nuclear power is a source of energy for many people all over the world. In nuclear power plants, a nuclear fission chain reaction takes place to produce energy. The reaction rate is controlled by control rods that absorb excess neutrons produced in the chain reaction without undergoing nuclear fission reactions themselves. These control rods are made of different metals and alloys. The nuclear material is kept in fuel rods that are placed between the control rods. By moving the control rods, the rate of nuclear reaction in the fuel rods can be controlled. The energy from the nuclear reaction is put through a heat exchanger to create steam to turn a turbine. The most-used radioactive material is uranium.

Review a description of some of the considerations in using nuclear power in Nuclear Energy.

 

Unit 8 Vocabulary

  • Alpha particle
  • Beta particle
  • Carbon dating
  • Chain reaction
  • Control rod
  • Daughter element
  • Fuel rod
  • Half-life
  • Nuclear decay
  • Nuclear fission
  • Nuclear fusion
  • Radioactive dating
  • Rate equation