9.2.E - Nuclear Chemistry


Radioactive materials that release alpha, beta or gamma radiation have a range of uses in both industry and medicine. The are used as diagnostic tools in both industry and medicine and are used in the treatment of diseases such as cancer in medicine. Radioisotopes have an unstable nucleus and release energy by ejecting particles or electromagnetic radiation from the nucleus. They are made in nuclear reactors where radioactive decay processes are artificially induced by bombarding nuclei with neutrons. Some radioisotopes can be made by smashing the nuclei of atoms together in particle accelerators through a process known as nuclear fusion. Many artifically synthesised elements called transuranic elements have been made in this way in particle accelerators.

In this unit you will learn to:

  • Describe the structure of the nucleus using atomic notation
  • Distinguish isotopes of an element and represent them using atomic notation
  • Explain what makes a nucleus unstable
  • Describe the processes of alpha, beta and gamma decay using chemical equations
  • Describe the properties of alpha, beta and gamma radiation
  • Oultine ways to detect radiation
  • Describe how transuranic elements are produced
  • Describe some uses of radioisotopes in industry and medicine


The Nucleus

Atomic Structure

The structure and atomic notation for the element helium.

In chemistry, electrons take up much of the air time because they are principally responsible for the chemical properties and reactions of substances. However, in the study of nuclear chemistry we turn our attention to the nucleus and the changes that sometimes take place there when atoms change from one element into another.

The nucleus of an atom contains the protons and neutrons. An element is defined by the number of protons it contains. Oxygen is oxygen because it has eight electrons while carbon is carbon because it contains six electrons. The neutrons play another role that we will come to later. The atomic number of an element tells us how many protons there are in the nucleus of a particular atom of that element. The mass number or atomic weight tells us how many nucleons there are in an atom of the element. A nucleon is any particle found in the nucleus so the mass number tells us how many protons and neutrons there are in total in the nucleus. To work out the number of neutrons you simply subtract the atomic number from the atomic mass number. Both of these values can be determined for any element from the periodic table.

The atomic notation for an element uses the symbol of the element with a subscript on the left of the symbol showing the atomic number and a superscript showing the atomic mass as shown in the diagram above for helium. Elements are sometimes also written as helium-4, for example. The number at the end represents the mass number.



A video from TED Ed on the relative size of the atom and nucleus.

 

Isotopes

Atoms of the same element with different mass numbers are called isotopes as they contain the same number of protons but different numbers of neutrons. For example, three naturally occurring isotopes of uranium are uranium-234, uranium-235 and uranium-238 and they each have 142, 143 and 146 neutrons, respectively.

Different isotopes will have different abundances. For example, 99.3% of naturally occurring uranium is uranium-238, 0.7% is uranium-235 and only a trace is uranium-234. All elements have isotopes and the atomic number printed in the periodic table represents an average number that is weighted according to the abundance of each isotope. This is the reason why most atomic masses are not whole numbers but have decimal values.

Nuclear Stability

The belt of stability shows a plot of all of the stable
isotopes of the known elements. This graph shows
that as the number or protons in the nucleus
increases, the number of neutrons needed to
maintain stability increases at an increasing rate.

Not every isotope of an element will have a stable nucleus. The stability of a particular nucleus depends on a balancing act between the force of replusion between the positively charged protons and a force of attraction between nucleons known as the strong nuclear force. If these two forces are balanced, the nucleus will be stable. In many isotopes, the repulsive force between the protons is larger than the strong nuclear force and this makes the isotope unstable. Unstable isotopes are called radioisotopes because they undergo radioactive decay in a bid to achieve a stable nuclear configuration.

A stable nucleus is one where there is an appropriate ratio of protons and neutrons. The more protons packed into the nucleus, the more neutrons are needed to bind the nucleus together. Stable nuclei with low atomic numbers (up to about 20) have approximately equal numbers of protons and neutrons. For nuclei with higher atomic numbers, the number of neutrons exceeds the number of protons. Indeed, the number of neutrons necessary to create a stable nucleus increases more rapidly than the number of protons as shown in the graph. As such, the neutron-to-proton ratios of stable nuclei increase with increasing atomic number.

Since unstable nuclei undergo radioactive decay and turn into stable isotopes, the most abundant isotopes present on Earth are those that are stable. If you round the mass number in the periodic table off to a whole number, this will generally tell you the mass number of the most abundant and stable isotope. Every element below lead (atomic number 82) will have at least one isotope that is stable. Above lead, all of the isotopes of every element are unstable and are, therefore, radioisotopes.

 

Radioactive Decay

The relative penetration of alpha, beta and gamma radiation.

Radioisotopes have unstable nuclei due to an imbalance between the forces of attaction and repulsion in the nucleus. This imbalance is due to a lack of neutrons or too many many protons and the nucleus changes to address this problem. Radioactive decay is the process by which a nucleus will eject particles or electromagntic radiation to address the imbalanceand/ or release excess energy. This process occurs naturally for radioisotopes and keeps on occuring until a stable nuclear configuration is achieved.

A summary of the properties and decay process for each type of decay is shown in the table below.

Property Alpha Beta Gamma
Identity helium nucleus electron gamma photon
Symbol
Charge +2 -1 0
Mass large small no mass
Pentrating Ability stopped by a sheet of paper stopped by a sheet of aluminium stopped by a thick sheet of lead
Ionising Ability high medium low
Example
Nuclear Process Two neutrons and two protons are ejected from the nucleus A neutron is converted to a proton and electron and the electron is ejected from the nucleus Gamma decay usually accompanies beta decay and some of the excess energy of the nucleus is released as a gamma photon

 



A video shoing the properties and equations of alpha, beta and gamma radiation.

 

Detecting Radiation

A typical Geiger counter used to
detect alpha, beta and gamma
radiation.

A variety of methods have been devised to detect emission from radioactive sources. Becquerel discovered radioactivity because of the effect of radiation on photographic plates. Photographic plates and film have long been used to detect radioactivity. The radiation affects photographic film in much the same way that x-rays do. With care, the film can be used to give a quantitative measure of activity. The greater the extent of exposure to the radiation, the darker the area of the developed negative. People who work with radioactive substances carry film badges to record the extent of their exposure to radiation.

Radioactivity can also be detected and measured using a device known as a Geiger counter. The operation of a Geiger counter is based on the ionisation of matter by radiation. The ions and electrons produced by the ionising radiation permit conduction of an electric current. The counter consists of a metal tube filled with gas. It has a cylinder that can be penetrated by radiation. In the centre of the tube is a wire connected to one terminal of a source of direct current. The metal cylinder is attached to the other terminal. Current flows between the wire and the metal cylinder when ions are produced by entering radiation. The current pulse created when radiation enters the tube is amplifies; each pulse is counted as a measure of the amount of radiation.

Click here to find out about some other ways to detect radiation.

Half-Life

A graph showing how the percentage of carbon-14 remaining is
related to its half-life. Carbon-14 undergoes beta decay and is used
in radioactive carbon dating.

Radioisotopes do not give out radiation at the same rate forever. The radiation steadily decreases exponentially. The time taken for the radiation to drop by half is called the half-life of the radioactive isotope. For example, the half-life of uranium-238 is 4.5 billion years. That is, in 4.5 billion years, half of the uranium-238 on the earth will have decayed into other elements. In another 4.5 billion years, half of the remaining uranium-238 will have decayed. One fourth of the original material will remain on Earth after 9 billion years. Some radioisotopes have very short half-lives. Barium-137, used in the diagnosis of stomach problems, has a half-life of only 2.6 minutes. Other radioisotopes have extremely long half-lives such as uranium-238 described above. Radioactive emission of alpha particles, beta particles and gamma ray photons is the result of radioactive decay. Radioactive decay occurs randomly over space and time. One measure of the rate of decay is half-life. Half-life is defined as the time taken for half of the unstable nuclei to decay as shown in the graph on the right.

Artificially Induced Fission


This animation shows a single neutron absorbed by uranium-235 which
then splits into xenon and strontium, releasing three other neutrons.

Fission is a process where a large nucleus transmutates in to two or more smaller nuclei. Fission can be artificially triggered by bombarding a nucleus with neutrons. Heavy nuclei break up under such bombardment into a pair of lighter nuclei. As the nucleus breaks, the fragments fly apart under electrostatic repulsion and heat up the environment by collisions with neighbouring particles. The fragments are also in a highly excited state and its nucleons rearrange into configurations appropriate to medium weight nuclei by release of gamma radiation. As these medium weight nuclei would have smaller neutron proton ratios, some neutrons convert to protons by beta emission. As stated, some nuclei can be made unstable by firing an extra neutron into them. This is called a fission reaction because the nuclei split into two fragments. When the nucleus decays, more neutrons are ejected. If these neutrons cause further fissions to occur a chain reaction occurs. Controlled chain reactions take place in nuclear reactors, which use a fissionable material such as uranium-235.

Commerical and Medical Radioisotopes

The sequence of reactions involved in producing
cobalt-60 in a nuclear reactor.

Most synthetic isotopes used commerically, in medicine or scientific research are produced using neutrons as projectiles in nuclear reactors as described above. Because neutrons are neutral, they are not repelled by the nucleus and do not need to be accelerated like charged particles in order to cause nuclear reactions. Also, because they have no charge, it is not possible for them to be accelerated by particle accelerators. The necessary neutrons are produced by the reactions that occur in nuclear reactors as described above. Cobalt-60, for example, used in radiation therapy for cancer, is produced by neutron capture. Iron-58 is placed in a nuclear reactor where it is bombarded by neutrons. The following sequence reactions taking place place to produce the cobalt-60 are shown on the right.

Transuranic Elements



Neptunium-239 and Plutonium-239 are transuranic elements that can be
synthesised in a nuclear reactor using uranium-238 and neutrons via beta
decay with neptunium as an intermediate.

Artificially induced fission has been used to produce the elements above atomic number 92. These are known as transuranic elements because they occur immediately following uranium in the periodic table. Elements 93 (neptunium, Np) and 94 (plutonium, Pu) were first discovered in 1940. They were produced by bombarding uranium-238 with neutrons as described above. However, most transuranic elements with larger atomic numbers are normally formed in small quantities in particle accelerators (see next section).

Nuclear Fusion

Fusion is a process whereby two smaller nuclei or particles are fused together to form a larger nucleus.

Particle Accelerators

The structure of a typical particle accelerator used to produce
roentgenium-272 . The nickel nucleus would make many trips
around the accelerator before it strikes the bismuth target.

Charged particles, such as protons or nuclei, must be moving very fast in order to overcome the electrostatic repulsion between the two positively charged nuclei. The higher the nuclear charge, the faster the nucleus or particle must be moving to bring about a nuclear fusion reaction.

Many methods have been devised to accelerate charged particles using strong magnetic and electrostatic fields. These particle accelerators are popularly called "atom smashers" and bear such names as cyclotrons and synchrotrons. They use magnetic fields to accelerate charged particles to speeds near the speed of light.

At top speed, the fast moving nuclei smashes into the target nuclei and the two nuclei fuse together to make a heavier nucleus.

Transuranic Elements

In 1919, Rutherford bombarded nitrogen with alpha particles and the presence of oxygen was detected spectroscopically. Such artificial fusion reactions have been used to produce the elements above atomic number 92. These are known as transuranic elements because they occur immediately following uranium in the periodic table. Transuranic elements with larger atomic numbers are normally formed in small quantities in particle accelerators. Curium-242, for example, was formed when a plutonium target was struck with accelerated alpha particles. In 1994, a team of European scientists synthesised element 111 by bombarding a bismuth target for several days with a beam of nickel atoms (see above). Amazingly, their discovery was based on the detection of only three atoms of the new element. The nuclei are very short-lived and they undergo alpha decay within milliseconds of their synthesis. The same group of scientists also reported the synthesis of element 112 in 1996.

Commerical and Medical Radioisotopes

The production of iodine-123 in a particle
accelerator.

While most commerical and medical radioisotopes are made in nuclear reactors, a few are made in smaller particle accelerators called cyclotrons or synchrotrons. ANSTO in Sydney uses a particle accelerator and a type of fusion process to make these radioistopes. Iodine-123 is an example of a radioisotope produced in this way. It is a beta and gamma emitter and used to detect and treat thyroid disease. To make iondine-123, a particle accelerator is used to accelerate protons towards a xenon-124 target. Whe the proton is taken up by the xenon-124 nucleus, is becomes caesium-123. Caesium-123 then undergoes beta plus decay to produce xenon-123 and a positron. Beta plus decay produces the antiparticle of the electron (positron) instead of an electron. Xenon-12 then decays further to form iodine-123 and another positron.