Nuclear Chemistry

Distinguish between stable and radioactive isotopes and describe the conditions under which a nucleus is unstable

The definition of an isotope should have been encountered in the preliminary course, but is stated again below for convenience. Note that an unstable nucleus is really one that is ‘falling apart’, giving off radiation as it does so. This occurs for many reasons, but largely because the atom is growing too large for the nucleus to hold together. Keep this in mind, and the exact definition as well as the criteria listed below should prove easier to understand.

An isotope of an element is an atom with the same number of protons, but a different number of neutrons. For example, one isotope of hydrogen may have 1 proton and 1 neutron, but another isotope may have 1 proton and 2 neutrons.

Radioactive isotopes (commonly referred to as radioisotopes) have an unstable nucleus due to the particular number of neutrons they have, and emit radiation as they spontaneously disintegrate due to their unstable nuclei.

Instability will generally occur if

  • The atomic number of the element is greater than 82, where lead is the 82nd
  • The n:p ratio (neutron to proton ratio) lies outside the zone of stability, which is 1:1 for elements with atomic numbers less than 20, and increasingly greater than 1 for higher atomic numbers.

figure 21

Remember- A radioisotope is simply an isotope of an element with an unstable nucleus. This means that its neutron to proton ratio is not within the zone of stability.

 

Process information from secondary sources to describe recent discoveries of elements

Be prepared to name at least one recent discovery such as Darmstadtium for the purposes of this dotpoint

Of the 25 transuranic elements to be created, only the first three were produced within nuclear reactors (Those with the atomic numbers 93, 94, and 95). The remaining transuranic elements were created by accelerating a small nucleus within a particle accelerator to collide with a heavy nucleus. New discoveries are hard to verify, as some have life spans significantly less than one second.

One transuranic element which has been created is Americium, which is produced by the bombard- ment of Pu-239 with neutrons. Americium is often used in smoke alarms.

One transuranic element which has been discovered far more recently is darmstadtium, an element discovered in Darmstadt, Germany. Previously known as ununnilium, darmstadtium has an atomic number of 110 and is produced by bombarding Lead-208 with Nickel-64. This radioisotope decays within microseconds as it is highly unstable, with its more stable isotopes such as Darmstadtium-281 having a half-life of around 11 seconds.

 

figure 22

Although many other radioisotopes are commonly used in society, this dotpoint requires a ‘recent’ discovery. As such, darmstadtium is a safe option, as it was only verified by IUPAC within the last decade.

 

Describe how transuranic elements are produced

Transuranic elements are all elements with an atomic number of 93 or higher, and all have unsta- ble nuclei. Remembering how one transuranic element is produced, perhaps neptunium, is highly recommended.

Transuranic elements are elements with an atomic number over 92, i.e. past that of uranium. As uranium is the heaviest natural element, all transuranic elements are artificially produced. These are produced by bombarding certain nuclei with neutrons. Some isotopes will ‘split’ when hit by the neutrons in a process known as fission, while others will ‘absorb’ the neutron, resulting in a larger atomic weight.

More recently, transuranic elements have been produced through the use of machines known as cyclotrons, or linear accelerators. In these cases, a high speed, positively charged particle such as a helium or carbon nuclei is bombarded against a larger nuclei.

For example, in the production of neptunium: Uranium-235 is first bombarded with neutrons to form Uranium-236

figure 23

Further neutron capture creates Uranium-237, which then decays to form Neptunium-237 through beta decay:

 

figure 24

The mass of the reactants should equal to the mass of the products. The same holds true for the atomic numbers of both sides.

Remember- Transuranic elements are produced by either fission through neutron bombardment or cyclotrons.

 

Describe how commercial radioisotopes are produced

This dotpoint closely resembles the previous dotpoint 1.5.3, but focuses rather on the production of commercial radioisotopes. As such, be prepared to list at least one example. Although this may seem demanding now, you will find that you will need to recall at least two radioisotopes and their relevant methods of production by the end of this topic. As such, this dotpoint should pose little trouble if the later dotpoints are learnt satisfactorily.

On a commercial level, nuclear reactors are often used. Suitable target nuclei are placed in the reactor core and bombarded with neutrons to produce the desired isotope. Sometimes this isotope may decay further into other isotopes.

As mentioned in the previous dotpoint, cyclotrons may also be used to create radioisotopes. However, such machines are significantly more costly.

In the commercial production of radioisotopes, consideration must be given to the life span of the radioisotope. As such, reactors such as Lucas Heights, presently run by ANSTO, the Australian Nuclear Science and Technology Organisation, may supply Sydney destinations with radioisotopes, but destinations further away may not be able to receive such isotopes due to their short half-lives.

One example of a commercial radioisotope is Technetium-99, which is produced in Lucas Heights. It is produced by bombarding Molybdenum-98 with a neutron to form Molybdenum-99. Molybdenum-99 then decays via beta decay to form Technetium-99.

 

figure 25

 

Particle accelerators are also used to produce commercial radioisotopes. Technetium-99 can is also produced this way: Heavy hydrogen (An isotope of hydrogen, 2H, also known as Deuterium) is accelerated and bombarded onto target Molybdenum-98, producing Technetium-99 and an excess neutron. This is done in the Royal Prince Alfred Hospital in the National Medical Cyclotron.

The main difference between nuclear reactors and particle accelerators is that reactors produce neutron rich isotopes, and accelerators produce neutron deficient isotopes.

Many radioisotopes decay extremely quickly. As such, commercial production facilities must be located relatively close to where they will be used, and importing or exporting radioisotopes is altogether impossible for many.

Remember- Commercially, most radioisotopes are produced in nuclear reactions or cyclotrons.

 

Identify instruments and processes that can be used to detect radiation

You will often be expected to be able to list at least three methods of detecting radiation, giving one or two in detail. Be sure you understand what the question requires from you, as some may simply require a list, whereas others may require a detailed explanation.

‘Photographic film’ offers a simple method of detecting radiation, as it darkens when in contact with radiation. This is due to a reaction within the silver halide crystals within the film. It is common for handlers of radioisotopes to wear radiation badges made of photographic film as a security precaution.

The ‘scintillation counter’ is a method of detecting non-ionising radiation (low energy radiation unable to ionise atoms). The radiation transfers energy to a solvent molecule, and then to a fluorescent molecule in order to give off light. Light is given off as excited electrons jump up to higher-energy shells when energy is absorbed, and give off light as they return to their ground state. This light is then passed through a photomultiplier, which in turn emits an electrical pulse, which is recorded by a counter.

The ‘cloud chamber’ is another means of detecting radiation. It consists of an air space with supersaturated water or alcohol vapour. When radiation passes through the device, the air is ionised, with the ions serving as nucleation points upon which the vapour may form droplets. These droplets appear as ‘clouds’ inside the chamber. Different forms of radiation which form different paths as they travel through the chamber. Alpha particles will leave straight lines, beta particles will leave a fainter zig-zagging path, and gamma rays will leave an even fainter path.

The ‘Geiger-Muller’ tube is an effective way of detecting ionising radiation. The radiation enters through a thin mica window at the end of the tube, and ionises a gas molecule within. The electron knocked out then accelerates towards the central electrode, ionising more gas as it proceeds. The molecules shed further electrons and become positive ions, moving towards the negative outer casing. The flow of electrons forms an electrical pulse when they come into contact with the central electrode. This pulse is amplified and used to generate clicks in an audio amplifier, or measured using an electronic digital counter.

 

figure 26

Identify one use of a named radioisotope in industry and in medicine (including ‘Describe the way in which the above named industrial and medical ra- dioisotopes are used and explain their use in terms of their chemical properties’)

Industry- Cobalt 60

Cobalt-60 is used for several specialised purposes in an industrial context. One example relates to its use in gauging metal thickness, and another is to find faults within objects such as metal pipes. Both of these uses are relatively similar, as both rely upon the detection of the gamma rays emitted by the radioisotope. A source of Cobalt-60 is placed on one side of the object within a sealed container, and photographic film is placed on the other side. Both variances in thickness and the presence of defects in the metal are then identified by the changes in the level of radiation exposure indicated on the photographic film. This is caused by a darkening of the silver halide crystals as the gamma rays strike the surface of the film.

With a half-life of 5.3 years, Cobalt-60 can be left within a container inside metal pipes and other parts so that routine maintenance checks can be carried out without a frequent requirement to replace the radioisotope. Relatively low emission of radiation also limits the potential damage to anyone working with the radioisotope.

The production and decay of Cobalt-60 is shown in the following equations:

 

figure 27

 

Medicine- Technetium 99m

Technetium 99m is used for more than 80% of all medical tracer diagnosis. Emitted gamma radiation ca be picked up by equipment and converted into an image on a monitor, effectively ‘tagging’ the desired area.

With a half-life of 6 hours, Technetium 99m is ideal for such use, as the patient’s exposure to the radiation is minimised. The energy released is also low energy gamma radiation, minimising any tissue damage. However, most useful of all is Technetium 99m’s ability to be combined with other compounds to study different areas of the body. For example, it can be combined with tin to attach itself to red blood cells to examine the heart and blood vessels. Technetium 99m is not used by any part of the body, and is therefore not absorbed, further minimising radiation exposure. This makes it suitable to study highly radiation-sensitive organs such as the brain, kidney, bones, liver, and spleen.

The production and decay of Technetium-99m is shown in the following equations:

figure 28

Note that figure 29 is beta ‘radiation’.

Also note that radioistopes are chosen after considering a variety of factors. As a general rule, the half-life (And thus length of exposure), penetrating power, and chemical properties of the substance are the three main questions which must be covered. Relate the use of the radioisotope to each category.

 

Use available evidence to analyse benefits and problems associated with the use of radioactive isotopes in identified industries and medicine

You will find that, as with most of this course, the following points place a heavy reliance upon com- mon sense. As such, learn the previous dotpoints well, particularly taking time to note a radioisotope used in medicine and industrially, and you will find that this dotpoint is relatively simple to answer.

Benefits:

  • Medical applications of Tc-99m have reduced costs and provided a convenient, non-invasive method of
  • A greater understanding of diseases and infections are possible through the examination of images which can be obtained by sensors monitoring the radioisotope travelling through a patient.
  • Within an industrial context, tracer have provided a simply, cheap, and effective method of gauging metal thickness and identifying structural
  • Isotopes can be chosen with half-lives appropriate to their purpose. In medicine, short half- lives are desired so that the radiation exposure is minimised, whereas longer half-lives may be desired in industrial settings to avoid the need to frequently replace the

Problems:

  • Radiation can cause damage to organic tissue, disrupting normal cellular processes, DNA, and proteins, potentially leading to abnormalities such as tumours, genetic mutations, or
  • Some radioisotopes are chemically similar to elements within the human For example, Strontium-90 is chemically similar to calcium, and may replace the calcium within bones, potentially causing leukemia.
  • Further problems may Without further research, the long-term effects of certain types of radiation, such as the irradiation of food, are unknown.
  • Many radioisotopes are costly to produce, and entirely unrealistic to transport as their use is highly limited by their half-lives.

Remember- Much of the allure of using radioisotopes industrially and in medicine is that the benefits are proven, yet the problems have not been fully identified. Be prepared to note this point when conducting any analysis of the use of any current or future radioisotopes