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Section 1: Essentials (physical chemistry)

CHAPTER 3: RADIOACTIVITY
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Band of Stability
Nature of a, b and g radiation
Predicting type of radioactivity
Properties of a, b and g radiation
Kinetics of radiation
Applications of radioactivity

3.1. BAND OF STABILITY

3.1.1. All nuclei with more than 83 protons are unstable. Elements with 83 protons or fewer may also have unstable nuclei, depending on the ratio of neutrons to protons.

If number of neutrons is plotted against number of protons for stable nuclei, a characteristic graph is obtained (FIG. 3.1.).

The plot produces a band of stable nuclei called the band of stability. A section of the band is shown in more detail at element 34, which has six stable nuclei.

Elements which lie outside the band undergo radioactive decay. This produces a new nucleus which may or may not be radioactive itself. The process continues until a stable nucleus is formed. This can be seen more clearly by understanding nature of three main types of radiation.
 

3.2. a, b, AND g RADIATION

3.2.1. Nature of a radiation: This is the loss by an unstable nucleus of two protons and two neutrons as a single a-particle. An a-particle is therefore a helium nucleus:

Note that the ratio of neutrons:protons changes because they are removed in a different ratio from that which exists in the parent atom.

However, the Thallium nucleus produced by the decay is still unstable and it must undergo the second type of radiation (b) before a stable nucleus results.

3.2.2. Nature of b radiation: When b-radiation occurs, the ratio of neutrons:protons is reduced because a neutron changes into a proton. At the same time an electron is produced, and this is lost from the nucleus as a, so-called, b-particle:

3.2.3. Nature of g-radiation: During a- and b-decay, excess energy may be released as high frequency electromagnetic radiation known as g-radiation.

3.3. PREDICTING THE TYPE OF RADIATION

3.3.1. Up to element 82, nuclei which have too high a ratio of neutrons:protons undergo b-radiation, but not a-radiation. Above 82, elements with too high a ratio can undergo either a- or b-radiation.

Note also, that elements with too low a ratio of neutrons:protons undergo a different type of decay in which a proton is converted into a neutron, and a positron is released:

However, such nuclei are not naturally occuring, but they may be produced by nuclear reactions.

3.4. SUMMARY OF THE PROPERTIES OF a-, b-, and g-RADIATION

3.4.1. Properties of a-radiation:

i) Nature: Fast moving helium nuclei, thus positively charged.

ii) Behaviour in an electric field: Deflected towards the negative plate.

iii) Behaviour in a magnetic field: Deflected according to Fleming's left hand rule (FIG. 3.5.):

Note that the direction of flow of the a-particles = the direction of flow of conventional current.

iv) Ionising power: a-particles have a powerful ionising effect on any gases they pass through.

v) Penetrating power: a-radiation is absorbed by 7cm of air or by a sheet of paper.

3.4.2. Properties of b-radiation:

i) Nature: Fast moving electrons, thus negatively charged.

ii) Behaviour in an electric field: Deflected towards the positive plate, and deflected to a greater extent than a-particles owing to the low mass of an electron.

iii) Behaviour in a magnetic field: Deflected according to Fleming's left hand rule, and thus in the opposite direction to a-radiation, as well as to a greater extent.

iv) Ionising power: b-particles are less ionising than a-pariticles as predictable from their lower mass and lower kinetic energy.

v) Penetrating power: b-radiation can travel a few metres through air, and through thin sheets of metal. The denser the metal, the thinner the sheet that can be penetrated.

3.4.3. Properties of g-radiation:

i) Nature: High frequency electromagnetic radiation.

ii) Behaviour in an electric field: Unaffected.

iii) Behaviour in a magnetic field: Unaffected.

iv) Ionising power: Weakly ionising.

v) Penetrating power: g-radiation can pass through several kilometers of air through up to 15cm of lead.

3.5. KINETICS OF RADIOACTIVE DECAY

3.5.1. Radioactive elements decay according to first order kinetics (section 8.1.): the rate is proportional to the number of radioactive atoms present, and the half-life is constant (section 9.1.2. and table 9.1.).

In this context, half-life is the time taken for half the original number of radioactive atoms to disintegrate. During this period, the intensity of radiation will obviously fall to half its original value.

3.5.2. An equation: If you wish to remember an equation describing the rate of radioactive decay, remember this one:

By the way, it follows that N = ½N₀ when t = t_½ (the half-life)

You may have the misfortune of encountering such a mathematical manoeuvre. Hopefully, your examiners will not require you to take part in one.

3.6. APPLICATIONS OF RADIOACTIVITY

3.6.1. Carbon dating: The concentration of radioactive ¹⁴carbon dioxide in the atmosphere is assumed to have been constant throughout history (about 1 molecule in 10,000).

During their lifetime, living organisms absorb radioactive carbon, either during photosynthesis (plants) or indirectly via feeding (animals) on other living organisms. It is therefore assumed that throughout history the proportion of radioactive to non-radioactive carbon in living organisms has been constant.

When a living organism dies, it stops absorbing radioactive carbon and the radioactive carbon decays with a half-life of 5570 years. By measuring the ratio of radioactive to non-radioactive carbon in material derived from living organisms, it is therefore possible to estimate its age since death.

3.6.2. Tracers and labelling: The fate of a molecule in a living organism can be traced by labelling the molecule with a radioactive isotope. In this method, one atom in each of the molecules to be traced is replaced with a radioactive isotope.

For example, a particular carbon atom in each molecule of a sample of glucose can be replaced by ¹⁴carbon. (In fact, replacement is not 100%.) If the glucose is fed to an organism, the fate of the glucose ¹⁴carbon atom can be traced by detecting and locating the b-radiation. This can provide information about the types of molecule produced from the glucose, and the location of those molecules within the organism and its cells.

However, labelling is not exclusive to biochemistry and medicine. For example, by replacing the oxygen atoms in an ester with ¹⁸oxygen, it is possible to determine which bond is broken during ester hydrolysis (section 22.3.6.i.).

3.7. QUESTIONS

1) Account for the different behaviour of a-, b-, and g-radiation in

i) an electric field, ii) a magnetic field.

2) Comment on the following statements:

i) Isotopes are radioactive atoms of an element.

ii) Half-life is half the time taken for a sample of a radioactive element to decay totally.

iii) There is more similarity between g-radiation and light, than there is between g-radiation and a-radiation.

3) Fill in the missing data (indicated by question marks) in the following schemes. You will need a periodic table to identify the unamed elements.

Will the final nucleus at the end of each chain be stable?

4) How valid are the assumptions on which carbon dating is based? (The ¹⁴carbon isotope is produced in the atmosphere by the bombardment of nitrogen by cosmic rays.)

5) How would you use ¹⁸oxygen to determine which bond is broken during ester hydrolysis?

Unless otherwise stated, all materials in this web version of chapter 3 are © 2007 Adrian Faiers MA (Oxon) MCIPR