Spark image


Isotopes are particles which have the same position in the Periodic Table, that is, they are atoms of the same chemical element but their nucleon numbers are different. Isotopes of an element have nuclei with the same number of protons but different numbers of neutrons. Neon, for instance, has three isotopes with nucleon numbers of 20, 21 and 22, corresponding respectively to 10, 11 and 12 neutrons in the nucleus. The most common isotopes of uranium are uranium-235 and uranium-238 (143 and 146 neutrons respectively.)

It is important to realise that since the number of electrons is identical for all isotopes of the same element, the chemical properties of isotopes of the same element are identical. Since the structure of the nuclei are different, however, their nuclear properties will be different and, since their relative atomic masses are different, some of their physical properties are different as well. For example, the boiling point of 'heavy water' (water containing the isotope of hydrogen with a neutron in the nucleus) is 104 oC.

In 1906 isotopes were discovered in radioactive elements (although their nature was not understood) and in 1912 Thomson discovered the three isotopes of neon with the nucleon numbers shown above.

Two isotopes of carbon are shown in the following diagram

The following table shows some of the more common isotopes of a few elements:

Element Nucleon numbers of isotopes
Hydrogen 1,2,3
Helium 3,4
Carbon 12,14
Oxygen 16,17,18
Neon 20,21,22
Calcium 40,42,44
Iron 56,57
Mercury 198,199,200,201,202
Lead 206,207,208
Uranium 235,238

Separation of isotopes

There are several methods for separating isotopes.

(a) Centrifuge
Due to the difference in the masses of the two isotopes of uranium, a centrifuge method can be used to separate them. The mass difference is three neutron masses and this is sufficient to make this method effective.

(b) Gaseous diffusion
This method is used when the difference in mass is small, for example one neutron mass as in the case of hydrogen (1p) and deuterium (1p, 1n). It is also used to enrich uranium.

(c) Electromagnetic/electrostatic deflection
Where a very high purity is required the sample may be built up particle by particle, by deflection and collection in a mass spectrometer.
An ion current of 0.1 mA gives 6.21x1014 particles per second, assuming that the ions are singly charged. To produce 10-4 mole of the sample by this method would take 1.33 days!

Isotopes and relative atomic masses

The following in an extract from an article by Dr F.W.Aston first published in Nature in 1920.
In the atomic theory put forward by John Dalton in 1801 the second postulate was 'Atoms of the same element are similar to one another and equal in weight'. For more than a century this was regarded by chemists and physicists alike as an article of scientific faith. The only item among the immense quantities of knowledge acquired during that productive period which offered the faintest suggestion against its validity was the inexplicable mixture of order and disorder among the elementary atomic weights [relative atomic masses].The general state of opinion at the end of the last century may be gathered from the two following quotations from Sir William Ramsay's -address to the British Association at Toronto in 1897:

'There have been almost innumerable attempts to reduce the differences between atomic weights to regularity by contriving some formula which will express the numbers which represent the atomic weights with all their irregularities. Needless to say such attempts have in no case been successful. The idea has been advanced that what we call the atomic weight is a mean; that when we say the atomic weight of oxygen is 16 we merely state that the average atomic weight of oxygen is 16; and it is not inconceivable that a certain number of oxygen molecules have a weight somewhat higher than 32 and a certain number have a lower weight.'

This idea was placed on an altogether different footing some ten years later by the work of Lord Rutherford and his colleagues on radioactive transformations. The results of these led inevitably to the conclusion that there must exist elements which have chemical properties identical for all practical purposes, hut the atoms have different weights. This conclusion has been recently confirmed in a most convincing manner by the production in quantity of specimens of lead from radioactive and other sources, which, although perfectly pure and chemically indistinguishable, give atomic weights differing by amounts quite outside the possible experimental error. Elements differing in mass but chemically identical have been called isotopes by Professor Soddy.

The work of Sir J.J.Thomson before the war led to the belief that neon also existed as a mixture of two isotopes with atomic weights of 20 and 22, the accepted atomic weight being 20.2. The methods available were not accurate enough to distinguish between 20 and 20.2 with certainty but in 1913 a diffusion experiment gave positive results, an apparent change in density of 0.7 per cent between the lightest and heaviest fractions being obtained after many thousands of operations.

By the time work was started again after the war the isotope theory had been generally accepted so far as the radioactive elements were concerned and a good deal of theoretical speculation had been made as to its applicability to the elements generally. As separation by diffusion is at best extremely slow and laborious attention was again turned to positive rays in the hope of increasing the accuracy of measurements to the required degree.

[A description of the Aston mass spectrograph then follows. I will continue the extract at the point where the results are considered.]

By far the most important result obtained from this work is the generalisation that, with the exception of hydrogen, all atomic weights so far measured are exactly whole numbers on the scale 0 = 16. Hydrogen is found to be 1.008, which agrees with the value accepted by the chemists. This exception from the whole number rule is not unexpected, as on the Rutherford 'nucleus' theory the hydrogen atom is the only one not containing any negative electricity in its nucleus.

The results which have been so far obtained with eighteen elements make it possible that the higher the atomic weight of an element, the more complex it is likely to be, and that there are more complex elements than simple. It must be noticed that, though the whole number rule asserts that a pure element must have a whole number atomic weight, there is no reason to suppose that all elements having atomic weights approximating to integers are therefore pure.'

1. Why was the existence of isotopes initially thought to be unlikely?

2. Write an account of Rutherford's work on radioactive decay.

3. Assuming that neon occurs as two isotopes of atomic weights 20 and 22, what proportion of naturally occuring neon is neon-20 if the atomic weight of neon is 20.2? (Ignore other isotopes)

4. What is the modern unit for 'atomic weights', and why was it chosen?

5. Why did hydrogen appear to be an exception to the whole number rule?

Uses of radioisotopes

Radioactive isotopes can be very useful. They are used in:

1. Medicine for both treatment and diagnosis
2. Archaeological and geological dating using carbon 14 or uranium
3. Fluid flow measurement - water, blood, mud, sewage etc.
4. Thickness testing of materials such as polythene
5. Radiographs of metal castings
6. Sterilisation of food and insects
7. Tracers in fertilisers used in agriculture
8. Smoke alarms in houses
9. Tracing phosphate fertilisers using phosphorus 32
10. Checking the silver content of coins
11. Atomic lights using krypton 85
12. Testing for leaks in pipes

© Keith Gibbs