NUCLEAR STRUCTURE AND STABILITY

1. Revision

The nuclei of all atoms contain positively charged particles, called PROTONS, and neutral particles called NEUTRONS (1H is an exception - its nucleus consists only of a single proton).

ISOTOPES of a given element always contain the same number of protons, but differing numbers of neutrons. Thus, naturally occuring carbon consists of three isotopes, namely, 12C, 13C, and 14C, all having six protons in their nuclei. Neutral atoms have no net charge, due to the fact that the charge due to the protons is exactly balanced by an equal number of negatively charged ELECTRONS found outside the nucleus of the atom. (For more details, refer to the Grade 10 topic "The Structure of Atoms").

Source: Wikipedia (Author: Napi1kenobi)

2. Nuclear stability

A particular atom with a specific number of protons and neutrons is called a NUCLIDE. About 2500 nuclides have been identified, some occurring naturally, others being ptoduced synthetically. Only about 270 of these are stable, in that they are not RADIOACTIVE. Radioactive nuclides are said to undergo RADIOACTIVE DECAY. In the process, new nuclides are produced, and energy is released.

The ratio of the number of protons (Z) to the number of neutrons (A-Z) () has a bearing on the stability of a nuclide. For light elements, a ratio of Z/(A-Z) ~ 1 gives rise to a stable nucleus. As the atomic number increases, the nucleus requires additional neutrons for stability. The elements technetium, Tc, (Z = 43) and promethium, Pm, (Z = 61), as well as all elements with an atomic number greater than 84 do not have stable nuclides.

The diagram on the right () shows the distribution of radioactive and stable nuclides. The dark blue region is called the REGION OF STABILITY.

The reason why some nuclei are stable and others are not is outside the scope of this discussion.

3. Radioactive decay

Nuclides that undergo radioactive decay are known as RADIOACTIVE ISOTOPES or RADIOISOTOPES. While there are about a dozen different ways in which nuclides can decay, the two most two common types of radioactive decay are discussed below ().

These are α-particle emission and β-particle emission. Both of these types of decay may be accompanied by the emission of very high-energy electromagnetic radiation which is known as γ ("gamma") rays.

Radioactivity was discovered in by 1896 by Becquerel (). The SI unit of radioactivity, the BECQUEREL, Bq is a rate of radioactive decay equal to one decay per second. The non-SI unit, the CURIE, Ci equals 3.70x1010 decays per second (). Thus, 1 Ci = 3.70x1010 Bq

4. α-Particle emission

This occurs when a positive α-PARTICLE leaves the nucleus. Since the α-particle is simply the nucleus of a helium atom with mass number 4, the DAUGHTER NUCLIDE which results from this type of decay has a mass number 4 atomic mass units and an atomic number 2 less than the PARENT NUCLIDE

In the above example, the isotope of uranium with mass number 238 (the parent nuclide) decays to the isotope of thorium with mass number 234 (the daughter nuclide) and an α-particle.

This type of radioactive decay occurs frequently with heavy elements or elements that have too many protons for stability. Plutonium-239, 239Pu, and americium-241, 241Am, are pure α-emitters. α-Particles are not very penetrating, having a range of only a few centimeters in air.

5. β-Particle emission

This occurs when one neutron in the nucleus is converted to a proton and a high-energy electron, called a β-particle. Since the mass of the β-particle is negligibly small compared to nucleons, this type of decay does not change the mass number, but as the number of protons increases by 1, the daughter nuclide will have an atomic number 1 larger than the parent nuclide.

Many isotopes of light elements that have too many neutrons for stability are subject to this type of decay (). β-Particles are fairly penetrating, and may have a range in air up to several metres.

6. γ-ray emission

Certain radioactive nuclides emit very penetrating electromagnetic radiation known as γ-RAYS. These rays are in fact very high energy photons. For example cobalt-60 decays by losing a β-particle, leaving a nickel-60 nucleus in a high energy state, shown as 60Ni* in the diagram on the right. The nickel nucleus then very rapidly loses 2 γ-photons to stabilise its nucleus to the normal (ground) state.

7. The half-life of radioisotopes

The time taken for a given sample of a radioisotope to decay so that its radioactivity is one half the initial amount is called the HALF-LIFE, t, of that isotope. The half-life is a constant for the isotope, and shows enormous variations from one isotope to another. For example, 17F (fluorine-17) has a half-life of 70 seconds, while 238U has a half-life of 4.51 x 109 years. 14C (carbon-14) has a half-life of 5730 years.

The long-lived nature of some waste products of nuclear reactors poses grave ecological problems, as some of these radioisotopes remain dangerously active for tens of thousands of years.

There is no evidence that the rate of radioactive decay of isotopes is significantly altered by the chemical () or physical environment of the radioactive atom. Further, there is compelling evidence that the rate of decay of radioisotopes is the same now as it was in the very distant past.

8. Additional questions