Nuclear Stability and Radioactive Decay


With a discussion of radioisotopes comes the topic of nuclear stability. The nucleus of a radioisotope is unstable. In an attempt to reach a more stable arrangement of its protons and neutrons, the nucleus will spontaneously decompose to form a different nucleus. If the number of neutrons changes in the process, a different isotopes is formed. If the number of protons changes in the process, then an atom of a different element is formed. This decomposition of the nucleus is referred to as radioactive decay. During radioactive decay an unstable nucleus spontaneosly decomposes to form a different nucleus, giving off radiation in the form of atomic partices or high energy rays. This decay occurs at a constant, predictable rate that is referred to as half-life. A stable nucleus will not undergo this kind of decay and is thus non-radioactive.

CHEM WINDOW - Half-life

Why are the nuclei of radioisotopes unstable? In order to answer this question we must examine how the number of protons and neutrons in a nucleus are related to its stability and how this relates to radioactive decay.

The figure below shows a plot in which stable nuclei are positioned according to the number of protons (Z) and the number of neutrons (A-Z) that they contain. The stable (non-radioactive) nuclides are shown to reside in the zone of stability. Nuclei of atoms that do not contain a number of protons and neutrons that allows then to be plotted in this region are unstable and they will spontaneously decay until a nucleus is formed that does not reside in this stable zone.

Radioactive nuclei can undergo decomposition in a variety of ways. The spontaneous decay process can produce particles as in the case of alpha, beta, or positron emission. The alternate form of emission is that of electromagnetic radiation such as x-rays or gamma-rays.

CHEM WINDOW - Electomagnetic Spectrum

When alpha, beta, or positrons are emitted from the nuclei of a radioactive atom, it changes into a nucleus of another element. Scientists refer to this as transformation. Emission of gamma rays results only in a release of energy, not in transformation.

Alpha particles

An alpha particle is simply a helium nuclei (He) which is ejected with high energy from an unstable nucleus. This particle, which consists of two protons and two neutrons, has a net positive charge. Although emitted with high energy, alpha particles lose energy quickly as they pass through matter of air and therefore, do not travel long distances. They can even be stopped by a piece of paper or the outer layers of human skin. These slow moving particles are generally the product of heavier elements.

Example : 23892U ----> 42He + 23490Th

What would the radioactive decay of 22688Ra look like?

Beta particles

Beta particles are identical to electrons and thus have a charge of (-1). This type of decay process leaves the mass number of the nuclei unchanged. A beta particle is minute in comparison to that of an alpha particle and has about one hundred times the penetrating ability. Where an alpha particle can be stopped by a piece of paper a beta particle can pass right through. It takes aluminum foil or even wood to stop a beta particle. The electron that is released was not present before the decay occured, but was actually created in the decay process itself.

Example : 3215P ----> 0-1e + 3216S

Note that the mass number is unchanged and a new element is formed. So what was the effect of this Beta particle production? It actually changed a neutron into a proton. Notice that this new element will be down and to the right on the zone of stability plot.

Positron

This type of particle production is just the opposite of Beta particle decay.

Example : Na ----> 0 1e + Ne

Notice that is still has the same zero mass as an electron but an opposite charge. This is what is known as an antiparticle of the electron.

What happens when a positron collides with an electron? Annihilation!!
This can be shown by the following reaction:

Example : 0-1e + 01e ----> 2

Gamma Rays

As the name implies, these are not particles but high energy photons and can be found on the electromagnetic spectrum. They are very similar to x-rays but have a shorter wavelength and therefore more energy. The penetrating ability of gamma rays is much greater than that of alpha or beta particles. They can only be stopped by several centimeters of lead or more than a meter of concrete. In fact, gamma rays can pass right through the human body. Gamma rays often accompany other processes of decay such as alpha or beta. An example of this was our previous representation of an alpha particle process.

23892U ----> 23490Th + 200 + 42He

A ramification of alpha or beta particle production is that the newly formed nucleus is left in a state of excess energy. A way for the nucleus to release this excess energy is by emitting gamma rays. Since gamma rays have no mass, and are waves rather than particles, the elements atomic number does not change after emission.

Fill in the blanks :

12553I ----> 125Xe + 0-1e + 200

22688Ra ----> + 42He + 200


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