Chemistry, Third edition

400 21 · NUCLEAR AND RADIOCHEMISTRY

Definition of radioactivity

Atomic nuclei are said to be unstablewhen they spontaneously disintegrateand simul-

taneously give off nuclear radiation. This phenomenon is called radioactivity. The

key ideas in this definition are:

1. Spontaneous

Radioactivity is spontaneous. This means that it does not require any help to start or

to continue. The rate of disintegration does notdepend upon temperature. This is in

contrast to chemicalreactions, whose rates are often drastically affected by changes in

temperature. The radioactive decay of the nucleus of an atom is unaffected by the

presence of other atoms and cannot be catalysed. For example, uranium-235 decays

at the same speed (and into the same products) whether it is pure uranium, combined

as uranium oxide (UO 2 ) or as uranium fluoride (UF 6 ).

2. Disintegration

In radioactive decay, one nuclidechanges into another. We can represent this as

parent nuclide daughter nuclide nuclear radiation

where the term ‘nuclide’ means an atomof a particular isotope. As a result of this

change, the number of protons or neutrons (or both) in the parent nuclide changes.

Where the number of protons changes, a new element is formed. Radioactive

changes are therefore very different from chemical reactions, in which atoms are

neither destroyed nor created and where reaction simply involves the rearrangement

of atoms to form new molecules.

3. Nuclear radiation

The term ‘nuclear radiation’ includes particles (such as alpha particles, high-energy

electrons and nuclei), and also high-frequency light known as gamma rays. See Table

21.1 and Fig. 21.2.

Table 21.1The main types of nuclear radiation

Name Symbol Nature Penetration

Alpha particle^42 He^2 or helium nuclei (i.e. helium relatively massive but travel only

ions) a few cm in air and are stopped

by thin card

Beta particle  10 eor highly energetic (and fast travel up to 5 m in air; stopped

moving) electrons by thin metal plate

Positron  10 eor highly energetic positively as beta particle

charged electrons

Gamma rays  very short wavelength very penetrating; requires thick

electromagnetic radiation; lead plate and/or thick concrete

travels at the speed of light shielding

Neutrons*^10 n relatively massive particles as gamma rays

Fission fragments* – highly ionized nuclei of relatively massive and possess

medium and large atoms, enormous energies; stopped

e.g. U^20  by thick lead and concrete

shielding

* Produced during nuclear fission.