Chemistry, Third edition

21 · NUCLEAR AND RADIOCHEMISTRY

More about nuclear radiation

Alpha particles

Alpha particles are doubly ionized helium atoms, i.e. helium nuclei. Since a helium

nucleus consists of two protons and two neutrons, the loss of an alpha particle from

a nucleus reduces the number of protons by two and the number of neutrons by two.

Therefore the mass number Afalls by four and the atomic number Zfalls by two.

The emission of alpha particles is represented by the general equation

Z

AX A 4

Z 2 Y

2 + 4

2 He

2 

where X is the parent nuclide and Y is the daughter nuclide. Both Y^2 and^42 He^2 

become electrically neutral by losing or gaining electrons from the container or the

surrounding air.

An example of alpha emission is provided by the decay of radium-226:

226

88 Ra—

222

86 Rn

2  4

2 He

2+

88 protons 86 protons 2 protons

138 neutrons 136 neutrons 2 neutrons

88 electrons 88 electrons 0 electrons

Notice that the total charge, the sum of the mass numbers, and the sum of the

atomic numbers, are equal on both sides. This is true of all nuclear equations.

Alpha particles are so bulky that they do not usually travel more than 10 cm in air.

They are easily stopped by card, brick or thin metal sheet. However, their mass and

speed (typically about 10^7 ms^1 ) means that they possess considerable kinetic

energy and so cause substantial ionisation, and if brought into close contact with

human tissue (for example, by ingestion) they usually cause much more damage

than beta or gamma radiation.

Beta particles

Beta particles are high-energy electrons which are ejected from the nucleus. Since

there are not normally any electrons in the nucleus, the beta particles must have been

produced in the nucleus during radioactive decay. It is now known that during this

process, a neutron changes into a proton and an electron. From this it follows that the

atomic number (the number of protons) of the nuclide increases by one, while the

mass number (the number of protons and neutrons) remains the same. The electron

escapes from the nucleus and is now called a beta particle. The general equation is

A

ZX Z 1

AY+

 1

(^0) e
where the atomic number of the electron is (for the purposes of a balanced nuclear
equation) regarded as 1. An example of beta emission is the radioactive decay of
strontium-90:
90
38 Sr——
90
39 Y
+
 1
(^0) e
38 protons 39 protons 0 protons
52 neutrons 51 neutrons 0 neutrons
38 electrons 38 electrons 1 electron
The positively charged yttrium ion will gain an extra electron from the surroundings.
Beta particles vary greatly in their energies and speeds, and (therefore) in their
penetration of matter. Some beta particles travel at speeds at 99% the speed of light,
and as a consequence penetrate several metres of air.
21.3
402
Alpha decay
Complete the equation
(^21884) Po ?? (^42) He2+
Exercise 21A
Beta decay
Complete the equation
(^21583) Bi ?? 10 e
Exercise 21B