Ionising radiation

Isotopes share the same chemical properties as they have equal numbers of electrons. However, different isotopes of the same element will have different physical properties due to the difference in the mass and stability of the nucleus.

Protons are held in the nucleus by a strong nuclear force. The strength of this force is so vast that it has the ability to keep the nucleus together even when the repulsive force between the protons attempts to move them away from each other. The main factor affecting the balance between these forces is the number of neutrons present in the nucleus. The stability of the nucleus depends on the quantity of the protons and neutrons it contains. An abundance of neutrons or a lack of neutrons can cause the nucleus to lose stability.

Some isotopes of an element are unstable which means that they tend to break down or decay. As these isotopes decay, they emit radiation in a process known as radioactive decay. This process occurs randomly, meaning you cannot predict when or which unstable nuclei will decay.

Ionising radiation

The radiation emitted during radioactive decay is known as ionising radiation because it causes electrons to be ‘knocked off’ or removed from atoms. This forms positively charged ions as shown in the diagram below:

There are four types of ionising radiation which you need to be able to describe the nature and effects of. These are:

• Alpha radiation
• Beta radiation
• Gamma radiation
• Neutron radiation

Alpha radiation

Alpha particles are emitted from the nucleus of the atom during radioactive decay in a process known as alpha radiation. Alpha particles have the lowest penetrating power of the three types of ionising radiation. They only travel a few centimetres in air and are stopped by paper.

As alpha particles are so small, they can easily ionise other particles. Alpha particles are therefore described as being strongly ionising. An alpha particle contains two protons and two neutrons, the same as a helium nucleus. Alpha particles are therefore represented as:

Alpha particles have a relative mass of 4 and a relative charge of +2 due to the two positively charged protons. As an atom emits an alpha particle, its atomic mass and nuclear charge decrease. For example, an atom of radium-226 has a mass of 226, an atomic number of 88 and a nuclear charge of +88 from 88 positively charged protons in the nucleus.

When radium-226 emits an alpha particle, its mass decreases to 222, atomic number decreases to 86 and the nuclear charge decreases to +86. Each element has a specific atomic mass and proton number. When the proton number changes, the identity of the element also changes.

When the atomic mass of radium-226 decreases to 222, a new element is formed. Using the periodic table, this new element is identified as radon-222. We can represent the changes that occur during radioactive decay by using a nuclear equation. Nuclear equations are written in the same way as the equations used to represent chemical reactions. On the left you have the substances you start with, an arrow is used to show the direction of the change, and on the right you have the substances produced from the change. When writing nuclear equations, the mass and charge on both sides of the equation must be balanced.

For example, the nuclear equation used to represent the radioactive decay of radium-226 to radon-222 would be written as:

The mass on both sides is 226 and the charge on both sides is neutral.

Beta radiation

Beta particles  are fast-moving electrons which are released by the nucleus during radioactive decay. This emission is known as beta radiation. As beta particles are smaller and have a lower relative charge than alpha particles, they are able to move faster and further. Beta particles can travel a few metres through the air. They also have a higher penetrating power than alpha particles, and can penetrate through paper. Beta particles are stopped by absorption by aluminium foil. As beta particles are electrons, they have virtually zero mass and are negatively charged with a relative charge of -1. Beta particles can be represented as:

During the process of beta radiation, one of the neutrons in the nucleus of the atom turns into a proton and releases an electron in the process. This electron is the beta particle which is then emitted from the atom. The atom has become an ion with a charge of +1 as it now has more positively charged particles than negatively charged particles. The mass of the atom does not change because it still has the same overall number of nucleons in the nucleus.

Beta radiation increases the atomic number and the charge on the nucleus but no changes to the mass number occur. As with alpha radiation, we can represent the changes which occur during beta radiation by using a nuclear equation. For example, if carbon-14 undergoes the process of beta radiation, one of the neutrons is converted into a proton and a beta particle is released. The atomic mass remains 14 but the proton number changes from 6 to 7. As the atomic number has changed, the identity of the element has also changed. Using the periodic table, we can see that the element with an atomic mass of 14 and a proton number of 7 is nitrogen. The nuclear equation for this change is:

Gamma radiation

Gamma rays ( are electromagnetic waves which have very short wavelengths. They are emitted from the nucleus during radioactive decay. This emission is known as gamma radiation. Waves have no mass, so gamma rays are able to travel over very long distances through air. They have the highest penetrating power of the three ionising radiation types and go through paper and aluminium. Gamma rays can be stopped by thick sheets of lead or concrete. Gamma rays have very weak ionising power and tend to pass through and past atoms rather than ionising them. This means that no change in atomic mass or atomic number occurs during gamma radiation.

The table below summarises the ionising and penetrating powers of alpha particles, beta particles and gamma rays.

Neutron radiation

Neutron radiation is caused by the release of neutrons from the nucleus of atoms during the process of nuclear fission or nuclear fusion. These processes will be discussed in detail later in the chapter.