Physics 2.5

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What happens when radioactive substances decay, and the uses and dangers of their emissions

Learning Objectives:

understand the properties of different types of nuclear radiation

understand what happens to radioactive substances when they decay

understand the structure of the atoms from which they are made

Things to remember:

Radioactive substances emit radiation from the nuclei of their atoms

The use of radioactive sources depends on their penetrating power and half-life

New evidence can cause a theory to be re-evaluated

According to the nuclear model, most of the atom is empty space

5.1 Atomic structure

The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons.

The relative electric charges of protons, neutrons and electrons.

Proton 1

Neutron 1

Electron 1/2000

The relative masses of protons, neutrons and electrons.

Proton +1

Neutron 0

Electron -1

In an atom the number of electrons is equal to the number of protons in the nucleus.

The atom has no overall electrical charge.

Atoms may lose or gain electrons to form charged particles called ions.

The atoms of an element always have the same number of protons, but have a different number of neutrons for each isotope.

The total number of protons in an atom is called its atomic number; the total number of protons and neutrons in an atom is called its mass number.

5.2 Atoms and radiation Some substances give out radiation from the nuclei of their atoms all the time, these substances are radioactive. There is random nature of radioactive decay.

b) The origins of background radiation.

Additional guidance:

Knowledge and understanding should include both natural sources, such as rocks and cosmic rays from space, and man-made sources such as the fallout from nuclear weapons tests and nuclear accidents.

c) Identification of an alpha particle as two neutrons and two protons, the same as a helium nucleus, a beta particle as an electron from the nucleus and gamma radiation as electromagnetic radiation.

d) Higher Tier only: Nuclear equations to show single alpha and beta decay.

Additional guidance:

Candidates will be required to balance such equations, limited to the completion of atomic number and mass number. The identification of daughter elements from such decays is not required.

e) Properties of the alpha, beta and gamma radiations limited to their relative ionising power, their penetration through materials and their range in air.

f) Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not.

Additional guidance:

All candidates should know that alpha particles are deflected less than beta particles and in an opposite direction.

Higher Tier candidates should be able to explain this in terms of the relative mass and charge of each particle.

g) The uses of and the dangers associated with each type of nuclear radiation.

h) The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.