The atomic number in a nucleus is denoted by Z. The mass number is denoted by A. Isotopes are atoms of the same element with different numbers of neutrons.
An isotope of an element, X, which has Z protons and A protons and neutrons, is represented by the symbol .
A = atomic mass (number of protons + neutrons)
Z = atomic number (number of protons)
X = chemical symbol (as shown on the periodic table)
An unstable nucleus becomes more stable by emitting an α (alpha) or β (beta) particle or by emitting γ (gamma) radiation.
- An α particle consists of 2 protons and 2 neutrons. It relative mass is 4 and its relative charge is 2.
- When an unstable nucleus emits an α particle, its atomic number goes down by 2 and its mass number goes down by 4.
- A β particle is an electron created and emitted by a nucleus which has too many neutrons compared with protons. A neutron in its nucleus changes into a proton and a β particle which is instantly emitted at high speed by the nucleus.
- The relative mass of a β particle is effectively zero and its relative charge is -1.
- When an unstable nucleus emits a β particle, its atomic number goes up by one but its mass number stays the same (the neutron changes into a proton).
Radioactive Decay (cont.)
γ radiation is emitted by some unstable nuclei after an α particle or a β particle has been emitted. γ radiation is uncharged and has no mass. So it does not change the number of protons or the number of neutrons in a nucleus.
Origins of background radiation
Background radiation is ionising radiation from space (cosmic rays), from devices such as X-ray tubes and from radioactive isotopes in the environment. Some of these isotopes are present because of nuclear weapons testing and nuclear power stations. But most of it is from substances in the Earth.
The 'Plum Pudding' Model of the Atom
Before the nucleus was discovered in 1914, scientists didn't know what the structure of the atom was. They did know atoms contained electrons and they knew these are tiny negatively charged particles but they didn't know how the positive charge was arranged in an atom, although there were different models in circulation. Some scientists thought the atom was like a 'plum pudding' model with:
- Positive charged matter in the atomo evenly spread about (like in a pudding)
- Electrons buried inside (like plums in the pudding)
An early model (scientific idea) about the structure of the atom was called the plum pudding model. In this model, the atom was imagined to be a sphere of positive charge with electrons dotted around inside it - like plums in a pudding.
Scientific models can be tested to see if they are wrong by doing experiments. An experiment carried out in 1905 showed that the plum pudding model could not be correct.
Alpha Scattering Experiment
Rutherford, Geiger and Marsden devised an alpha particle scattering experiment, in which they fired alpha particles at thin gold foil.
Most of the alpha particles passed straight through the foil - most of the atom is just empty space. Some of the alpha particles were deflected through small angles - suggested that nucleus has a positive charge. A few rebound through very large angles - suggests that nucleus has a large mass and very large positive charge.
Alpha Scattering Experiment (cont.)
Nuclear power reactors use a reaction called nuclear fission which is the splitting of an atomic nucleus. It occurs when a neutron collides with and splits a uranium-235 nucleus or a plutonium-239 nucleus. Naturally occurring uranium is mostly uranium-238 (non-fissionable). Most nuclear reactors use 'enriched' uranium that contains 2-3% uranium-235.
For fission to occur:
- Uranium-235 or plutonium-239 nucleus must absorb a neutron
- Nucleus then splits into two smaller nuclei (radioactive)
- 2 or 3 neutrons
- energy is released
- The neutrons produced go on to produce further fissions (chain reaction)
Energy released in such a nuclear process is much greater than the energy released in a chemical process such as burning. In a nuclear reactor the process is controlled, so one fission neutron per fission on average goes on to produce further fission.
Nuclear Fission (cont.)
Nuclear fusion is the joining of two atomic nuclei to form a single, larger nucleus. During the process, energy is released, but only if the relative mass of the product nucleus is no more than about 55. Fusion is the process in which energy is released in stars.
There are enormous problems with producing energy from nuclear fusion in reactors. Nuclei approaching each other will repel one another due to their positive charge. To overcome, the nuclei must be heated to very high temperatures to give them enough energy to overcome the repulsion and fuse. Because of the enormously high temperatures involved, the reaction cannot take place in a normal 'container', but has to be contained by a magnetic field.
Nuclear Fusion (cont.)
Hydrogen-1 nuclei fuse with hydrogen-2 nuclei to make helium-3 nuclei
In a fusion reactor:
- The plasma is heated by passing a very large electric current through it
- The plasma is contained by a magnetic field so it doesn't touch the reactor walls. If it did, it would go cold and fusion would stop.