Atoms are very small, having a radius of about 1 × 10-10 metres.
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GCSE Physics Revision
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GCSE Physics revision
The structure of an atom
Atoms and isotopes
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Apply the specified scientific knowledge of the structure of an atom to a relevant example.
Analyse an unfamiliar example of the structure of an atom using the specified scientific ideas.
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Key knowledge
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Structure of the Atom
- An atom consists of a nucleus containing protons and neutrons, surrounded by electrons arranged in shells.
- The radius of an atom is approximately 1 imes 10^{-10} m, while the nucleus is about 10,000 times smaller at 1 imes 10^{-14} m.
- Protons have a relative charge of +1 and relative mass of 1; neutrons have no charge and relative mass of 1; electrons have a relative charge of −1 and a negligible relative mass of approximately rac{1}{1800}.
- The atomic (proton) number gives the number of protons, and in a neutral atom the number of electrons equals the number of protons; the number of neutrons equals the mass number minus the atomic number.
- Electrons fill shells in order: the first shell holds a maximum of 2 electrons, and the second and third shells each hold a maximum of 8 electrons.
Isotopes
- Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
- The three isotopes of hydrogen each have one proton but differ in their neutron count: 0, 1, or 2 neutrons.
- Isotopes have the same chemical properties because they have the same number of electrons, but their physical properties differ due to different masses.
Development of the Atomic Model
- Before the discovery of electrons, atoms were thought to be tiny, indivisible solid spheres.
- The discovery of electrons led to the plum pudding model, in which electrons are embedded in a ball of positive charge.
- In the alpha particle scattering experiment, a beam of alpha particles (helium nuclei with a 2+ charge) was fired at thin gold foil; most passed straight through, some were deflected, and a few were reflected back.
- The results showed that most of the atom is empty space, with a small, dense, positively charged nucleus at the centre, replacing the plum pudding model with the nuclear model.
- Niels Bohr adapted the nuclear model to propose that electrons orbit the nucleus at specific distances (shells), supported by calculations that matched experimental observations.
- James Chadwick later provided evidence for the existence of neutrons in the nucleus, completing the modern model of the atom.
Radioactive Decay and Types of Nuclear Radiation
- Unstable nuclei undergo radioactive decay, emitting radiation to become more stable, and activity is measured in becquerels (Bq) using a Geiger–Müller tube.
- Alpha (α) particles consist of 2 protons and 2 neutrons (identical to a helium nucleus), travel only a few centimetres in air, are stopped by paper or skin, and have high ionising power.
- Beta (β) particles are high-speed electrons emitted when a neutron turns into a proton; they travel about 1 m in air, are stopped by a few millimetres of aluminium or tin foil, and have medium ionising power.
- Gamma (γ) rays are electromagnetic radiation from the nucleus; they can travel kilometres in air, require thick lead or several metres of concrete to be blocked, and have low ionising power.
- Neutrons can also be emitted from the nucleus during radioactive decay.
Nuclear Equations
- In alpha decay, the mass number of the nucleus decreases by 4 and the atomic number decreases by 2, producing a new element and an alpha particle ^{4}2He.
- For example, radon-219 (²19}{86}Rn) undergoes alpha decay to form polonium215 (²15}{84}Po) plus an alpha particle.
- In beta decay, a neutron becomes a proton and an electron; the mass number stays the same but the atomic number increases by 1, producing a new element.
- For example, carbon-14 (^{14}{6}C) undergoes beta decay to form nitrogen-14 (^{14}{7}N) plus a beta particle (^{0}-1e).
- Gamma emission involves no change in mass number or atomic number, so nuclear equations are not used for gamma emission alone.
Half-Life and Calculations
- Radioactive decay is random; it is not possible to predict when or which nucleus will decay next.
- The half-life is defined as the time taken for the count rate or activity of a radioactive substance to fall to half of its initial value.
- A half-life graph plots count rate against time, and the half-life is read off by finding the time taken for the count rate to halve.
- For example, a substance with a half-life of 10 hours and an initial activity of 100% will fall to 50% after 10 hours, 25% after 20 hours, 12.5% after 30 hours, and 6.25% after 40 hours.
- Carbon-14 has a half-life of 5,700 years; if a bone contains 25% of its original carbon-14, it has undergone 2 half-lives, making it 2 imes 5700 = 11{,}400 years old.
Contamination, Irradiation, and Background Radiation
- Contamination is the unwanted presence of radioactive material on or in an object or person, making that object a source of radiation itself.
- Irradiation is the exposure of an object to radiation from an external source; removing the source ends the exposure and the object does not become radioactive.
- Background radiation is always present and comes from natural sources (radon gas, rocks, food, drink, cosmic rays) and artificial sources (medical X-rays, nuclear weapons testing, nuclear accidents).
- When measuring radiation from a source, background radiation must be subtracted from the total count to find the true count rate from the source.
- Radiation dose is measured in sieverts (Sv); 1 Sv equals 1,000 millisieverts, and dose reflects the risk of harm from radiation exposure.
- Safety precautions when working with radioactive materials include limiting exposure time, using lead aprons or screens, and wearing a radiation detection badge.
Uses of Nuclear Radiation (Triple Science)
- A radioactive tracer containing a gamma source is injected into the bloodstream to allow doctors to image internal organs, as gamma radiation has low ionising power and can penetrate body tissue to be detected externally.
- The tracer must have a short half-life to minimise the time cells are exposed to radiation, reducing damage to healthy tissue.
- Gamma radiation can destroy tumours by targeting them from multiple directions, so healthy tissue receives a low dose while the tumour receives a high, cell-destroying dose.
- The gamma source used for cancer treatment should have a long half-life so it can be used for multiple treatment sessions.
- Smoke alarms use an alpha source with a long half-life; alpha radiation's low penetration means it stays within the detector, and the long half-life ensures the alarm works reliably for many years.
Nuclear Fission and Nuclear Fusion (Triple Science)
- Nuclear fission is the splitting of a large, unstable nucleus (such as uranium or plutonium) when it absorbs a slow-moving neutron, releasing energy, two smaller nuclei, two or three neutrons, and gamma radiation.
- The released neutrons can be absorbed by other nuclei, triggering further fission events in a chain reaction; nuclear reactors control this chain reaction to generate electricity, while nuclear weapons use an uncontrolled chain reaction.
- Nuclear fusion is the joining of two light nuclei (such as hydrogen) to form a heavier nucleus, with some mass converted into energy of radiation.
- The mass of the heavier nucleus produced in fusion is less than the combined mass of the two original nuclei, because some mass is converted to energy.
- Fusion requires extremely high temperatures and pressures to give nuclei enough kinetic energy to overcome their mutual electrostatic repulsion and fuse together.
- Fusion occurs naturally in the cores of stars, but recreating the necessary conditions on Earth remains a major scientific and engineering challenge.