Topic 4: Atomic structure

Everything around you is made of atoms, tiny units with a dense central nucleus surrounded by moving electrons. This topic traces how scientists pieced together the atom's structure, then explores unstable nuclei that give out radiation. You will also meet half-life calculations and the two ways nuclei release huge amounts of energy.

The structure of the atom and its size

An atom has a central nucleus containing protons and neutrons, with electrons arranged in shells (energy levels) around it. Protons carry a positive charge, electrons carry an equal negative charge, and neutrons have no charge, so a neutral atom has equal numbers of protons and electrons. Almost all the mass sits in the nucleus, even though the nucleus is far smaller than the whole atom. A typical atom has a radius of about 1 times 10 to the power minus 10 metres, while the nucleus is roughly 1 times 10 to the power minus 14 metres across, about ten thousand times smaller. Electrons can move to a higher energy level by absorbing electromagnetic radiation, and they drop back down by emitting it. If an outer electron is removed, the atom becomes a positively charged ion.

Development of the atomic model

Before electrons were discovered, atoms were pictured as tiny solid spheres that could not be divided. After J. J. Thomson discovered the electron, the plum pudding model suggested the atom was a ball of positive charge with negative electrons dotted inside it, like fruit in a pudding. Rutherford's team then fired positively charged alpha particles at a thin sheet of gold foil. Most passed straight through, but a few were deflected and a tiny number bounced almost straight back. This could only be explained if the positive charge and most of the mass were concentrated in a very small central nucleus, leading to the nuclear model. Niels Bohr later refined this by proposing that electrons orbit the nucleus at fixed distances in specific energy levels, which matched experimental observations. Further work showed the nucleus contains positive protons, and James Chadwick's experiments about twenty years after the nuclear model was suggested provided evidence for the neutron.

Atomic number, mass number and isotopes

The atomic number (proton number) is the number of protons in an atom and defines which element it is. The mass number is the total number of protons plus neutrons. You can find the number of neutrons by subtracting the atomic number from the mass number. Atoms of the same element always have the same number of protons but can have different numbers of neutrons. These different forms are called isotopes. For example, carbon always has 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Isotopes have the same chemical behaviour because they have the same electron arrangement, but they differ in mass and some are unstable.

Radioactive decay and the three types of radiation

Some isotopes have unstable nuclei that break down at random, giving out radiation to become more stable. This is radioactive decay, and it is a completely random process, so you cannot predict when a particular nucleus will decay. Activity is the rate at which a source decays, measured in becquerel (Bq), where one becquerel is one decay per second. There are three main types of nuclear radiation. An alpha particle is two protons and two neutrons (the same as a helium nucleus); it is the most ionising but has the lowest penetrating power, stopped by a sheet of paper or a few centimetres of air. A beta particle is a fast-moving electron emitted from the nucleus when a neutron turns into a proton; it is moderately ionising and is stopped by a few millimetres of aluminium. Gamma radiation is electromagnetic radiation given out by the nucleus; it is the least ionising but the most penetrating, needing thick lead or metres of concrete to reduce it. A nucleus may also emit a neutron. Because alpha is highly ionising, it is the most dangerous if a source gets inside the body.

Nuclear equations

Nuclear equations show how the nucleus changes during decay, using atomic and mass numbers that must balance on both sides. When alpha decay happens, the nucleus loses 2 protons and 2 neutrons, so the mass number falls by 4 and the atomic number falls by 2. The alpha particle is written as a helium nucleus with mass number 4 and atomic number 2. When beta decay happens, a neutron changes into a proton, so the atomic number increases by 1 while the mass number stays the same; the emitted beta particle is written as an electron with mass number 0 and a charge of minus 1. Gamma emission carries away energy only, so it does not change the atomic number or the mass number. To check any equation, make sure the totals of the top numbers (mass) match and the totals of the bottom numbers (charge) match on each side.

Half-life and net decline

The half-life of a radioactive isotope is the time it takes for the number of unstable nuclei in a sample to halve, or equally the time for the activity (count rate) to fall to half its starting value. Because decay is random, half-life is an average and is the same regardless of how much you start with. A useful idea is the net decline, the fraction of the original that remains after a whole number of half-lives. Worked example: a sample has an activity of 800 Bq and a half-life of 5 hours. After 5 hours it falls to 400 Bq (one half-life), after 10 hours to 200 Bq (two half-lives), after 15 hours to 100 Bq (three half-lives), and after 20 hours to 50 Bq (four half-lives). After those four half-lives, 50 out of 800 remains, so the net decline can be written as the ratio 50 to 800, which simplifies to 1 to 16, meaning the activity has dropped to one sixteenth of the original.

Contamination versus irradiation and safety

Irradiation is when an object is exposed to radiation from an outside source. The object itself does not become radioactive, and the exposure stops as soon as the source is removed or shielded. Contamination is when radioactive atoms get onto or into an object or person, for example as dust or liquid. The hazard then stays for as long as those atoms keep decaying, and they can be hard to remove. To protect people, sources are kept far away, handled with tongs or remote tools, stored in lead-lined containers, and the time spent near them is kept short. Findings about the dangers of radiation are checked by other scientists through peer review, which helps make conclusions more reliable and trusted.

Nuclear fission and fusion

Nuclear fission is the splitting of a large, unstable nucleus, usually uranium or plutonium, into two smaller nuclei. It is normally triggered when the nucleus absorbs a neutron. Fission releases energy and gives out two or three more neutrons, which can go on to split further nuclei. If this continues it produces a chain reaction. In a nuclear reactor the chain reaction is controlled so the energy is released steadily; in a nuclear weapon it is uncontrolled. Nuclear fusion is the joining of two light nuclei, such as hydrogen isotopes, to form a heavier nucleus. Fusion releases even more energy than fission and is the process that powers the Sun. Some of the mass is converted into the energy of the radiation released. Fusion is very hard to achieve on Earth because the nuclei must be forced together at extremely high temperatures and pressures.