6.1.1 Structure of the atom

Atoms have a tiny, massive nucleus

1. An atom has a very small, positively charged nucleus at its centre.
2. The nucleus contains protons and neutrons.
3. Negatively charged electrons surround the nucleus at distances that determine the overall size of the atom.
4. The positive charge of the nucleus comes from its protons because neutrons have no electric charge.
5. Almost all the atom's mass is concentrated in the nucleus because protons and neutrons are far more massive than electrons.
6. The nuclear radius is much smaller than the radius of the whole atom, so most of an atom's volume lies outside its nucleus.

Atomic size is about one ten-billionth of a metre

1. The typical size of an atom or small molecule is of order $10^{-10}m$.
2. This is $0.0000000001m$ or $0.1nm$.
3. An order of magnitude is an estimate, so different atoms do not all have exactly the same radius.

A complete description links structure, charge and scale

1. Name the nucleus and state that it contains protons and neutrons.
2. State that negatively charged electrons surround the nucleus.
3. Add that the nucleus is much smaller than the atom but contains almost all its mass.

6.1.2 Isotopes: atomic and mass number

Proton and mass numbers describe a nucleus

1. The atomic number, also called the proton number, identifies the element because every atom of one element has the same number of protons.
2. A nucleus containing six protons is carbon, and its positive nuclear charge is $+6$ in relative units.

1. The mass number, also called the nucleon number, counts all nucleons in the nucleus.
2. The relationship is $A=Z+N$, where $A$ is mass number, $Z$ is atomic number and $N$ is number of neutrons.
3. Rearranging gives $N=A-Z$.

Isotopes share protons but differ in neutrons

1. Isotopes have the same atomic number and the same positive nuclear charge because they contain the same number of protons.
2. They have different mass numbers because their nuclei contain different numbers of neutrons.
3. Changing the neutron number makes a different isotope, while changing the proton number makes a different element.
4. Carbon-12 has six protons and six neutrons, while carbon-13 has six protons and seven neutrons.

Nuclear notation places both numbers on the left

1. An isotope is written as ${}_Z^A\mathrm{X}$, where $\mathrm{X}$ is the element symbol.
2. The mass number $A$ is the upper-left number and the atomic number $Z$ is the lower-left number.
3. For carbon-13, ${}_6^{13}\mathrm{C}$ shows thirteen nucleons in total and six protons.

Comparisons must name both similarities and differences

1. Two nuclei are isotopes when their lower-left numbers are equal and their upper-left numbers are different.
2. A full explanation states that they have the same number of protons but different numbers of neutrons.

6.1.3 Sub-atomic particles: masses and charges

Relative values compare particle mass and charge

1. A proton has relative mass $1$ and a neutron has relative mass $1$.
2. An electron and a positron each have relative mass about $\frac{1}{1840}$, so their contribution to atomic mass is very small.
3. A proton has relative charge $+1$, a neutron has $0$, an electron has $-1$ and a positron has $+1$.
4. Relative mass and relative charge are ratios, so they have no units.

Each sub-atomic particle has a distinct combination

1. Proton: relative mass $1$, relative charge $+1$ and located in the nucleus.
2. Neutron: relative mass $1$, relative charge $0$ and located in the nucleus.
3. Electron: relative mass about $\frac{1}{1840}$, relative charge $-1$ and found outside the nucleus.
4. Positron: relative mass about $\frac{1}{1840}$ and relative charge $+1$.
5. A positron is the electron's antiparticle, so it has the same mass as an electron but the opposite charge.

Equal proton and electron numbers make an atom neutral

1. Each proton contributes $+1$ and each electron contributes $-1$, so equal numbers give a total charge of $0$.
2. Neutrons do not affect overall charge because each neutron has relative charge $0$.
3. The total relative charge can be found from $Q=N_p-N_e$, where $N_p$ and $N_e$ are the numbers of protons and electrons.

6.1.4 Electron orbits and energy changes

Electrons occupy set orbits around the nucleus

1. Electrons occupy orbits at set distances from the nucleus rather than any distance between them.
2. An electron in an orbit farther from the nucleus has more energy than one in an orbit closer to the nucleus.
3. A change of orbit requires a fixed energy transfer equal to the energy difference between the two orbits.

Absorption moves an electron farther out

1. When an atom absorbs electromagnetic radiation, energy is transferred to an electron.
2. The electron gains energy and moves to an allowed orbit farther from the nucleus.
3. The electron can make the change only when the absorbed radiation supplies the correct energy difference.

Emission moves an electron closer in

1. When an electron moves to an allowed orbit closer to the nucleus, it loses energy.
2. The atom emits the energy difference as electromagnetic radiation.
3. The electron remains in the atom after emission; it has changed orbit rather than being used up.

Losing outer electrons forms a positive ion

1. Outer electrons can be removed from an atom, but the number of protons in the nucleus does not change.
2. Losing a negatively charged electron leaves an excess of positive charge.
3. If a neutral atom loses one electron, its relative charge becomes $+1$, and losing two electrons gives $+2$.
4. Ion formation changes the number of electrons, not the identity of the element, because the proton number remains fixed.

6.2.1 Types of nuclear radiation

Radiation from unstable nuclei

1. Every atom has a nucleus made of protons and neutrons, with electrons arranged in shells around it.
2. The protons and neutrons are held together by an attractive force that acts only over a very short range inside the nucleus.
3. In some nuclei that attraction does not balance the electrostatic repulsion between the positive protons, so the nucleus is unstable.
4. An unstable nucleus becomes more stable by emitting nuclear radiation, and this emission is called radioactive decay.
5. The five emissions to recall are alpha, $\alpha$, beta minus, ${\beta }^{-}$, beta plus, ${\beta }^{+}$, gamma, $\gamma$, and neutron radiation.
6. Alpha radiation is a particle of two protons and two neutrons thrown out of the nucleus.
7. Beta minus radiation is a fast-moving electron that is created in the nucleus at the moment of decay and then emitted.
8. Beta plus radiation is a positron, a particle with the same mass as an electron but a relative charge of $+1$.
9. Gamma radiation is a high-energy electromagnetic wave emitted from the nucleus, so it has no mass and no charge.
10. Neutron radiation is a neutron emitted from the nucleus, so it also carries no charge.
11. All five emissions come out of the nucleus, never from the electron shells that surround it.

Decay is random and spontaneous

1. Radioactive decay is random, so you cannot pick out one nucleus in a sample and say that it will be the next to decay.
2. You also cannot predict the exact moment at which a chosen nucleus will decay.
3. Every unstable nucleus of the same type has the same chance of decaying in the next second, no matter how long it has already existed.
4. Decay is spontaneous, so it is not started or speeded up by heating, cooling, crushing, lighting or reacting the material chemically.
5. Because decay is random, a detector records a different count in each equal time interval even when the source and the detector have not been moved.
6. A real sample contains an enormous number of nuclei, so the average rate of decay across the whole sample is steady and predictable even though single decays are not.
7. Calling decay random describes the timing of each decay, not any uncertainty about which radiations can be emitted.

Ionising radiation

1. An atom is electrically neutral because it contains equal numbers of positive protons and negative electrons.
2. When ionising radiation passes close to an atom it can transfer enough energy to pull an outer electron away.
3. The atom is then left with more protons than electrons, so it becomes a positive ion.
4. The four ionising radiations to recall are $\alpha$, ${\beta }^{-}$, ${\beta }^{+}$ and $\gamma$.
5. Any one of those four is a correct answer when a question asks you to name an ionising radiation.
6. Neutron radiation is emitted by unstable nuclei but is not one of the four named ionising radiations.
7. Ionisation is also what makes detection possible, because the ions and free electrons produced can be used to make an electrical pulse in a detector.

6.2.2 Background radiation

What background radiation is

1. A detector switched on in an ordinary room still records counts even though no radioactive source is present.
2. Those counts come from background radiation, which is always there in the surroundings.
3. The level is low, so it is far weaker than the radiation from a sealed source held close to a detector.
4. Most background radiation is natural and comes from radioactive materials in rocks, soil, air, food and the body, together with radiation arriving from space.
5. A smaller share comes from human activity, mainly medical uses of radiation.
6. A detector cannot tell background radiation apart from radiation coming from a source, so the background count rate must be measured and then subtracted from every reading.

Background radiation from Earth

1. Rocks and soil contain small amounts of naturally occurring radioactive isotopes, mainly uranium, thorium and potassium.
2. As those unstable nuclei decay they release ionising radiation into the surroundings, so the ground itself is a weak source.
3. Granite holds more uranium than most other rocks, so areas built on granite, such as parts of Cornwall and Aberdeenshire, have a higher background level than average.
4. Bricks, stone and concrete are made from rock, so building materials raise the background level measured inside a house.
5. Uranium in rock decays through a series of steps that produces radon, a radioactive gas that escapes from the ground into the air.
6. Radon collects in enclosed spaces such as cellars, mines and poorly ventilated rooms, and it is the largest single natural contributor to the average dose received in the UK.
7. Houses in high-radon areas are fitted with underfloor fans or sealed floors so that the gas is drawn away rather than allowed to build up indoors.
8. Food and drink contain radioactive potassium-40, so eating and drinking adds a small amount of radioactive material to the body.
9. All living things contain radioactive carbon-14, which means the human body is itself a weak source of background radiation.

Background radiation from space

1. Cosmic rays arrive at the Earth continuously from every direction in space.
2. When a cosmic ray strikes a nucleus high in the atmosphere it produces a shower of other fast-moving particles that travel down towards the ground.
3. The atmosphere absorbs much of this radiation, so only part of it reaches sea level.
4. The higher you go, the less atmosphere lies above you to absorb the radiation, so cosmic radiation increases with altitude.
5. People living in mountain regions, and aircrew who spend long working hours at cruising altitude, receive more cosmic radiation than people at sea level.
6. Cosmic rays also create carbon-14 in the upper atmosphere, which then passes into plants, animals and people through food chains.

Human-made sources

1. Medical procedures are the largest human-made contribution, including X-ray images, computed tomography scans and radioactive tracers used in hospitals.
2. Nuclear power stations and the industries that supply and reprocess their fuel release small, monitored amounts of radioactive material.
3. Fallout from past nuclear weapons testing and from nuclear accidents left long-lived radioactive isotopes in soil and dust across large areas.
4. Together these human-made sources add far less to the average dose than natural sources do.

Why the level varies

1. Background radiation is not the same everywhere, because the sources are not spread evenly over the Earth.
2. Places built on granite have more uranium beneath them, so more radon is released and the background count rate is higher.
3. Places at high altitude receive more cosmic radiation because less of it has been absorbed by the air above.
4. Poorly ventilated rooms, cellars and mines let radon collect instead of dispersing, so the level indoors can be higher than outdoors.
5. Some occupations raise exposure, for example mining, flying aircraft and operating X-ray equipment in a hospital.
6. Because the level varies from place to place, the background count rate must be measured in the same room, with the same detector, as the experiment it will be subtracted from.

6.2.3 Detecting and measuring radioactivity

Detecting radioactivity

1. Ionising radiation cannot be seen, heard, smelt, tasted or felt, so a detector is always needed to show that it is present.
2. Every detector works by responding to the ionisation that the radiation causes as it passes through a material.
3. The two methods to be able to describe are photographic film and the Geiger-Muller tube.
4. Photographic film records the total exposure built up over hours, days or weeks.
5. A Geiger-Muller tube gives an immediate reading, count by count, as the radiation arrives.
6. A count rate is worked out from a count and a time using $count\; rate=\frac{count}{time}$, giving an answer in counts per second.

Photographic film

1. Photographic film is coated with an emulsion that contains grains of light-sensitive silver salts.
2. Ionising radiation passing through the emulsion ionises the silver salts and changes the grains it hits, in the same way that light does.
3. The film is sealed inside a light-proof wrapper, so any change to the emulsion must have been caused by radiation rather than by light.
4. When the film is later developed, every changed grain turns into dark metallic silver and that part of the film appears blackened.
5. The greater the exposure to radiation, the more grains are changed, so the darker the developed film becomes.
6. Comparing the darkening with an unexposed control film shows both that radiation was present and roughly how much of it was absorbed.
7. A film badge worn by a radiographer or a nuclear worker holds the film behind several windows made of different materials, typically an open window, plastic, aluminium and lead.
8. Radiation that darkens the film behind the lead window must be highly penetrating, so comparing the darkening behind each window shows which types of radiation the worker met.
9. The badge is collected and developed at set intervals, so it gives a permanent record of the dose received but no warning at the time.

The Geiger-Muller tube

1. The tube is a sealed metal cylinder holding argon gas at low pressure, with a thin mica window at one end that is thin enough to let alpha and beta radiation through.
2. A thin wire anode runs along the centre of the tube and the metal wall acts as the cathode, with a potential difference of several hundred volts between them.
3. When ionising radiation enters the tube it ionises argon atoms, producing positive ions and free electrons.
4. The free electrons are accelerated towards the central wire by the electric field and gain enough energy to ionise further argon atoms on the way.
5. This chain of ionisation, called an avalanche, turns a single entering particle into a large number of charge carriers.
6. The charge arriving at the wire causes a brief pulse of current in the circuit connected to the tube.
7. The pulse is passed to a counter or ratemeter, which registers one count and often produces an audible click.
8. Dividing the count by the counting time gives the count rate, and a larger count rate means more radiation is reaching the detector.
9. The tube records only the radiation that actually enters its window, so the count rate is always smaller than the number of decays happening in the source.

Comparing the two methods

1. Both methods respond to ionisation, but they give different kinds of information.
2. Photographic film shows nothing until it has been developed, so it cannot warn anyone while the exposure is happening.
3. A Geiger-Muller tube responds instantly, so it can be used to search for a source or to watch a count rate change from moment to moment.
4. Film records the total dose collected over a long period, which a single reading from a tube cannot show.
5. Film is small, cheap and needs no power supply, so it is convenient to wear as a badge all day.
6. A tube needs a high-voltage supply and a counter, so it is used as laboratory or survey equipment rather than as personal monitoring.
7. Film behind different absorbers shows which types of radiation were present, while a tube shows this only when absorbers are deliberately placed in front of the window.

6.2.4 Properties of alpha, beta and gamma radiation

What the three radiations are

1. An alpha particle has a relative mass of $4$ and a relative charge of $+2$, and is written ${}_2^4\alpha$ or ${}_2^{4}He$.
2. It is by far the heaviest of the three emissions and leaves the nucleus at roughly $5\%$ of the speed of light.
3. A beta particle has a relative charge of $-1$ and a relative mass of about $\frac{1}{1836}$, and is written ${}_{-1}^0\beta$ or ${}_{-1}^{0}e$.
4. The electron is created inside the nucleus at the moment of decay, so it does not come from the electron shells around the atom.
5. Beta particles are emitted at speeds up to about $99\%$ of the speed of light, which makes them much faster than alpha particles.
6. A gamma ray has no mass and no charge and is written ${}_0^0\gamma$.
7. Gamma radiation is part of the electromagnetic spectrum, so it travels at the speed of light, $3\times 10^{8}m/s$.
8. Alpha and beta emissions are charged particles while gamma is an uncharged wave, and that difference is what makes their behaviour in matter so different.

Ionising power

1. A charged particle ionises an atom by attracting or repelling an outer electron strongly enough to pull it away as the particle goes past.
2. Alpha is the most strongly ionising, because its charge of $+2$ exerts a large force on electrons and its heavy mass keeps it slow, so it spends longer beside each atom.
3. A single alpha particle can create tens of thousands of ion pairs before it comes to rest.
4. Beta is moderately ionising, because its charge is only $-1$ and its very high speed means it sweeps past each atom in a much shorter time.
5. Gamma is the least ionising, because it has no charge at all and only occasionally interacts with an atom.
6. Every ionisation transfers energy out of the radiation, so a strongly ionising radiation runs out of energy over a short distance.

Penetrating power

1. Alpha is the least penetrating, with a range in air of only a few centimetres.
2. A single sheet of paper, or the dead outer layer of skin, absorbs alpha radiation completely.
3. Beta has a medium penetrating power and travels up to about a metre through air.
4. Beta passes straight through paper but is absorbed by roughly $3mm$ of aluminium.
5. Gamma is the most penetrating and can travel many metres through air.
6. Gamma passes through paper and through thin aluminium, so several centimetres of lead or metres of concrete are needed to cut it down substantially.
7. Gamma radiation is never absorbed completely by a shield, so thicker shielding reduces the intensity that gets through rather than removing it.
8. Ionising power and penetrating power run in opposite orders, because the radiation that ionises most strongly gives up its energy soonest and therefore stops soonest.
9. In order of increasing penetrating power the sequence is $\alpha$, then $\beta$, then $\gamma$.
10. In order of increasing ionising power the sequence is $\gamma$, then $\beta$, then $\alpha$.

6.2.5 The changing model of the atom

How scientific models change

1. A model of the atom is accepted only for as long as it explains all of the experimental evidence available at the time.
2. A useful model makes a prediction that can be tested, so an experiment can be designed to check whether the model is right.
3. When an experiment produces a result that the model cannot explain, the model has to be changed or replaced.
4. New results are published and repeated by other scientists before a new model is accepted, which is why each change took years rather than days.
5. The model of the atom moved through three stages: the plum pudding model, then the nuclear model, then the Bohr model.

The plum pudding model

1. Before the electron was discovered, the atom was pictured as a solid sphere that could not be divided into anything smaller.
2. The discovery of the electron showed that atoms contain small, negatively charged particles.
3. An atom has no net charge, so the rest of the atom had to carry an equal amount of positive charge.
4. The plum pudding model described the atom as a ball of positive charge with negative electrons dotted through it, like pieces of fruit set in a pudding.
5. In this model there is no nucleus, and both the positive charge and the mass are spread evenly through the whole volume of the atom.
6. Spread-out charge can only exert a weak force on a passing particle, so the model predicted that a fast positive particle fired at a thin sheet of atoms would go through with a very small deflection at most.

Rutherford alpha particle scattering

1. Working in Rutherford's laboratory, Geiger and Marsden aimed a narrow beam of alpha particles at a very thin sheet of gold foil.
2. Gold was chosen because it can be beaten into a foil only a few atoms thick, so most alpha particles pass through only a small number of atoms.
3. The apparatus was held inside an evacuated chamber, so the alpha particles were not absorbed by air before they reached the foil.
4. A movable zinc sulfide detector was placed around the foil and gave a tiny flash of light wherever an alpha particle struck it.
5. Most alpha particles passed straight through the foil with no measurable change of direction.
6. A small number were deflected through small angles as they went through.
7. About one in eight thousand was deflected through more than $90^{\circ }$, and a few came almost straight back towards the source.
8. The plum pudding model could not explain the large deflections, because a positive charge spread through the whole atom could never push an alpha particle backwards.

What the results showed

1. Most alpha particles passed straight through, so most of the atom must be empty space.
2. Some alpha particles were pushed off course, so the atom must hold a concentrated region of positive charge that repels a positive alpha particle.
3. Only a very small fraction came close enough to that region to be turned through a large angle, so the region must be extremely small compared with the whole atom.
4. An alpha particle is heavy and fast, so only something with a much larger mass could reverse its direction, which showed that this small central region holds almost all of the mass of the atom.
5. The small, dense, positively charged centre was named the nucleus.

The nuclear model and the Bohr model

1. The nuclear model places a tiny positive nucleus at the centre of the atom with the electrons somewhere outside it.
2. Almost all of the mass sits in the nucleus, while almost all of the volume of the atom is empty space.
3. A typical atom has a diameter of about $1\times 10^{-10}m$, and its nucleus is roughly $10000$ times smaller than that.
4. The nuclear model left a problem unsolved, because an electron free to circle at any distance should lose energy steadily and spiral into the nucleus.
5. Bohr answered this by proposing that electrons can only orbit at certain fixed distances from the nucleus, called shells or energy levels.
6. An electron staying in one shell does not lose energy, so the atom is stable and does not collapse.
7. The Bohr model also explained why an atom only absorbs and emits particular frequencies of light, because an electron changes energy only when it moves between two shells.
8. Calculations of the energies of those shells matched measurements of the light given out by real atoms, which is why the Bohr model replaced the simple nuclear model.