Atomic structure – GCSE Combined Science Physics

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Picture a football stadium with one small pea sitting on the centre spot. If the whole stadium is an atom, the pea is its nucleus. That is how tiny the centre of an atom really is, compared with the space around it.

Almost all of the weight of the atom is packed into that one pea. The rest of the stadium is mostly empty space, with a few very light electrons whizzing about in it. It sounds strange, but it is true for every atom in your body, in your phone, in your bike and in the air around you.

Now think of a tall tower of building blocks that is a bit wobbly. Sooner or later it drops a block and throws some energy out, and then it sits a little steadier than before. Some atoms behave in just this way, and we call it radioactivity, which is the word you will see in every exam.

In this lesson you find out what an atom is made of, what a radioactive atom throws out, and how quickly it happens. You will also see why doctors can use radiation to help patients, and why they take such care with it.

By the end you will be able to do four things. You will name the parts of an atom, say what charge each has, and explain why scientists changed their picture of the atom. You will use two numbers to find how many neutrons an atom has. You will work out how much of a radioactive source is left after some half-lives. You will also say how people stay safe around radiation.

Inside the atom

An atom is the tiny building block that everything is made from. Every atom has a nucleus in the middle, with electrons moving around it. An atom is only about 1 × 10⁻¹⁰ metres across, which is far too small to see, and its nucleus is more than 10 000 times smaller again.

The nucleus is made of two kinds of particle, called protons and neutrons. A proton has a positive charge, and a neutron has no charge at all, so we say it is neutral. Charge is the property that makes things push or pull on each other, like the static that makes a balloon stick to a wall.

Electrons have a negative charge, and they move around the nucleus in layers called shells. An electron is about 2000 times lighter than a proton, while a proton and a neutron have almost exactly the same mass. That is why nearly all of the mass of an atom is in its nucleus.

A normal atom has the same number of protons as electrons. The positive and negative charges cancel out, like +3 and −3 adding up to zero, so the atom has no overall charge. We call an atom like this neutral, and it is the usual state for an atom.

The electrons are arranged at different distances from the nucleus. These are called energy levels, and the levels further out have more energy. An electron can absorb electromagnetic radiation and jump up to a higher level, further from the nucleus. It can also fall to a lower level, closer to the nucleus, and when it does it gives out electromagnetic radiation.

Atoms are very small. The radius of an atom is about 1 × 10⁻¹⁰ m, which is 0.1 nm. The nucleus is much smaller still, about 1 × 10⁻¹⁴ m across, which is around 10 000 times smaller than the atom. Most of the atom is empty space, but almost all of its mass is in the nucleus.

How our picture of the atom changed

Scientists build models to explain what they see. When new evidence does not fit a model, the model is changed or replaced. The atom is a good example, because our picture of it has changed several times.

Before the electron was found, atoms were thought to be tiny spheres that could not be divided. Then, in 1897, the electron was discovered. This is a tiny negative particle that is part of every atom, so atoms could be divided after all.

The discovery of the electron led to the plum pudding model. The atom was a ball of positive charge with negative electrons embedded in it, like plums in a pudding. The positive and negative charges cancelled, so the whole atom was neutral.

The next test was the alpha particle scattering experiment. Scientists fired alpha particles at a very thin sheet of gold foil and watched where they went. The plum pudding model predicted that the alpha particles would pass straight through with only tiny changes of direction.

The results were very different. Most alpha particles went straight through, so the atom is mostly empty space. A few were deflected through large angles, and a tiny number bounced almost straight back. Alpha particles are positive, so they were repelled by something positive. The positive charge and most of the mass must be squeezed into a tiny centre, the nucleus. This nuclear model replaced the plum pudding model.

Niels Bohr then adapted the nuclear model. He suggested that electrons orbit the nucleus at specific distances, which we call energy levels. His calculations agreed with what scientists saw in experiments.

Later experiments showed that the positive charge of a nucleus can be divided into a whole number of smaller particles, each with the same charge. These were named protons. About 20 years after the nucleus was accepted, James Chadwick found the evidence for neutrons, particles with no charge, in the nucleus.

Proton number, mass number and isotopes

Every element has its own number of protons, and this never changes. Carbon atoms always have 6 protons, and oxygen atoms always have 8. This is called the proton number, or the atomic number, and it decides which element an atom is, so change the protons and you change the element.

The mass number is the total number of protons and neutrons in the nucleus. Electrons are so light that they do not count towards it. So to find the neutrons, you take the protons away from the mass number, because the mass number counts both kinds of particle.

In words: number of neutrons = mass number − proton number. In symbols: neutrons = A − Z, where A is the mass number and Z is the proton number. There is no unit, because these are just counts of particles, and you use it whenever you are given two of the three numbers.

Atoms are written with the mass number at the top and the proton number at the bottom. Carbon-14 is written ¹⁴₆C. The top number 14 is the mass number, and the bottom number 6 is the proton number, so it has 14 − 6 = 8 neutrons.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Carbon-12 has 6 neutrons, and carbon-14 has 8 neutrons. Both are carbon, because both have 6 protons, but the second one is heavier because of its two extra neutrons.

Isotopes of one element behave the same way in chemistry, because they have the same number of electrons. The difference is inside the nucleus, where some isotopes are stable and others, like carbon-14, are unstable and radioactive. Do not mix isotopes up with ions, which are atoms that have gained or lost electrons while the nucleus stays the same. An atom turns into a positive ion if it loses one or more outer electrons.

Radioactive decay and nuclear equations

Some nuclei are unstable, because they have the wrong mix of protons and neutrons. An unstable nucleus changes to become steadier, and it throws out radiation as it does. This is called radioactive decay, and it happens without any help from heat or chemistry.

Decay is random, so you cannot say when one particular nucleus will go. Think of popcorn in a hot pan, where you cannot tell which kernel pops next, but the whole pan pops at a steady rate. In the same way, a big sample of atoms decays at a predictable overall rate.

This radiation is ionising, which means it can knock electrons off atoms and turn them into ions. That can damage living cells, or even cause cancer, which is why we treat radiation with respect.

There are four types of nuclear radiation. An alpha particle (α) is 2 protons and 2 neutrons, the same as a helium nucleus. A beta particle (β) is a fast electron thrown out of the nucleus when a neutron turns into a proton. A gamma ray (γ) is electromagnetic radiation from the nucleus. A neutron (n) can also be thrown out.

Their properties differ. Alpha is strongly ionising, travels only a few centimetres in air and is stopped by paper. Beta is less ionising, travels about a metre in air and is stopped by a few millimetres of aluminium. Gamma is weakly ionising and travels a long way in air. Thick lead or a lot of concrete is needed to reduce it. The more strongly a type ionises, the less far it goes.

Decay changes the nucleus, so we write it as a nuclear equation. An alpha particle is ⁴₂He and a beta particle is ⁰₋₁e. The mass numbers must add up to the same total on both sides, and so must the proton numbers. For alpha decay, ²²⁶₈₈Ra becomes ²²²₈₆Rn + ⁴₂He. Check: 222 + 4 = 226 and 86 + 2 = 88. For beta decay, ¹⁴₆C becomes ¹⁴₇N + ⁰₋₁e.

So alpha decay lowers both the mass number, by 4, and the proton number, by 2. Beta decay leaves the mass number alone but raises the proton number by 1. Gamma emission changes neither, because only energy leaves. Gamma often comes out just after an alpha or beta decay.

A positron is the antiparticle of the electron. It has the same mass as an electron but a charge of +1. In beta-plus decay a proton in the nucleus changes into a neutron and a positron is emitted. The mass number stays the same and the atomic number goes down by 1. For example, carbon-11 decays to boron-11 and emits a positron.

In beta-minus decay a neutron changes into a proton and an electron, and the electron is emitted. The mass number stays the same and the atomic number goes up by 1.

Half-life

The activity of a source is how many of its nuclei decay each second, and it is measured in becquerels, Bq. One becquerel is one decay per second. A detector such as a Geiger counter measures a count rate, which is the number of counts it picks up each second from the source.

Half-life is the time it takes for the number of unstable nuclei to halve. It is also the time for the activity or the count rate to halve. Imagine 80 crisps in a bowl, where every minute someone eats half of what is left, so you get 80, 40, 20, 10 and so on, and it never quite reaches zero.

Decay is random, so half-life only works for a large number of nuclei. We cannot say when one nucleus will decay. We can say how long it takes for half of a big sample to go.

In words: count rate after n half-lives = starting count rate ÷ 2ⁿ. In symbols: final = start ÷ 2ⁿ, where n is the number of half-lives. You use it when you are given a time and a half-life, and the number of half-lives is the total time ÷ the half-life, as long as both are in the same unit.

Work in three steps. First, find n by dividing the time by the half-life, and then work out 2ⁿ, which is 2, 4, 8 and 16 for n = 1, 2, 3 and 4. Last, divide the starting value by that number, so 12 days with a half-life of 3 days gives n = 4 and you divide by 16.

On a graph the curve falls steeply at first and then flattens out. To find a half-life from a graph, read the time it takes for the value to drop to half of where it started. A detector also picks up background radiation, so if you are given the background count rate you take it away first.

Higher tier: you can write the fall as a ratio. After 1 half-life the count rate is half the start, a ratio of 2 : 1. After 3 half-lives it is one eighth of the start, so the start to end ratio is 8 : 1. The ratio after n half-lives is 2ⁿ : 1.

Contamination, irradiation and staying safe

Radiation is all around us at a low level, and this is called background radiation. It comes from rocks and the radon gas they give off, from cosmic rays from space, from some foods and from medical scans. The amount depends on where you live, and some places have much more radon than others.

Irradiation means being exposed to radiation from a source outside your body. It is like standing near a fire, where you feel the heat but there is no fire inside you. The irradiated object does not become radioactive, and moving away or putting a shield in the way stops it.

Contamination means that radioactive atoms get onto or into you or an object. It is like getting muddy, because the mud stays on you until you wash it off. The atoms keep giving out radiation until they are removed or have decayed. The hazard comes from the decay of those atoms, and the type of radiation they give out changes how serious it is.

Alpha, beta and gamma are different hazards. Outside the body, alpha is hardly a risk, because skin and a few centimetres of air stop it. Gamma and beta can get through, so irradiation from them matters more. Inside the body, alpha is the most dangerous, because it is strongly ionising and gives all its energy to the cells nearby.

To limit a dose from a source, keep it in a lead-lined box and pick it up with tongs. Stay as far away as you can, and spend as little time near it as possible. Never point it at anyone. To avoid contamination, wear gloves and protective clothing. Workers wear badges that measure how much radiation they have received.

Early researchers suffered because they did not know the dangers, and rules for protection came in the 1930s. Today, studies into the effects of radiation on people must be published and shared. Other scientists can then check the findings. This is called peer review.

Radiation is also useful, so we choose a source to match the job. A smoke alarm uses an alpha source, because alpha ionises the air in a small gap and cannot get out of the case. A medical tracer uses a gamma source with a short half-life of a few hours, so it can be tracked from outside and soon stops being radioactive. Radiation can also be aimed at a tumour to kill cancer cells.

Recap

An atom has a tiny nucleus of protons and neutrons, with electrons in energy levels around it. Protons are positive, neutrons are neutral and electrons are negative, and nearly all of the mass is in the nucleus. The number of neutrons is the mass number minus the proton number.

The model changed as evidence arrived: the plum pudding model came after the electron, alpha scattering led to the nuclear model, Bohr put electrons in energy levels, and later work found protons and neutrons.

Isotopes have the same proton number and different neutron numbers. Unstable nuclei decay at random, giving out alpha, beta, gamma or neutrons. Alpha lowers the mass number by 4 and the proton number by 2, beta raises the proton number by 1, and gamma changes neither.

Alpha is stopped by paper, beta by aluminium and gamma by thick lead. Half-life is the time for the count rate to halve, and after n half-lives you divide by 2ⁿ. Irradiation comes from outside, while contamination means radioactive atoms are on or in you.

In an exam you might be asked to calculate the neutrons from two numbers, or the count rate after some half-lives. You might be asked what happens to the mass number and the proton number in a decay, or why alpha scattering ended the plum pudding model. Other favourites are why a tracer has a short half-life, why alpha is the most dangerous inside the body, and the difference between irradiation and contamination.

When you do a half-life question, always show the number of half-lives first, then the division. Write units on your answer, such as counts per second or Bq. Method marks are given for the working, even if the final number goes wrong.