IGCSE Co-ordinated Sciences · Area 3 · Physics

Nuclear Physics

Nuclear physics covers what is inside the atom and what happens when a nucleus changes: the nuclear model, nuclide notation, isotopes, fission and fusion, background radiation, alpha, beta and gamma emission, decay equations, half-life and radiation safety.

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Syllabus coverage

  • Describe the structure of an atom in terms of a positively charged nucleus and negatively charged electrons in orbit around the nucleus
  • Know how atoms may form positive ions by losing electrons or form negative ions by gaining electrons
  • Describe how the scattering of alpha particles by a sheet of thin metal supports the nuclear model of the atom, by providing evidence for a very small nucleus surrounded by mostly empty space, a nucleus containing most of the mass of the atom, and a nucleus that is positively charged
  • Describe the composition of the nucleus in terms of protons and neutrons, and state the relative charges of protons, neutrons and electrons as +1, 0 and -1 respectively
  • Define the terms proton number (atomic number) Z and nucleon number (mass number) A, and be able to calculate the number of neutrons in a nucleus
  • Use the nuclide notation A/Z X to represent a nucleus
  • Explain what is meant by an isotope and state that an element may have more than one isotope
  • Describe the processes of nuclear fission and nuclear fusion as the splitting or joining of nuclei, to include the nuclide equation and a qualitative description of the mass and energy changes without values
  • Know the relationship between the proton number and the relative charge on a nucleus, and the relationship between the nucleon number and the relative mass of a nucleus
  • Know what is meant by background radiation, and know the sources that make a significant contribution to it, including radon gas in the air, rocks and buildings, food and drink, and cosmic rays
  • Know that ionising nuclear radiation can be measured using a detector connected to a counter, use count rate measured in counts/s or counts/minute, and use measurements of background radiation to determine a corrected count rate
  • Describe the emission of radiation from a nucleus as spontaneous and random in direction, and identify alpha, beta and gamma emissions from the nucleus by recalling their nature, their relative ionising effects and their relative penetrating abilities
  • Describe the deflection of alpha-particles, beta-particles and gamma-radiation in electric fields and magnetic fields
  • Explain the relative ionising effects of alpha-particles, beta-particles and gamma-radiation with reference to kinetic energy and electric charge
  • Know that radioactive decay is a change in an unstable nucleus that can result in the emission of alpha-particles or beta-particles and/or gamma-radiation, know that these changes are spontaneous and random, and state that during alpha-decay or beta-decay the nucleus changes to that of a different element

Part of the IGCSE Co-ordinated Sciences syllabus 0654 / 0973.

The structure of the atomAn atom is mostly empty space. A tiny, dense, positively charged nucleus sits at the centre and electrons orbit aroundit.A simple atomelectron in orbitnucleusWhat is inside the atomprotonpositive charge, in the nucleusneutronno charge, in the nucleuselectronnegative charge, in orbitThe nucleus is tiny, but it holdsalmost all of the mass of the atom.Protons and neutrons are callednucleons. The electrons are arrangedin shells at increasing distance fromthe nucleus.Ions: losing and gaining electronsneutral atomequal numbers of protons and electronspositive ion (+)more protons than electronsnegative ion (-)more electrons than protonsloses an electrongains an electronAn atom that loses an electron becomes a positive ion; one that gains an electron becomes a negative ion.
The nuclear model of the atom: a small, dense, positively charged nucleus containing protons and neutrons at the centre, with negatively charged electrons in orbit around it, showing that most of the atom is empty space.

The Nuclear Model of the Atom

Every atom is a tiny positive nucleus of protons and neutrons with negative electrons in orbit around it. This topic covers that structure, how atoms form ions, the alpha particle scattering evidence for the nuclear model, proton and nucleon number, isotopes, and the fission and fusion of nuclei.

The structure of the atomAn atom is mostly empty space. A tiny, dense, positively charged nucleus sits at the centre and electrons orbit aroundit.A simple atomelectron in orbitnucleusWhat is inside the atomprotonpositive charge, in the nucleusneutronno charge, in the nucleuselectronnegative charge, in orbitThe nucleus is tiny, but it holdsalmost all of the mass of the atom.Protons and neutrons are callednucleons. The electrons are arrangedin shells at increasing distance fromthe nucleus.Ions: losing and gaining electronsneutral atomequal numbers of protons and electronspositive ion (+)more protons than electronsnegative ion (-)more electrons than protonsloses an electrongains an electronAn atom that loses an electron becomes a positive ion; one that gains an electron becomes a negative ion.

Radioactivity

Radioactivity is the spontaneous, random decay of unstable nuclei, which emit alpha particles, beta particles or gamma radiation. This topic covers background radiation and how it is measured, the three types of emission and how they are deflected, decay equations, half-life calculations, the uses of radioisotopes, and the safety precautions that keep the dose to living tissue low.

Key terms you must be able to define

Atom
The smallest particle of an element, made of a positively charged nucleus of protons and neutrons with negatively charged electrons in orbit around it; the atom is neutral overall.
Nucleus
The very small, dense, positively charged centre of an atom, containing the protons and neutrons and almost all of the mass of the atom.
Proton number, Z
The number of protons in the nucleus of an atom, also called the atomic number; it decides which element the atom is, and in a neutral atom it also equals the number of electrons.
Nucleon number, A
The total number of protons and neutrons (nucleons) in a nucleus, also called the mass number; the number of neutrons is A - Z.
Isotope
Atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same proton number and different nucleon numbers, for example chlorine-35 and chlorine-37.
Ion
A charged atom or group of atoms, formed by losing electrons to give a positive ion or by gaining electrons to give a negative ion; the number of protons does not change.
Nuclide
A particular nucleus described by its proton number and its nucleon number and written in nuclide notation as A/Z X, for example 235/92 U.
Alpha particle
A helium nucleus, made of 2 protons and 2 neutrons, with a relative charge of +2 and a relative mass of 4; alpha particles were used in the scattering experiment that established the nuclear model.
Background radiation
The low-level ionising radiation that is present in the environment at all times, from natural sources such as radon gas, rocks and buildings, food and drink and cosmic rays, together with a small contribution from medical and other artificial sources.
Count rate
The number of counts recorded by a detector connected to a counter each second or each minute, measured in counts/s or counts/min; it is proportional to the activity of the source.
Corrected count rate
The count rate due to the source alone, found by subtracting the background count rate from the count rate measured with the source present.
Alpha particle
A helium nucleus, made of 2 protons and 2 neutrons, with a charge of +2e and a mass number of 4; the most strongly ionising and the least penetrating emission, stopped by paper or a few centimetres of air.

Applied to the real world

Cambridge sets Co-ordinated Sciences in unfamiliar contexts - a hospital, a power station, a farm, a laboratory. Use these as models for how to apply the science, then write your own.

In a nuclear power station

A nuclear power station releases energy by the fission of uranium-235. A neutron is absorbed by a uranium nucleus, which splits into two smaller nuclei and releases two or three more neutrons, so a controlled chain reaction builds up. Control rods made of boron absorb surplus neutrons to keep the reaction steady, and a moderator slows the neutrons so that they are more likely to be absorbed. The energy released heats a coolant, which makes steam to drive the turbines.

In the core of the Sun

The Sun is powered by nuclear fusion. At temperatures of millions of degrees and enormous pressure, hydrogen nuclei are forced close enough together to join and form helium nuclei. The total mass of the helium produced is slightly less than the mass of the hydrogen that went into it, and that missing mass is released as energy, which is why the Sun has shone for billions of years.

In a hospital

Isotopes are used in medicine because different isotopes of the same element behave in the same chemical way but can be detected in different ways. A small quantity of a radioactive isotope is used as a tracer and its radiation is picked up outside the body to show how an organ is working, and other isotopes are used in radiotherapy, where the radiation is aimed at a tumour to kill the cancer cells.

In a smoke alarm

A smoke alarm contains a tiny amount of americium-241, an isotope that emits alpha particles. The alpha particles ionise the air inside the detector, so a small current flows between two electrodes. When smoke enters the chamber, the smoke particles absorb the alpha particles and the current falls, and the change in current sets off the alarm.

In archaeology

Living things take in carbon from the air, including a small proportion of the isotope carbon-14, which is radioactive. When the organism dies, no more carbon is taken in and the carbon-14 slowly decays, so the ratio of carbon-14 to ordinary carbon-12 in a sample shows how long ago the organism died. This is how wood, bone and charcoal thousands of years old are dated.

In a fusion laboratory

Fusion research aims to join hydrogen nuclei to make helium, because fusion releases far more energy per kilogram of fuel than fission and its fuel can be obtained from water. The problem is that both nuclei are positive and repel each other, so the fuel has to be heated to about 100 million degrees before the nuclei move fast enough to join. No fusion power station yet produces more energy than it uses.

Flashcards

Click a card to flip it. Mark honestly — the tracker uses this.

Q1What are the three particles that make up an atom, and where are they?
Protons and neutrons, which form the nucleus at the centre, and electrons, which orbit around the nucleus.
Q2State the relative charge of a proton, a neutron and an electron.
Proton +1, neutron 0, electron -1.
Q3Where is almost all of the mass of an atom, and why?
In the nucleus, because a proton and a neutron each have a relative mass of 1, while an electron has a relative mass of only about 1/1840.
Q4How does an atom form a positive ion?
By losing one or more electrons, so it is left with more protons than electrons.
Q5How does an atom form a negative ion?
By gaining one or more electrons, so it has more electrons than protons.
Q6What is the proton number, Z?
The number of protons in the nucleus, also called the atomic number; it decides which element the atom is and equals the number of electrons in a neutral atom.
Q7What is the nucleon number, A?
The total number of protons and neutrons in the nucleus, also called the mass number.
Q8How do you calculate the number of neutrons in a nucleus?
Number of neutrons = A - Z, the nucleon number minus the proton number.
Q9How is the nuclide 235/92 U described, and how many neutrons does it contain?
Uranium with nucleon number 235 and proton number 92, so it has 92 protons and 235 - 92 = 143 neutrons.
Q10What is an isotope?
Atoms of the same element with the same number of protons but different numbers of neutrons, so the same proton number and different nucleon numbers.
0 of 10 marked as known

Exam-style questions

9 questions, 28 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO11 mark
State the relative charge of a neutron.

Mark scheme — tick each point you actually made

Examiner tip: Relative charge is written with its sign where there is one, so a proton is +1 and an electron is -1. A neutron carries no charge at all, so its relative charge is 0.
DefineAO12 marks
Define the terms proton number and nucleon number.

Mark scheme — tick each point you actually made

Examiner tip: Both definitions must mention the nucleus. For nucleon number, do not write 'the number of particles in the nucleus' - it must be protons plus neutrons.
CalculateAO23 marks
An isotope of uranium is written in nuclide notation as 235/92 U. Calculate the number of neutrons in one atom of this isotope, and deduce the relative charge on its nucleus.

Mark scheme — tick each point you actually made

Examiner tip: Show the subtraction, then the answer. The relative charge on a nucleus is always positive and equal to the proton number, so do not leave out the plus sign.
ExplainAO24 marks
A sodium atom has proton number 11 and a chlorine atom has proton number 17. Explain how each atom forms an ion, and state the number of electrons in each ion.

Mark scheme — tick each point you actually made

Examiner tip: Only electrons are lost or gained, so the proton number stays the same. Give the number of electrons and the charge for each ion, because they are separate marks.
ExplainAO24 marks
In the alpha particle scattering experiment, alpha particles were fired at a very thin sheet of gold foil. Explain how the results of this experiment support the nuclear model of the atom.

Mark scheme — tick each point you actually made

Examiner tip: Link each observation to its conclusion. 'Most went straight through, so the atom is mostly empty space' scores, but describing what the particles did without the conclusion does not.
CompareAO23 marks
Chlorine has two common isotopes, written as 35/17 Cl and 37/17 Cl. Compare the two nuclides.

Mark scheme — tick each point you actually made

Examiner tip: Compare means say what is the same as well as what is different. State the neutron number for each nuclide, not just that they are different.
ExplainAO23 marks
Explain the relationship between the proton number and the relative charge on a nucleus, and the relationship between the nucleon number and the relative mass of a nucleus.

Mark scheme — tick each point you actually made

Examiner tip: Give the rule and a number that shows you can use it. Do not say the relative mass of the nucleus is the proton number - that is the relative charge.
DescribeAO25 marks
Describe nuclear fission and nuclear fusion, and explain how energy is released in each process. Include one nuclide equation in your answer.

Mark scheme — tick each point you actually made

Examiner tip: One equation is enough, and it must balance: nucleon numbers at the top left and proton numbers at the bottom left. For the mass change, say that the mass becomes energy, never that it is destroyed.
CalculateAO23 marks
A magnesium atom has proton number 12. Calculate the number of electrons in a neutral magnesium atom and in a magnesium ion with a charge of 2+, and explain how the ion is formed.

Mark scheme — tick each point you actually made

Examiner tip: In a neutral atom the number of electrons equals the proton number. For a 2+ ion, take two electrons away, and say that the ion is positive because there are now more protons than electrons.

Exam technique for this topic

  • Write the relative charges with their signs: proton +1, neutron 0, electron -1. Giving 1 without the plus sign for a proton, or -1 for a neutron, loses the mark.
  • Keep the two numbers apart: Z is the number of protons and A is the number of protons plus neutrons, so the number of neutrons is A - Z. Show the subtraction before you give the answer.
  • In nuclide notation the nucleon number goes top left and the proton number bottom left, for example 235/92 U. Write it consistently, and check that the numbers add up before you use them.
  • For isotopes, the mark is for the same number of protons and a different number of neutrons. If you change the proton number you have described a different element.
  • For alpha particle scattering, link each observation to its conclusion: most pass straight through means mostly empty space, a few are deflected through large angles means a small, dense, positively charged nucleus that holds most of the mass. Never say the alpha particles bounced off electrons.

Why students lose marks here

A student wroteA student wrote: 'The alpha particles that came straight back from the gold foil must have hit the electrons in the atoms.'
Why it loses marksAn electron has a relative mass of only about 1/1840, so it is far too light to turn a fast alpha particle round. Large deflections need a concentrated positive charge, not a light negative one, and this answer shows that the evidence for the nucleus has been missed.
The answer that scoresAn alpha particle is a helium nucleus with a relative mass of 4 and a relative charge of +2. An electron is about 1840 times lighter than a proton, so it cannot deflect an alpha particle through a large angle. The particles that came almost straight back had passed very close to a very small, dense, positively charged nucleus, which repelled them strongly because both the nucleus and the alpha particle are positive. Those rare large deflections are the evidence that the nucleus is small, massive and positive, while the many undeflected particles show that the atom is mostly empty space.
Alpha sourceGold foilDetector screenMost alpha particles pass straight through, a few are deflected, and very few bounce back. Thisshowed that the atom is mostly empty space with a tiny, massive, positively charged nucleus atits centre.
The apparatus used to scatter alpha particles: a radioactive source, a very thin gold foil in a vacuum, and a detector that can be moved round the foil to count the particles deflected through each angle.
A student wroteA student wrote: 'Chlorine-35 and chlorine-37 are isotopes because they have different numbers of protons.'
Why it loses marksThis is the wrong way round. Two atoms with different numbers of protons are different elements. Isotopes of the same element have the same proton number and differ only in the number of neutrons.
The answer that scoresIsotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Chlorine-35 and chlorine-37 both have 17 protons, so both are chlorine. Chlorine-35 has 35 - 17 = 18 neutrons and chlorine-37 has 37 - 17 = 20 neutrons. Because both have 17 electrons in a neutral atom, they have the same chemical properties, but their masses are different.
Nuclide notationA nuclide is written A/Z X. The mass number (nucleon number) A is written above the proton number (atomic number) Z, infront of the chemical symbol X.Reading 238/92 UU23892mass number Athe nucleon numberprotons + neutrons = 238proton number Zthe atomic numberthe number of protons = 92neutrons = A - Z= 238 - 92 = 146Isotopes of carbonC126carbon-126 protons, 6 neutronsC136carbon-136 protons, 7 neutronsC146carbon-146 protons, 8 neutronsSame proton number, different mass number.The mass number A is the total number of protons and neutrons (the nucleons) in the nucleus.The proton number Z is the number of protons; it decides which element the atom is.Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Nuclide notation: the nucleon number at the top left and the proton number at the bottom left of the chemical symbol, with the number of neutrons found by subtracting the two.
A student wroteA student wrote: 'A negative ion is formed when an atom gains protons.'
Why it loses marksA proton carries a charge of +1, so gaining protons would make an atom more positive, not negative, and it would also change the element. Ions form by the gain or loss of electrons only.
The answer that scoresA negative ion is formed when an atom gains one or more electrons. A chlorine atom has proton number 17, so it has 17 protons and 17 electrons; when it gains one electron it has 17 protons and 18 electrons, so there is one more negative charge than positive charge and the ion has a charge of 1-, written Cl-. The number of protons is unchanged when an ion forms.
A student wroteA student wrote: 'In a fusion power station, heavy nuclei are split to release energy.'
Why it loses marksThis confuses the two processes. Splitting a heavy nucleus is fission, which is what today's nuclear power stations use. Fusion is the joining of light nuclei, and it needs a temperature of millions of degrees that no power station has yet achieved usefully.
The answer that scoresNuclear fission is the splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei after it absorbs a neutron. It releases energy and more neutrons, so a chain reaction can build up, and this is the process used in a nuclear power station. Nuclear fusion is the joining of two light nuclei, such as hydrogen nuclei, to form a heavier nucleus such as helium, and it releases even more energy per kilogram of fuel. Fusion only happens at extremely high temperatures and pressures, as in the core of the Sun, so no fusion power station yet produces more energy than it uses.
A nuclear power station, where the energy released by the fission of uranium-235 heats a coolant and is used to raise steam to drive the turbines.

Where this fits in IGCSE Co-ordinated Sciences

This page is one topic of Cambridge IGCSE Co-ordinated Sciences 0654 / 0973. IGCSE Co-ordinated Sciences is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Co-ordinated Sciences syllabus page.