Definitions (mark-scheme wording)EquationsExam trapsExtended = Extended tier only
5.1aThe atom
Definitions
Atom
a small, dense, positively charged nucleus surrounded by negatively charged electrons in orbit.
Positive ion
an atom that has lost one or more electrons.
Negative ion
an atom that has gained one or more electrons.
Key points
The nucleus contains protons and neutrons and almost all the mass; electrons orbit far outside it.
Most of the atom is empty space; the nucleus is about 10-15 m across, the atom about 10-10 m.
A neutral atom has equal numbers of protons and electrons.
Extended in alpha scattering, most alpha particles passed straight through gold foil: the atom is mostly empty space.
Extendeda few were deflected: the nucleus is positively charged (repels alpha).
Extendedvery few bounced back: the nucleus is very small and contains most of the mass.
Lithium-7: 3 protons and 4 neutrons in the nucleus, 3 electrons in orbit.
Extended: alpha scattering. Most pass straight through, a few are deflected, very few bounce back.
Watch out!
Extended: bouncing back shows the nucleus is small and massive; deflection shows it is positive. Match each result to its conclusion.
An ion forms by losing or gaining electrons, never protons.
5.1bThe nucleus and isotopes
Definitions
Proton number (atomic number) Z
the number of protons in the nucleus.
Nucleon number (mass number) A
the total number of protons and neutrons in the nucleus.
Nucleon
a proton or a neutron.
Nuclide
a particular type of nucleus with a given number of protons and neutrons, written AZX.
Isotopes
atoms of the same element with the same number of protons but different numbers of neutrons.
Key points
Relative charges: proton +1, neutron 0, electron -1.
Z fixes the element.
Extended the relative charge on a nucleus equals its proton number, +Z.
Extended the relative mass of a nucleus equals its nucleon number A (proton and neutron each 1; electron about 1/1836, negligible).
Isotopes have the same chemistry (same electrons) but different mass; some are radioactive.
Nuclide notation: nucleon number A on top, proton number Z at the bottom.
Relative charge and mass of the particles in an atom.
Isotopes of carbon: same proton number (6), different neutron numbers (6 and 8).
Watch out!
A counts protons and neutrons: carbon-14 has 14 - 6 = 8 neutrons, not 14.
Isotopes differ in neutrons, not protons (a different Z is a different element).
5.1cNuclear fission and fusion Extended
Definitions
Nuclear fission
Extended the splitting of a large, unstable nucleus (e.g. uranium-235) into two smaller nuclei and two or three neutrons, releasing energy.
Nuclear fusion
Extended the joining of two light nuclei (e.g. hydrogen isotopes) to form a heavier nucleus, releasing energy.
Chain reaction
Beyond the syllabus: neutrons released by one fission cause further fissions.
Key points
Extended fission is started when a nucleus absorbs a neutron; used in nuclear power stations.
Extended fusion powers the Sun and other stars; it needs very high temperature and pressure.
Extended both release energy because the total mass of the products is slightly less than that of the reactants.
Extended nucleon number and proton number are conserved in the nuclide equation (top and bottom rows balance).
Extended: fission. A neutron splits U-235 into two smaller nuclei plus 2 or 3 neutrons, which can cause more fissions.
Extended: fusion. Two light nuclei join to form helium, releasing energy (the Sun).
Watch out!
Extended: fission splits heavy nuclei; fusion joins light nuclei. Do not swap the words.
Extended: check nuclide equations: the top numbers and the bottom numbers must each add up on both sides.
5.2aDetection of radioactivity
Definitions
Background radiation
ionising radiation from natural and artificial sources that is always present around us.
Count rate
the number of decays detected per unit time (counts/s or counts/minute).
Corrected count rate
Extended the measured count rate minus the background count rate.
Key points
Natural sources: radon gas (largest), rocks and buildings, cosmic rays, food and drink.
Beyond the syllabus: artificial sources: medical X-rays and scans, nuclear power and weapons testing.
Detect ionising radiation with a Geiger-Müller (GM) tube connected to a counter or ratemeter.
Extended measure the background count with no source present first, over a long time.
Extended always subtract the background from every reading before analysing a source (e.g. finding half-life).
A GM tube and counter record the counts in a measured time; repeat with no source to find the background.
Watch out!
Extended: forgetting to subtract the background before halving the count rate for half-life.
Background radiation is always present; a zero reading means the detector is off.
5.2bThe three types of nuclear emission
Definitions
Alpha particle (α)
a helium nucleus: two protons and two neutrons (charge +2).
Beta particle (β)
a fast-moving electron emitted from the nucleus (charge -1).
Gamma ray (γ)
high-frequency electromagnetic radiation emitted from the nucleus (no charge, no mass).
Ionising radiation
radiation that knocks electrons out of atoms, turning them into ions.
Radioactive emission
spontaneous and random (in time and in direction): it cannot be predicted when a particular nucleus will decay, and is not affected by temperature or chemistry.
Penetration: alpha stopped by paper or a few cm of air; beta by a few mm of aluminium; gamma only reduced by thick lead or concrete.
The more ionising the radiation, the less penetrating it is (it loses energy faster).
Extended in electric and magnetic fields alpha and beta deflect in opposite directions (opposite charges).
Extended beta deflects more than alpha (much smaller mass); gamma is not deflected (uncharged).
Extended alpha is most ionising: it has a large charge (+2) and a large kinetic energy, which it loses quickly in many ionising collisions; gamma (uncharged) ionises least.
Penetration: alpha stopped by paper, beta by a few mm of aluminium, gamma only reduced by thick lead.
Extended: between charged plates alpha bends slightly towards -, beta bends strongly towards +, gamma goes straight.
Summary of the three emissions.
Watch out!
Gamma is reduced by thick lead, never completely stopped.
Beta is an electron from the nucleus, not one of the orbiting electrons.
5.2cRadioactive decay
Definitions
Radioactive decay
a spontaneous and random change in an unstable nucleus that can result in the emission of α-particles or β-particles and/or γ-radiation.
Beta decay
Extended a neutron in the nucleus changes into a proton and an electron; the electron is emitted.
Key points
Alpha and beta decay change the nucleus into a different element (Z changes).
Extended unstable nuclei decay to become more stable.
Extended alpha decay: A falls by 4, Z falls by 2.
Extended beta decay: A unchanged, Z rises by 1; the number of excess neutrons falls.
Extended gamma emission: no change in A or Z; the nucleus just loses energy.
Extended isotopes with too many neutrons tend to decay by beta emission; very heavy nuclei by alpha.
Extended: decay equations. Top numbers and bottom numbers each balance.
Extended: decay moves a nucleus on a chart of A against Z: alpha 4 down and 2 left, beta 1 right, gamma not at all.
Watch out!
Extended: in beta decay the proton number goes up by 1 (a neutron becomes a proton); A stays the same.
Gamma emission does not change the element.
5.2dHalf-life
Definitions
Half-life
the time taken for half the nuclei of that isotope in any sample to decay.
Half-life (from activity)
the time taken for the count rate (activity) of a sample to fall to half its initial value.
Key points
Each half-life the number of undecayed nuclei (and the count rate) halves.
From a decay curve: read the time for the count rate to fall from any value to half that value.
The curve never reaches zero; after n half-lives (½)n of the sample remains.
Half-lives range from fractions of a second to billions of years; each isotope has a fixed value.
Extended subtract the background count rate from the data before finding the half-life.
Decay curve: 800 to 400 counts/min takes one half-life, 400 to 200 another of the same length.
Halving ladder: after 1, 2, 3, 4 half-lives, 1/2, 1/4, 1/8, 1/16 of the original remains.
Watch out!
After two half-lives a quarter remains, not none.
Read the half-life between any value and half of it, not just from the start.
5.2eUses and safety of radioactive sources
Definitions
Radioactive tracer
Extended a radioactive isotope put into a system so that its path can be followed by detecting its radiation.
Sterilisation
Extended killing bacteria on equipment or food using gamma radiation.
Key points
ExtendedSmoke alarms: alpha (ionises air, short range); long half-life so it lasts for years.
ExtendedThickness control: beta (partly absorbed by paper or foil); count rate changes with thickness.
ExtendedSterilising equipment, irradiating food to kill bacteria and treating cancer: gamma (penetrates, kills cells and bacteria).
ExtendedDiagnosing cancer (tracers): gamma emitter with a short half-life (hours), so it leaves the body quickly.
Beyond the syllabus: Dating: carbon-14 (half-life about 5700 years) in once-living material.
Dangers: ionising radiation causes cell death, mutations and cancer.
Safe handling: move sources with tongs, never point them at people, store them in lead-lined boxes.
Extended all safety precautions reduce exposure: shorten the time, increase the distance, use shielding to absorb the radiation.
Extended: thickness control: if the sheet gets thicker, fewer beta particles reach the detector and the rollers are adjusted.
Watch out!
Extended: tracers inside the body must be gamma (to escape) with a short half-life; alpha would be very harmful inside.
Extended: choose by both type (penetration) and half-life, and say why for each.