topic 4 - atomic structure

What is an atom made of?

A central nucleus containing protons and neutrons, surrounded by electrons.

What is the relative charge of a proton?

+1.

What is the relative charge of a neutron?

0.

What is the relative charge of an electron?

−1.

What is the relative mass of a proton?

1.

What is the relative mass of a neutron?

1.

What is the relative mass of an electron?

Very small, approximately 1/1836 of a proton.

Where is most of an atom's mass concentrated?

In the nucleus, because it contains the protons and neutrons.

What is the atomic number?

The number of protons in an atom's nucleus.

What is the mass number?

The total number of protons and neutrons in the nucleus.

How do you calculate the number of neutrons in an atom?

Number of neutrons = mass number − atomic number.

What is an isotope?

Atoms of the same element with the same number of protons but different numbers of neutrons.

Why do isotopes of an element have the same chemical properties?

They have the same number and arrangement of electrons.

What is the difference between an atom and an ion?

An atom is electrically neutral; an ion has gained or lost electrons and has an overall charge.

What is the nuclear model of the atom?

A model in which the atom has a tiny, positively charged nucleus containing most of its mass, with electrons around it.

What did Rutherford's alpha-scattering experiment show?

Most alpha particles passed straight through thin gold foil, but a few were deflected greatly, showing that atoms are mostly empty space with a tiny, dense, positively charged nucleus.

Why did most alpha particles pass straight through the gold foil?

Most of the atom is empty space.

Why were a few alpha particles deflected by large angles?

They came close to the positively charged nucleus and were repelled.

What did scientists discover about the atom's structure over time?

The model developed from a solid sphere to the nuclear model, then to a model with electrons in shells, and the discovery of neutrons helped explain nuclear mass.

What is radioactive decay?

The random, spontaneous emission of radiation from an unstable nucleus.

What does spontaneous mean in radioactive decay?

The decay happens by itself without being triggered by an external cause.

What does random mean in radioactive decay?

It is impossible to predict exactly when a particular unstable nucleus will decay.

What is an unstable nucleus?

A nucleus that can change by emitting radiation.

What are the three main types of nuclear radiation?

Alpha (α), beta (β) and gamma (γ).

What is an alpha particle?

A particle consisting of two protons and two neutrons, identical to a helium nucleus.

What is the charge of an alpha particle?

+2.

What is the relative mass of an alpha particle?

4.

What is the ionising ability of alpha radiation?

Very strongly ionising.

What is the penetrating ability of alpha radiation?

Low; it is stopped by paper or the outer layer of skin and travels only a few centimetres in air.

What is a beta particle in GCSE nuclear physics?

A fast-moving electron emitted from the nucleus when a neutron changes into a proton.

What is the charge of a beta-minus particle?

−1.

What is the relative mass of a beta particle?

Very small, approximately 1/1836.

What is the ionising ability of beta radiation?

Moderately ionising; less ionising than alpha but more than gamma.

What is the penetrating ability of beta radiation?

Moderate; it can pass through paper but is stopped by a few millimetres of aluminium.

What is gamma radiation?

High-frequency electromagnetic radiation emitted by an unstable nucleus.

What is the charge of gamma radiation?

0.

What is the mass of gamma radiation?

0; gamma radiation has no rest mass.

What is the ionising ability of gamma radiation?

Weakly ionising compared with alpha and beta.

What is the penetrating ability of gamma radiation?

Very high; thick lead or concrete reduces its intensity, but it is not always completely stopped.

Which type of radiation is the most ionising?

Alpha.

Which type of radiation is the most penetrating?

Gamma.

Which type of radiation is deflected most by electric or magnetic fields?

Beta is deflected strongly because it is light and charged; alpha is deflected less in the opposite direction, while gamma is not deflected.

Why is gamma radiation not deflected by electric or magnetic fields?

It has no charge.

What is ionisation?

The process by which an atom or molecule gains or loses electrons, forming an ion.

Why can ionising radiation damage living cells?

It can ionise atoms and molecules in cells, damaging molecules such as DNA and potentially causing mutations or cancer.

What is nuclear decay notation used for?

It shows how the numbers of protons and nucleons change during radioactive decay.

What happens to the mass number and atomic number during alpha decay?

Mass number decreases by 4 and atomic number decreases by 2.

What happens to the mass number and atomic number during beta-minus decay?

Mass number stays the same and atomic number increases by 1.

What happens to the mass number and atomic number during gamma emission?

Neither changes; the nucleus loses energy.

What happens to the nucleus during alpha decay?

It emits two protons and two neutrons, so it becomes a different element with atomic number two lower.

What happens to the nucleus during beta-minus decay?

A neutron changes into a proton and an electron; the electron is emitted, so atomic number increases by 1.

What happens during gamma emission?

An excited nucleus releases energy as a gamma photon without changing its proton or neutron numbers.

How do you complete a nuclear equation?

Conserve the total mass numbers and total atomic numbers on both sides of the equation.

What is the symbol for an alpha particle?

⁴₂He or α.

What is the symbol for a beta-minus particle?

⁰₋₁e or β⁻.

What is the symbol for gamma radiation?

⁰₀γ.

What is half-life?

The time taken for the number of undecayed radioactive nuclei, or the activity of a radioactive source, to halve.

Why is half-life described as random?

You cannot predict when an individual nucleus will decay, but the behaviour of a large sample can be predicted statistically.

Does radioactive decay depend on temperature, pressure or chemical state?

No; radioactive decay is not affected by ordinary physical conditions or chemical reactions.

How do you calculate the remaining activity after one half-life?

Halve the initial activity.

How do you calculate the remaining activity after two half-lives?

Divide the initial activity by 4.

How do you calculate the remaining activity after three half-lives?

Divide the initial activity by 8.

What fraction of a radioactive sample remains after n half-lives?

(1/2)ⁿ of the original undecayed nuclei or activity, assuming background radiation has been accounted for.

How can half-life be found from a decay graph?

Find the time taken for the activity or number of undecayed nuclei to fall to half a chosen value.

What is activity?

The number of radioactive decays per second, measured in becquerels (Bq).

What is one becquerel?

One radioactive decay per second.

What is count rate?

The number of counts recorded by a detector per second or per minute.

Why might a detector's count rate differ from the source's activity?

The detector does not detect every decay, and background radiation also contributes to the count rate.

What is background radiation?

Low-level ionising radiation that is always present in the environment.

What are sources of background radiation?

Radon gas from rocks and soil, cosmic rays, radioactive rocks and building materials, and small amounts of radioactive substances in food and living things.

How can background radiation affect a count-rate experiment?

It adds to the measured count rate even when the source is not present.

How do you correct for background radiation in an experiment?

Measure background count rate separately and subtract it from the count rate measured with the source.

What is radioactive contamination?

Unwanted radioactive material getting onto or into an object or person.

What is irradiation?

Exposure to radiation from a source without radioactive material necessarily being transferred.

What is the difference between contamination and irradiation?

Contamination means radioactive material is present on or inside something; irradiation means it has been exposed to radiation.

Can an object become radioactive simply because it has been irradiated?

Usually not; irradiation does not normally make an object radioactive, although certain nuclear reactions can induce radioactivity.

Why can contamination be particularly dangerous?

Radioactive material may continue emitting radiation while it remains on or inside the body; internal contamination can be especially hazardous.

How can exposure to radioactive sources be reduced?

Minimise exposure time, maximise distance and use suitable shielding.

Why does increasing distance from a radioactive source reduce exposure?

Radiation spreads out, so intensity generally decreases as distance increases.

What is the inverse-square relationship for a point source of radiation?

Intensity is proportional to 1 ÷ distance², where the inverse-square law applies.

How can alpha radiation be made safer to handle?

Keep the source contained and avoid inhalation or ingestion; alpha is stopped by paper or skin externally but can be dangerous inside the body.

What shielding is used for beta radiation?

A few millimetres of aluminium or another suitable material.

What shielding is used for gamma radiation?

Thick lead or concrete to reduce its intensity.

What are the medical uses of radiation?

Radioactive tracers can help diagnose conditions, and gamma radiation can be used in radiotherapy to destroy cancer cells.

Why are gamma emitters useful for medical tracers?

Gamma radiation can pass out of the body and be detected externally, while a suitable short half-life limits how long the radioactive material remains active.

Why must medical tracers have a suitable half-life?

They need to remain detectable long enough for the test but should decay quickly enough to limit the patient's radiation dose.

How can radiation be used to sterilise medical equipment?

Ionising radiation kills microorganisms on equipment without needing high temperatures.

How can radiation be used in industry?

It can be used to monitor the thickness of materials and to detect faults in metal structures.

How does beta radiation help monitor the thickness of paper?

A detector measures radiation passing through the paper; changes in count rate indicate changes in thickness.

What happens if paper becomes too thick in a beta thickness monitor?

Less beta radiation reaches the detector, so the count rate decreases; the system can be used to adjust production.

What are the risks of using ionising radiation?

It can damage cells and DNA, increasing the risk of mutations and cancer; high doses can cause serious tissue damage.

Why can radioactive sources be useful despite their risks?

They have important medical and industrial uses, and risks can be reduced through controlled use, shielding, distance and limited exposure time.

What is nuclear fission?

The splitting of a large unstable nucleus, usually after absorbing a neutron, into two smaller nuclei and releasing energy and more neutrons.

What is a chain reaction in nuclear fission?

Neutrons released by one fission event cause further fission events, releasing more neutrons.

How is a nuclear fission chain reaction controlled in a nuclear reactor?

Control rods absorb neutrons to regulate the rate of fission; a moderator slows neutrons in reactors that use one.

What is nuclear fusion?

The joining of two light nuclei to form a heavier nucleus, releasing energy.

Where does nuclear fusion occur naturally?

In stars, including the Sun.

Why are high temperatures needed for nuclear fusion?

The nuclei need enough energy to overcome electrostatic repulsion between their positive charges.

What is the difference between nuclear fission and nuclear fusion?

Fission splits a large nucleus; fusion joins small nuclei together.

What should you remember when comparing alpha, beta and gamma radiation?

Compare their charge, relative mass, ionising ability, penetrating ability and shielding.