Physics - Atomic Structure

What is the approximate radius of an atom?

About 1 × 10⁻¹⁰ m.

How large is the nucleus compared with the atom?

Its radius is under 1/10,000 of the atomic radius.

What particles make up the nucleus?

Protons and neutrons.

What electric charges do protons, neutrons and electrons have?

Proton +1, neutron 0, electron −1.

What are their relative masses?

Proton ≈1, neutron ≈1, electron very small (≈1/1836).

What is the atomic number?

Number of protons in the nucleus.

What is the mass number?

Total number of protons and neutrons.

How do you calculate neutrons in an atom?

Mass number − atomic number.

What is an isotope?

Atoms of one element with the same protons but different neutrons.

Why is an atom electrically neutral?

It has equal numbers of protons and electrons.

How is a positive ion formed?

An atom loses one or more electrons.

What was Dalton's early atomic model?

Tiny indivisible solid spheres.

What was the plum-pudding model?

Electrons embedded within a sphere of positive charge.

What did Rutherford's alpha scattering experiment show?

Atoms are mostly empty space with a tiny dense positive nucleus.

Why did alpha scattering disprove the plum-pudding model?

Some particles were deflected strongly, inconsistent with spread-out charge.

What did Bohr add to the nuclear model?

Electrons occupy fixed shells or energy levels around the nucleus.

Who demonstrated the neutron's existence?

James Chadwick.

What evidence led to changes in atomic models?

New observations such as electrons and alpha-particle scattering results.

What is radioactive decay?

Random emission of nuclear radiation from an unstable nucleus.

What is activity?

Number of nuclear decays per second.

What is the SI unit of radioactive activity?

Becquerel (Bq), one decay per second.

What is count rate?

Detector-recorded counts per second, which may be below true activity.

What is an alpha particle?

Two protons and two neutrons: a helium nucleus.

What is a beta-minus particle?

A high-speed electron emitted when a neutron changes to a proton.

What is gamma radiation?

High-energy electromagnetic radiation emitted by the nucleus.

Which radiation has the greatest ionising power?

Alpha particles.

Which of alpha, beta and gamma is most penetrating?

Gamma radiation.

What stops alpha particles?

Paper, skin or a few centimetres of air.

What reduces or stops beta radiation?

A few millimetres of aluminium or similar material.

What is used to reduce gamma radiation?

Thick lead or concrete shielding.

How do mass and atomic numbers change in alpha decay?

Mass number −4 and atomic number −2.

How do mass and atomic numbers change in beta-minus decay?

Mass number unchanged; atomic number +1.

How do mass and atomic numbers change after gamma emission?

Neither changes.

Why must nuclear equations balance?

Total mass numbers and atomic numbers are conserved.

What is a neutron's nuclear symbol?

¹₀n.

What is the nuclear symbol of an alpha particle?

⁴₂He (or ⁴₂α).

What is the nuclear symbol of a beta-minus particle?

⁰₋₁e (or ⁰₋₁β).

Why is radioactive decay unpredictable for a particular nucleus?

Each decay event is random.

What is half-life?

Time for undecayed nuclei or activity/count rate to fall to half its value.

What fraction remains after two half-lives?

One-quarter (¼) of the original radioactive nuclei.

What fraction remains after four half-lives?

One-sixteenth (1/16) of the original radioactive nuclei.

What is the general remaining-fraction formula after n half-lives?

Remaining fraction = (½)ⁿ.

What is radioactive contamination?

Unwanted radioactive material on or inside another substance.

What is irradiation?

Exposure to radiation from a source; it does not normally make objects radioactive.

Why can contamination be hazardous after the source moves away?

Radioactive atoms remain in/on the material and continue decaying.

How can irradiation risk be reduced?

Limit exposure time, increase distance and use shielding.

How does a long half-life affect a radioactive source?

Activity tends to be lower per nucleus but hazard may persist for years.

How does a short half-life affect a radioactive source?

It decays quickly, often with high initial activity for a given number of nuclei.

What causes natural background radiation?

Radioactivity in rocks/soil and cosmic rays; radon is a major source.

What causes artificial background radiation?

Medical exposure and small amounts from industrial or fallout sources.

Why do background radiation levels differ by location?

Rock composition, altitude and radon exposure vary.

How are radioactive tracers used in medicine?

Detect emitted radiation from a tracer inside the body to monitor organ function.

Why is gamma radiation useful for external imaging?

It can escape the body and be detected externally.

How is radiation used in radiotherapy?

Delivered at controlled doses to damage or destroy tumour cells.

What trade-off must be considered in radiation treatments?

Medical benefit versus risk of damage to healthy tissue.

What is peer review?

Independent scientists evaluate a study's methods and findings.

What is nuclear fission?

Splitting of a heavy unstable nucleus into smaller nuclei, releasing energy.

What usually triggers fission in a nuclear reactor?

A neutron is absorbed by a fissile nucleus.

What are products of neutron-induced fission?

Two smaller nuclei, more neutrons, gamma radiation and energy.

What is a nuclear chain reaction?

Neutrons from one fission trigger additional fissions.

Why are control rods used in nuclear reactors?

Absorb neutrons to control the chain-reaction rate.

How does a nuclear reactor use fission energy?

Heats coolant/steam which drives turbines and generators.

What is nuclear fusion?

Two light nuclei join to produce a heavier nucleus and release energy.

Why is fusion difficult to maintain on Earth?

Positive nuclei repel; extremely high temperatures are needed.

How does fusion differ from fission?

Fusion joins light nuclei; fission splits heavy nuclei.

Where does naturally sustained nuclear fusion occur?

In stars, including the Sun.