Atomic Structure

What is an atom made of?

A tiny central nucleus containing protons + neutrons, surrounded by electrons in energy levels.

What is the approximate radius of an atom?

About 1 x 10^-10 m.

What is the approximate radius of an atomic nucleus?

Less than 1/10,000 of the radius of an atom.

Where is most of an atom's mass?

In the nucleus.

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 - about 1/2000.

Why is an atom electrically neutral?

It has equal numbers of protons + electrons - charges cancel.

What is atomic number?

Number of protons in the nucleus.

What determines which element an atom is?

Number of protons - its atomic number.

What is mass number?

Total number of protons + neutrons in the nucleus.

How do you calculate the number of neutrons?

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 are isotopes still the same element?

They have the same number of protons.

How does an atom become a positive ion?

Loses one or more electrons.

How does an atom become a negative ion?

Gains one or more electrons.

Can the number of protons change when an ordinary ion forms?

No - only electrons are gained or lost.

What was the earliest model of the atom?

Atoms were thought to be tiny solid spheres that could not be divided.

What did discovery of the electron lead to?

Plum pudding model.

What was the plum pudding model?

Sphere of positive charge with negative electrons embedded throughout it.

What experiment challenged the plum pudding model?

Rutherford alpha-particle scattering experiment.

What happened to most alpha particles in Rutherford's experiment?

Passed straight through the thin gold foil.

What happened to some alpha particles in Rutherford's experiment?

Deflected through small angles.

What happened to a very small number of alpha particles?

Deflected backwards through large angles.

What did most alpha particles passing straight through show?

Most of the atom is empty space.

What did some alpha particles being deflected show?

Positive charge is concentrated in a small central nucleus.

What did the rare large deflections show?

The nucleus is very small, dense + contains most of the atom's mass.

What model replaced the plum pudding model?

Nuclear model - tiny positive nucleus surrounded by electrons.

How did Bohr improve the nuclear model?

Suggested electrons orbit the nucleus at specific distances/energy levels.

What did later experiments show about positive charge in the nucleus?

It is made of protons.

What did James Chadwick discover?

The neutron - explaining additional nuclear mass.

What is radioactive decay?

Unstable nucleus emits radiation to become more stable.

Is radioactive decay predictable for an individual nucleus?

No - radioactive decay is random.

What happens to the activity of a radioactive source over time?

It decreases as unstable nuclei decay.

What is activity?

Rate at which unstable nuclei decay.

What is the unit of activity?

Becquerel (Bq).

What does 1 Bq mean?

1 nuclear decay per second.

What are the three main types of nuclear radiation?

Alpha, beta + gamma.

What is an alpha particle?

Two protons + two neutrons - same as a helium nucleus.

What is a beta particle in beta-minus decay?

A high-speed electron emitted from the nucleus.

What is gamma radiation?

Electromagnetic radiation emitted from the nucleus - no mass + no charge.

What happens in alpha decay?

Nucleus loses 2 protons + 2 neutrons - mass number decreases by 4 and atomic number decreases by 2.

What happens in beta-minus decay?

A neutron changes into a proton + electron - electron is emitted.

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

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

What happens to mass and atomic numbers during gamma emission?

Neither changes - gamma has no mass or charge.

How is an alpha particle written in a nuclear equation?

Mass number 4, atomic number 2 - ⁴₂He.

How is a beta-minus particle written in a nuclear equation?

Mass number 0, atomic number -1 - ⁰₋₁e.

What must be conserved in a nuclear equation?

Total mass number + total atomic number on both sides.

Which radiation is most ionising?

Alpha.

Which radiation is least ionising?

Gamma.

Which radiation is least penetrating?

Alpha - stopped by paper/skin and travels only a few cm in air.

How penetrating is beta radiation?

Moderately penetrating - passes through paper but is stopped by a few mm of aluminium.

Which radiation is most penetrating?

Gamma - reduced by thick lead or concrete.

How do alpha, beta and gamma compare for ionisation?

Alpha - strongest. Beta - medium. Gamma - weakest.

How do alpha, beta and gamma compare for penetration?

Alpha - lowest. Beta - medium. Gamma - highest.

What is half-life?

Time taken for the number of undecayed nuclei, or activity/count rate, to fall to half its initial value.

What happens after one half-life?

1/2 remains.

What happens after two half-lives?

1/4 remains.

What happens after three half-lives?

1/8 remains.

How can remaining activity after several half-lives be calculated?

Keep halving the original activity once for each half-life.

How do you find the number of half-lives elapsed?

Number of half-lives = total time / half-life.

How can half-life be determined from a graph?

Choose an activity/count - find time for it to fall to half - repeat using another section to check.

Does radioactive activity ever suddenly become zero after a fixed number of half-lives?

No - it keeps decreasing statistically.

What is background radiation?

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

What are natural sources of background radiation?

Radon gas from rocks/soil + cosmic rays from space.

What are artificial sources of background radiation?

Medical radiation + small contributions from nuclear industry/weapons testing.

Why does background radiation vary with location?

Geology changes radon levels + altitude changes exposure to cosmic rays.

What is irradiation?

Exposing an object/person to radiation from an external source.

Does irradiation make an object radioactive?

No.

What is radioactive contamination?

Unwanted radioactive material gets on or inside an object/person.

Why can contamination be especially dangerous?

Radioactive material may remain close to or inside the body and continue irradiating tissue.

What is the key difference between contamination and irradiation?

Contamination - radioactive material is present. Irradiation - exposure to radiation without radioactive material being transferred.

How can ionising radiation damage living cells?

Ionises atoms/molecules - can damage or kill cells and cause DNA mutations.

What can DNA mutations caused by radiation lead to?

Cancer.

What determines the hazard from a radioactive source?

Type of radiation, dose/exposure + half-life.

Why can alpha radiation be especially dangerous inside the body?

Highly ionising - deposits lots of energy over a short distance in tissue.

Why is alpha usually less dangerous outside the body?

Low penetration - stopped by skin.

Why can gamma be hazardous outside the body?

Highly penetrating - can reach internal tissues.

What properties should a radioactive medical tracer have?

Gamma emitter + short enough half-life to reduce long-term exposure.

Why is gamma suitable for medical tracers?

Can pass out of the body and be detected externally + is relatively weakly ionising.

How can radioactive tracers be used?

Introduced into body - movement/accumulation detected using radiation outside body.

How is radiation used in radiotherapy?

High doses are targeted at cancer cells to kill them.

Why must benefits and risks of medical radiation be compared?

Radiation can diagnose/treat disease but ionising radiation can damage healthy tissue and increase cancer risk.

Why is peer review important when evaluating radiation risks?

Other scientists assess methods, evidence + conclusions before findings are accepted.

What is nuclear fission?

Splitting of a large unstable nucleus into two smaller nuclei, releasing energy + usually neutrons. (Physics only)

Which nuclei are commonly used for induced fission?

Uranium-235 + plutonium-239. (Physics only)

How can nuclear fission be induced?

Nucleus absorbs a neutron - becomes unstable - splits into two smaller nuclei. (Physics only)

What is released during nuclear fission?

Two smaller nuclei + 2 or 3 neutrons + energy. (Physics only)

What is a nuclear chain reaction?

Neutrons released by one fission cause further nuclei to undergo fission. (Physics only)

Why can a fission chain reaction continue?

Each fission releases neutrons that can trigger more fissions. (Physics only)

Where does the energy released in fission come from?

Products have slightly less total mass - some mass is converted to energy. (Physics only)

What is nuclear fusion?

Two light nuclei join to form a heavier nucleus, releasing energy. (Physics only)

Where does nuclear fusion occur naturally?

Stars.

Why does fusion require extremely high temperatures?

Positive nuclei strongly repel - high temperature gives them enough kinetic energy to get close enough to fuse. (Physics only)

Why does nuclear fusion release energy?

The new nucleus has slightly less mass than the original nuclei combined - some mass is converted to energy. (Physics only)

What is the key difference between fission and fusion?

Fission - large nucleus splits. Fusion - small nuclei join. Both can release energy.