P4: Atomic Structure

What is the approximate radius of an atom?

About 1 × 10⁻¹⁰ metres.

Describe the basic structure of an atom.

A positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons.

How does the radius of a nucleus compare to the radius of an atom?

The nucleus radius is less than 1/10 000 of the atom's radius.

Where is most of the mass of an atom concentrated?

In the nucleus.

How are electrons arranged in an atom?

At different distances (energy levels) from the nucleus.

What happens when an electron absorbs electromagnetic radiation?

It moves to a higher energy level (further from the nucleus).

What happens when an electron emits electromagnetic radiation?

It moves to a lower energy level (closer to the nucleus).

What is the atomic number of an element?

The number of protons in the nucleus of an atom of that element.

What is the mass number of an atom?

The total number of protons and neutrons in the nucleus.

In a neutral atom, how does the number of electrons compare to the number of protons?

They are equal, giving the atom no overall charge.

What are isotopes?

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

How is an ion formed from an atom?

An atom becomes a positive ion by losing one or more outer electrons.

What did scientists think atoms were like before the discovery of the electron?

Tiny indivisible spheres.

What is the plum pudding model of the atom?

Atoms are balls of positive charge with negative electrons embedded throughout (like raisins in a pudding).

What discovery led to the plum pudding model?

The discovery of the electron.

What experiment replaced the plum pudding model?

The alpha particle scattering experiment (Geiger-Marsden experiment).

What were the results of the alpha particle scattering experiment?

Most alpha particles passed straight through, some were deflected slightly, and a very small number bounced straight back.

What conclusions were drawn from the alpha scattering experiment?

The atom is mostly empty space, and its mass and positive charge are concentrated in a tiny central nucleus.

What did Niels Bohr contribute to the atomic model?

He suggested electrons orbit the nucleus at specific distances (energy levels); his calculations agreed with experimental observations.

What evidence led to the discovery of the proton?

Later experiments showed the positive charge of any nucleus could be subdivided into whole-number units of the same positive charge, each called a proton.

Who provided evidence for the neutron, and how long after nuclear model was accepted?

James Chadwick - about 20 years after the nucleus became an accepted scientific idea.

Why did the plum pudding model need to be replaced?

New evidence from the alpha scattering experiment could not be explained by the plum pudding model; a new nuclear model was required.

What is radioactive decay?

The random process by which an unstable nucleus emits radiation to become more stable.

What is activity?

The rate at which a source of unstable nuclei decays, measured in becquerels (Bq).

What is count-rate?

The number of decays recorded each second by a detector such as a Geiger-Muller tube.

What is an alpha particle?

Two protons and two neutrons (the same as a helium nucleus).

What is a beta particle?

A high-speed electron ejected from the nucleus as a neutron turns into a proton.

What is a gamma ray?

Electromagnetic radiation emitted from the nucleus.

Which type of radiation has the greatest ionising power?

Alpha radiation.

Which type of radiation has the greatest penetrating power?

Gamma radiation.

What stops alpha radiation?

A few centimetres of air or a thin sheet of paper.

What stops beta radiation?

A few millimetres of aluminium.

What significantly reduces gamma radiation?

Several centimetres of lead or metres of concrete.

What is the range of alpha particles in air?

A few centimetres.

In a nuclear equation for alpha decay, how do mass number and atomic number change?

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

In a nuclear equation for beta decay, how do mass number and atomic number change?

Mass number stays the same; atomic number increases by 1 (a neutron becomes a proton).

In gamma emission, how do mass number and atomic number change?

Neither changes - no particles are emitted.

What is the half-life of a radioactive isotope?

The time for the number of nuclei of that isotope to halve, or for the count rate/activity to fall to half its initial value.

Is radioactive decay a predictable or random process?

Random - you cannot predict when any individual nucleus will decay.

A source has an initial count rate of 800 Bq. After 3 half-lives, what is the count rate?

800 → 400 → 200 → 100 Bq. The count rate is 100 Bq.

What is radioactive contamination?

The unwanted presence of radioactive material on or in other materials; hazardous because the contaminating atoms continue to decay.

What is irradiation?

Exposing an object to nuclear radiation; the irradiated object does not itself become radioactive.

Why is contamination considered more hazardous than irradiation in many situations?

Contaminating material stays in contact with (or inside) the body, continuously emitting radiation; irradiation stops when the source is removed.

Why should findings of studies into radiation effects on humans be published and peer-reviewed?

So other scientists can check the results, identify errors, and build a reliable scientific consensus.

Give two natural sources of background radiation.

Rocks (e.g. granite/radon gas) and cosmic rays from space.

Give two man-made sources of background radiation.

Fallout from nuclear weapons testing and nuclear accidents.

What factors can affect a person's radiation dose?

Occupation (e.g. nuclear industry workers) and location (e.g. living in a granite area or at high altitude).

What unit is radiation dose measured in?

Sieverts (Sv); often expressed as millisieverts (mSv).

Why does the hazard of a radioactive material depend on its half-life?

A long half-life means the material remains radioactive for longer; a short half-life means intense radiation for a shorter time.

Give two medical uses of nuclear radiation.

Exploration of internal organs (e.g. tracers) and control or destruction of unwanted tissue (e.g. radiotherapy for cancer).

Why must the source used for medical tracers have a short half-life?

To minimise the patient's radiation dose after the investigation is complete.

Why is gamma used for medical tracers rather than alpha?

Gamma penetrates tissue and can be detected outside the body; alpha would not escape the body and would cause more ionisation damage.

What is nuclear fission?

The splitting of a large, unstable nucleus (e.g. uranium-235 or plutonium-239) into two smaller nuclei, releasing energy and neutrons.

What must happen for fission to be induced?

The unstable nucleus must first absorb a neutron.

What particles are released during fission?

Two or three neutrons and gamma rays; all products have kinetic energy.

What is a chain reaction?

Neutrons released by fission trigger further fissions in other nuclei, releasing more neutrons and sustaining the process.

How is a chain reaction controlled in a nuclear reactor?

Control rods absorb excess neutrons to keep the rate of fission steady.

What is the difference between a nuclear reactor and a nuclear weapon?

In a reactor the chain reaction is controlled; in a nuclear weapon it is uncontrolled.

What is nuclear fusion?

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

What happens to some mass during nuclear fusion?

Some mass is converted into energy (radiation).

Why is nuclear fusion difficult to achieve on Earth?

The nuclei must be brought close enough to fuse, requiring extremely high temperatures and pressures to overcome electrostatic repulsion.