CP6 - Radioactivity

Structure of an atoms

A nucleus of neutrons and positive protons surrounded by orbiting negative electrons.

Where is most of the mass of an atom found?

In the nucleus

Size of atoms and small molecules

Of the order 10 to the -10

Plum pudding model of the atom

A spherical ‘blob’ of positive matter containing small negative electrons.

Isotope

Atoms with the same proton number but a different number of neutrons.

How to distinguish between atoms?

Different atoms will have a different number of protons and electrons (unless they are ions).

Relative Charge and Relative mass of all the sub-atomic particles.

Protons - charge = 1, mass = 1

Neutrons - charge = 0, mass = 1

Electrons - charge = -1, mass = 1/1836

Why do all atoms have no overall charge?

They have the same number of positive protons as negative electrons.

Where are electrons found inside of atoms?

Electrons are found in distinct energy levels or shells.

When can electrons change energy levels (orbit)?

They can be raised to a higher energy level if they absorb a specific energy photon or moved to a lower energy level if they release a specific energy proton.

What is an ion?

An atom that has lost or gained electrons.

How does ionisation occur?

If an electron absorbs enough energy to be released from the atom.

What is some evidence for the Bohr model of the atom?

When we excite an atom only certain frequencies of light are released.

Background radiation

Background radiation is radiation that is around us all the time.

How do we correct radiation measurements with background radiation?

Take the recorded value and subtract the recorded background radiation measurement.

Sources of background radiation.

Radon Gas, medical equipment, the ground and buildings, cosmic rays, food and drink, and nuclear power.

How can photographic film be used to detect radioactivity?

Photographic film will change colour when radiation is incident on it.

How does a Geiger-muller tube work?

Inert had in a tune is ionised and the release ld electron is detected as a current.

Relative atomic mass and charge of a positron

Relative mass is 1/1836 and charge is +1.

Five types of radiation that can be released from a nucleus?

Alpha, beta -, beta +, gamma and neutron.

What are Alpha and Beta - particles?

Alpha is a helium nucleus (2 protons and 2 neutrons).

Beta - is an electron.

Gamma radiation

Gamma is a high energy electromagnetic wave.

Penetration and ionisation strength of Alpha, Beta and Gamma radiation.

Alpha - weak penetration strength, high ionisation.

Beta - medium penetration, medium/high ionising.

Gamma - very penetrating, low ionising.

How far can Alpha, Beta and Gamma rays penetrate?

Alpha - thin piece of paper

Beta - sheet aluminium

Gamma - thick lead

What happens in the nucleus during beta - decay?

A neutron turns into a proton and an electron.

What occurs in the nucleus during beta + decay?

A proton turns into a neutron and a positron.

What happens in the nucleus during alpha decay?

2 protons and 2 neutrons are ejected from the nucleus.

What happens to the nucleus after decay?

The nucleus rearranges and gamma radiation is emitted.

How does the activity of a substance change over time?

The activity will decrease.

Half life

The time it takes for the number of radioactive nuclei (activity) to decrease by half.

Hazards of ionising radiation

Can damage tissue and ionise atoms in cells, causing DNA mutations which could lead to cancer.

How do the dangers of ionising radiation depend on half life?

The shorter the half life of a radioactive source the lower the time a patient is exposed for, so there is less risk.

Precautions when handling radioisotopes.

Shielding is placed between source and handler, distance between source and handler is maximised and the time of exposure is minimised.

Contamination

When a radioactive substance gets on or inside another object.

Irradiation

When a object is exposed to radiation by an external source.

Why is contamination more dangerous than irradiation?

Contaminated objects will emit radiation themselves until the contamination is removed, irradiated objects will not.

What was Rutherford’s scattering experiment?

Alpha particles were fired at a thin piece of gold foil and were all expected to go through but:

Most went straight through: the atom is mostly empty space.

Some bounced back: Most of the mass must be in a small dense centre.

Some were deflected: The dense centre must contain the positive charge.