SP6 Radioactivity

SP6a Atomic Models

Particle theory/Kinetic theory - model that explains properties of solids, liquids and gases. Particles represented as spheres.
1897, J.J Thompson - found atoms have subatomic particles called electrons.
1900, J.J Thompson - made a model, 'plum pudding model', made of positively charged material with negatively charged electrons scattered.
1909 to 1913, Ernest Rutherford - 'gold foil experiment', he fired alpha particles at gold foil, expecting them to go through. Some bounced back, because of nucleus. He suggests that atoms are mostly space, most mass in nucleus with a positive charge and electrons moving around it.
Radius of nucleus - 1 x 10^-15m
Radius of atom - 1 x 10^-10m

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When atoms absorb energy, the electrons jump to higher levels. When they jump back down, they emit energy as electromagnetic radiation.
Bohr - suggests that electrons are in certain fixed objects, cannot be part-way between objects.
Ion - atom that has lost or gained electrons.
Ionising radiation - Radiation that causes electrons to escape.

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Background radiation - ionising radiation that is all around us.
Radon gas - radioactive gas produced by rocks that contain small amounts of uranium. Radon diffuses into the air and builds up in houses.
Food and Water - they naturally contain small amounts of radioactive substance.
Hospital - x-rays, gamma-ray scans and cancer treatments.
Cosmic-rays - high-energy, charged particles stream out of the sun and other stars. It is stopped in the upper atmosphere.
Photographic film - becomes darker as radiation reaches it. Film has developed to measure the amount of radiation.
Geiger-Muller tube - Radiation passing through the tube ionises gas inside it, allowing a short pulse of current. it connected to a counter, to count the pulses of current.

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The nucleus of a radioactive substance is unstable, easily change/decay. When it decays, radiation is emitted, causing nucleus to lose energy and become stable - random process.
Alpha particles(α or ⁴₂He) - 2 protons and 2 neutrons, like helium atom, charge +2. Travels few centimeters in air, very ionising and can be stopped by paper.
Beta particles (β- or ⁰_-1e) - high-energy, high-speed electrons, with mass of 1/1835, charge -1. Travels few meters in air, moderately ionising, stopped by 3mm thick aluminium.
Positrons (β⁺) - high-energy, high-speed particles, with mass of 1/1835 but charge of +1.
Gamma Rays (γ) - high-frequency electromagnetic waves (travel speed of light), no charge. Travels few kilometres in air, weakly ionising, stopped by thick lead.
Penetrate - pass through materials
As alpha particles are emitted at high speed and their high mass, good at transferring energy, good at ionising atoms. As they ionise, they lose energy. They produce many ions in short distance, they lose energy and have a short penetration distance.

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Whe unstable nucleus changes, atomic number changes. If happens, atom becomes different element.
Alpha particle - mass number goes down by 4, atomic number goes down by 2.
Beta (minus) - neutron changes into a proton and electron. Electron ejected, so atomic number increases by 1, no change to mass.
Beta (plus) - proton becomes neutron and a positron. Atomic number goes down by 1 but mass number doesn't change.
Neutron - mass number goes down by 1 but atomic number stays same.
Gamma radiation - when subatomic particles re-arrange, gamma ray ejected to be stable.

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Activity - number of nuclear decays per second, becquerels (Bq).
Radioactive decay - random process, cannot predict when it will happen.
Half life - time taken for half the unstable nuclei in a sample of radioactive isotope to decay.
Half life allows us to predct the activity of a large number of nuclei.
After decays, nucleus become stable. More stable the nucleus, lower its activity. The half-life of an isotope is also a meaure of how long it takes for activity to halve.

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