Radioactivity 5 th Year Chemistry

Recap Henri Bacquerel – uranium and photographic plate Marie Curie – pitchblende, polonium and radium Define Radioactivity? Spontaneous disintegration of certain unstable nuclei accompanied by the emission of radiation

α-particles Positively charged Consist of two protons and two neutrons The He nuclei Amercium-241 used in smoke detectors is an α-emitter Stopped by a sheet of paper

Β-particles Formed by the disintegration of a neutron into a proton and electron. Proton stays in nucleus – mass no. stays the same Atomic number is increased by 1

Β-particles Electrons emitted in a fast moving stream Higher penetration than α-particles, 2- 3mm of aluminium foil Stopped by skin, harmful if they get inside the body Carbon-14 used for age determination

γ-rays Not a particle – radiation A Unstable nuclei can release photons of high energy electromagnetic radiation called γ-rays Fast moving and of the greatest penetrating power – stopped by thick lead or concrete Cobalt 60 – cancer treatment and food irradiation

Chemical reaction Vs Nuclear reaction Elements combine to form other compounds Compounds break up into their constituent elements Elements in compounds rearrange to form different compounds Cause changes in the nucleus involving protons and neutrons Cause elements to change into other elements

Chemical reactions

Nuclear Reaction

Radioisotopes What is an isotope? Atoms of the same element, which differ in the number of neutrons in the nucleus What is a radioisotope? Unstable radioactive isotopes

Half Life The time taken for half of the radioactive atoms present to decay Can vary from seconds to millions of years E.g. Radium-214 has a half life of 20 minutes, Radium-226 has a half life of 1,620 years Nuclear power waste – half life?

Half Life Graph

Background Radiation Source of radiation Percentage Radon Gas (α-emitter)60% Rocks & Soil (γ-rays)10% Food & Drink10% Cosmic rays10% Medical e.g. Cobalt-609% Weapons Tests0.2% Nuclear Waste<0.1%

Carbon Dating Used to determine the age of carbon containing objects When plant/animal is alive it will have C-12 and C-14 in ratio the same as air When organism dies ratio changes with time due to decay of C-14. Extent of the change allows age to be calculated

Cobalt- 60 Cancer Treatment Gamma ray emitter Half life 5.23 years Radiotherapy – treatment of cancer with radiation Cancerous cells more susceptible to damage by ionising radiation than normal cells Ionising radiation is directed on to the tumour from different directions Tumour dose is high but normal tissue receives a much lower, less harmful dose

Cobalt - 60 Gamma rays kill micro-organisms in food May also involve inhibiting sprouting, controlling ripening and pasteurising foods Fears of genetic modification

Americium-241 Emitted α-particles ionise the air molecules, conduct an electric current between two terminals Smoke clings to ionised air molecules and slows them down Current decreases and a transistor switch activates the alarm This type of alarm not used these days

Practice Nuclear reactions