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physics topic 4: atomic structure - Coggle Diagram
physics topic 4: atomic structure
developing the model of the atom
JJ Tomspson in 1897 - plum pudding model. - a ball of positive charge with electrons embedded in it.
Rutherford and Marsden experiment
1905 bombardment of thin gold foil with alpha particles. if plum pudding model was correct the heavy positively charged alpha particles would have passed through the gold foil. not all did.
some alpha particles were deflected so they must have come close to a concentration of charge unlike the spread out charge in plum pudding model .
some reflected back meaning: - they must have been repelled by the same charge that the alpha particles carried. - the repelling charge must have been much heavier than the alpha article or else it would have passed through.
in conclusion the mass of the atom was concentrated in a central nucleus which is positively charged and the electrons surround this nucleus.
Neil's Bohr - adaptation that electrons orbit the nucleus at specific distances.
finally Chadwick carried out many experiments which led to the evidence of the neutron
radiation
unstable atomic nuclei are unstable and give out radiation to become more stable. a random process.
The activity of a radioactive source is the rate at which it decays. measured in becquerels. 1 Bq is equivalent to one decay a second.
count rate is the number of decays recorded each second by a Geiger muller tube and counter. 1bq = 1 count a second.
alpha beta gamma.
alpha: two neutrons and two protons (same as a helium nucleus) ejected from the nucleus. very likely to be absorbed and cause damage if passing through living cells. strong ionising power. absorbed by a few centimetres of air or a thin sheet of paper.
beta - a high speed electron. ejected from the nucleus as a neutron turns into a proton. likely to cause damage if absorbed by living cell and can penetrate the body to inner organs. medium ionising power. passes through air and paper but absorbed by a few millimetres of aluminium.
gamma - electromagnetic radiation. emitted from the nucleus. likely to pass through living cells without being absorbed and causing ionisation. in waves. poor ionising power. very penetrating and needs many centimetres of lead or many metres of concrete to absorb most of it.
contamination.
the unwanted presence of materials containing radioactive atoms on other materials. hazard of contamination due to decay of the contaminating ions.
irradiation: the process of exposing an object to nuclear radiation. can be deliberate or accidental. doesn't cause the object to become radioactive.
protect against unwanted irradiation by: - using sources of the lowest activity possible for the shortest amount of time possible
wearing appropriate protective clothing such as a lead apron.
using tongs to handle sources.
half-life.
half life of radioactive isotope is the: - average time for half of the nuclei to decay.
the time it takes for the count rate or activity of a sample containing the isotope to fall to 50% of its original value.
on a graph. to find half life take the counts per second. half it and go along the graph and down to find time.
half life known then activity of a source after a certain amount of time. 2 years and initial activity is 800Bq what is activity after 6 years.
2=0.5
4=0.75
8=0.125
final count rate = 1/8th x initial activity = 1/8 x 800 =100Bq
isotopes with a short half life: - very unstable, emit radiation quickly. exposure hazardous. don't remain radioactive for long. Isotopes with a long half life: - more stable and remain radioactive for a long time, emit radiation slowly so exposure is less hazardous.
nuclear equations
alpha particle is 4,2He beta particle 0,-1e.
when an alpha particle is emitted the mass number of the element is reduced by 4 and the atomic number is reduced by 2. e.g 219,86Radon -> 215,84Polonium + 4,2He
beta decay: mass number doesn't change, atomic number increased by 1. neutron turns into a proton and electron and electron is transmitted as the beta particle. e.g. 12,6Carbon -> 14,7Nitrogen + 0,-1e