Atomic Models
Describe the plum-pudding model of the atom
Negative electrons embedded in a positively charged sphere
The charges of the subatomic particles are:
Electron =
Proton =
Neutron =
Electron = -1
Proton = +1
Neutron = 0
2 protons and 2 neutrons
Half-life is...
the time taken for activity to halve.
OR
the time taken for the number of unstable nuclei to halve.
Why is beta radiation not used in smoke detectors?
Beta particles are not stopped by smoke particles
tiny indivisible spheres.
Isotopes of an element have the same number of _________ but different numbers of __________
Isotopes of an element have the same number of protons but different numbers of neutrons
Radioactivity is when...
unstable nuclei emit ionising radiation
The activity of a sample decreases from 8Bq to 2Bq in 30hours
What is the half-life of the sample?
15 hours
How do doctors use radioactive sources to locate issues in the body.
Patient is injected with a gamma source. Source travels to relevant location.
Gamma waves leave the body and can be detected to show doctors where it is.
Who discovered the neutron?
James Chadwick
Zirconium has an atomic number of 40 and an atomic mass of 91.
How many protons/neutrons/electrons does an atom of Zr-91 have?
Protons = 40
Electrons = 40
Neutrons = 51
What stops each type of ionising radiation?
Alpha - 5cm air / paper / skin
Beta - 5mm Al / metres of air
Gamma - metres of concrete, Pb, km of air
A radioactive source with a half life of 2.5years has 136 000 unstable nuclei.
How many unstable nuclei will remain after 10 years?
8500
How does the half-life of a source used for testing paper thickness compared to the half-life of a source used to locate a blocked blood vessel?
Industrial source should have a long half-life so it does not need to be replaced often.
Medical source should have a half-life long enough to use but short so radioactive exposure is minimised
Tungsten-184 has 110 neutrons.
How many protons/neutrons does Tungsten-186 have?
74 protons
112 neutrons
?42Ca -> 21? ? + -10e
2042 Ca -> 2142 Sc + -10e
After 215ms a radioactive source has an activity of 5.4Bq.
The source has a half life of 43ms.
What was the source's original activity?
172.8Bq
Workers in nuclear power stations are monitored to check if they are contaminated with radioactive material.
A worker stands 1 cm away from a radiation detector. The amount of radiation the worker emits is recorded.
Explain why the worker needs to stand close to the radiation detector.
The worker could be contaminated with alpha particles.
Alpha particles have a very short range (5cm) in air
Explain how Rutherford's experiment changed the model of the atom.
Firing alpha particles at atom thin gold foil - most particles went straight through, some were deflected, few came straight back.
Nucleus is not a spread out positively charged sphere but dense at the centre of the atom.
Magnesium-24 and Magnesium-25 are isotopes of Mg.
Compare the atomic structure of these isotopes.
Mg-24 has 12 neutrons however Mg-25 has 13 neutrons.
When experimenting with an unknown radioactive source, the following results were obtained:
No barrier - 433 count/s
Paper - 182 count/s
10mm Aluminium - 183 count/s
What type of source do you think this is? Why?
Alpha and Gamma source
Alpha as count rate decreases when paper is used as a barrier
Not beta as count rate was not affected by aluminium
Gamma as count rate was not zero with aluminium
Carbon-14 has a half-life of 5700 years. In living organisms, the percentage of carbon-14 is approximately 0.8%.
An archaeological dig has uncovered a skeleton where 0.05% of the carbon is carbon-14.
How old is the skeleton?
22 800 years
If people drink water contaminated with iodine-131, the iodine-131 builds up in the thyroid gland. This continues until the thyroid is saturated with iodine-131 and cannot absorb any more. The radiation emitted from the iodine-131 could cause cancer of the thyroid.
People likely to be drinking water contaminated with iodine-131 were advised to take tablets containing a non-radioactive isotope of iodine.
Suggest why this advice was given.
limits amount of iodine-131 / radioactive iodine that can be absorbed
reducing risk of damage/cancer (of the thyroid)