Radioactivity Flashcards
What is the earliest theory of the atomic structure?
In 1897, j.j Thomson suggested ‘the plum pudding model’. It means that atoms were spheres of positive charge with tiny negative electrons stuck in them.
What was the next version?
In 1909, Rutherford and Marsden fired a beam of alpha particles at a thin gold foil and they discovered that most of the mass was in the centre in a tiny molecule also that the nucleus must have a positive charge.
What was the next model?
The nuclear model (Bohr model), Niels Bohr proposed that the fixed orbits are at set distances from the nucleus. These where called energy levels. He suggested that electrons can only exist in these fixed orbits/shells.
What is our current model?
An atom is has a positively charged nucleus surrounded by negatively charged electrons. Virtually all the mass is in the nucleus and is 10 000 times smaller than the atom. The rest of the atom is empty space other than electrons. The electrons in its shells give the atom a size of 1 x 10 to the power of -10 meters.
How can an electron move up a higher energy shell?
By absorbing electromagnetic radiation and it moves to a partially filled shell. But this wears away as it loses its extra energy when the radiation leaves.
What happens when an atom is ionised?
An outer electron absorbs enough energy it can move out, leave the atom and become a free electron, it has become ionised. It is now a positive ion as it has more protons than electrons.
Why is ionising radiation?
It is any radiation that can knock electrons from atoms.
What is an isotope? What is the difference between stable and unstable isotopes?
It is an element that has a different number of protons to neutrons. Unstable isotopes tend to decay into other elements and give out radiation whereas stable isotopes don’t?
What is released when radioactive substances when they decay?
- alpha> is emitted from the nucleus from the nucleus and it is two neutrons and two protons.
- beta plus> is simply a fast moving electron from the nucleus.
- beta minus> is a fast moving positron (is a positive electron)
- gamma rays> after a nucleus has decayed, it goes under nuclear rearrangement and releases energy. Gamma carries away this energy.
Properties of products of ionising radiation
Alpha> they don’t get very far, only a few cm in air, and are absorbed by a thin sheet of paper. They are strongly ionising.
Beta minus> they are moderately ionising and has a range of few metres in air but is absorbed by a sheet of aluminium.
Beta plus> very similar but travels a shorter range as when it hits an electron it emits gamma radiation.
Gamma> they are weakly ionising and pass through rather than collide, they are absorbed by thick sheets of lead or metres of concrete. They have a far range traveling a long distance in air.
How are nuclear equations written?
Alpha> the mass number decreases by 4 and atomic decreases by 2
Beta minus> the mass number doesn’t change but the atomic number increase by 1
Beta plus> the mass number doesn’t change but the atomic number decrease by 1
Neuton emissions> the mass number decreases by 1 and the atomic number stays the same
Gamma> doesn’t change, only gets rid of excess energy
What is half life?
Radioactive sources give our radiation which is an entirely random process, half life is way of predicting how many will have decayed in a given time. It is measured in becquerels,Bq.
The half time is the average time taken for the number of radioactive nuclei in an isotope to halve.
How to work out half life?
E.g. 640Bq –> two hours later it has fallen to 40Bq
640➗2 = 320➗2=160➗2=80➗2=40
SO two hours is four half lives so it must be 30 mins
What is background radiation?
It is the low level radiation that we are exposed to day to day. It comes from 6 sources:
Space(Cosmic rays),nuclear waste,food,buildings,rocks and argon gas.
What is irradiation?
It is exposure to radiation. Irradiated by radiation doesn’t mean it becomes radioactive.
