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2a.
What is Radioactivity?
and why does it happen?
- The nucleus is composed of protons and
neutrons
and glued together by a strong attraction
BUT only certain combinations of n/p seem to be stable (see stability curve graph
on right). Any isotope of any element that does not lie in the
stability band with a stable n/p ratio is likely to be radioactive!
- Radioactivity
results from the random and spontaneous breakdown of the unstable nucleus of an atom.
- This breakdown is called radioactive decay
of the unstable nucleus of the atom (radioisotope).
- In the breakdown of the unstable nucleus, energy is released by the emission of alpha, beta and gamma
ionising radiation (see diagram below).
- Alpha particles are
positive, mass 4, 2 protons (+, single plus) combined with 2 neutrons.
- It is the equivalent of the nucleus of a
helium atom, 42He2+
- Beta particles are negative, mass 1/1850,
a negative electron (charge -, minus),
- Gamma photon, mass 0 (~zero), radiation is electrically neutral (charge 0 or zero).
- When an unstable nucleus splits a
different nucleus with a different number of protons is formed and
so a different element is formed (NOT possible in chemical changes, but
this is a nuclear reaction!).
- The breakdown of an unstable atom is referred to as
radioactive decay or radioactive disintegration.
- It is a random process meaning it is a
matter of chance which particular nucleus decays.
2b.
How
did they find out there were three types of atomic ionising radiation?
The basic experimental technique for
separating beams of a mixture of particles
-
A radium source was used, which
also contains other radio-isotopes and a lead casing was the only
safety precaution used!
- How people like Marie Curie and Ernest
Rutherford etc. survived beyond the age of 60 is a mystery!
- Anyway! it was found that the
original 'emission' from the radium source was split into three beams
by an electric or magnetic field (diagram on the right).
- (i) The alpha beam was attracted/deflected
towards a negative plate, showing it was positive, since opposite
charges attract/like charges repel - the rules of electric charge
interaction.
- (ii) The beta beam bent
towards a positive plate (showing it was negative).
- (iii) The gamma beam passed through
un-deflected (showing it had no charge).
- Note: Although the alpha
particles have the biggest charge of +2, the beta beam (smaller
charge of -1) is more easily deflected because of its much smaller
mass (nearly 8000 x smaller, mass ratio for He2+ : e-
is 4 : 1/1850).
- Note that in other nuclear
particle separation experiments, a proton (+), or any other positive
particle beam, bends towards the negative plate.
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