The half-life of a radioactive isotope is the amount of time required for half of the atoms in a sample to decay to atoms of a new element.
For example, Carbon-14 has a half-life of 5568 years. This means that if you start with a 10 gram sample, after 5568 years you will only have 5 grams left. At the end of another 5568 years, there will only be 2.5 grams left.
Scientists use this decay process to date items which have been dug up. It is also a useful guide to determine how soon it is safe to return to a site which has been contaminated by nuclear radiation.
We are going to look at a manual simulation (model) of this decay. You will be using approximately 300 'atoms'.
1 Carefully put all the atoms in a container and give it a good shake. This represents an isotope which we will call obversium. This will decay into a new element called reversogen, which can be shown by the white side of an atom.
2 Tip them out onto a table, and remove all the ones which fall white side up – the reversogen.
3 Count the ones which are still obversium (blue side up).
4 Record these results in a table like the one below or on an EXCEL spreadsheet. For the purposes of the simulation, assume that the time between each trial is one second.
5 Put the atoms that landed blue side up back into the container carefully, and repeat the above until you have no obversium left.
Example table:
| Time (secs) | Obversium left |
| 0 | 300 |
| 1 | 151 |
| 2 | 76 |
Obviously, the numbers you obtain will differ from those in the table, and will vary each time you perform the simulation.
Now open the Half-life spreadsheet. This spreadsheet simulates the decay of a very large number of atoms.
The spreadsheet is given the initial mass of an element measured in grams, and the probability of any one atom decaying each second.
It displays the mass of the undecayed element remaining as time passes, by using the probability to calculate the proportion of the mass which decays each second.