Radioactive Decay

Here is an atom with a stable nucleus. The atom itself may take place in a number of chemical reactions but these will only affect the electrons whizzing round the outside. The nucleus will not change.	Here is an atom with an unstable nucleus There are a number of reasons why it might be unstable which we will look at later. This atom, at some point in time in the future, will decay by emitting an alpha or beta particle.

Here is a sample which contains some radioactive atoms

The activity of the sample is the number of radioactive particles it emits every second. 

This is measured in counts per second (c.p.s) , or Bequerel (Bq).

Obviously not all the particles emitted by the source will go into the detector.

The activity is also the number of decays which take place inside the sample every second.

Notice that the graph of N against t is not a straight line. As time goes by the activity falls. We can understand why if we think of unstable atoms as dice.

Clearly, whether an unstable atom decays or not is a question of probability. We cannot predict exactly when any particular atom will decay, we can only express the probability that it will decay in a certain time.

Here are the half lives of some isotopes

If you had samples which contained the same number of atoms which would have the largest activity?