Radioactive decay is a spontaneous and random process.
A block of radioactive material will contain many trillions of nuclei and not all nuclei are likely to decay at the same time so it is impossible to tell when a particular nucleus will decay.
A spontaneous process means that it is not possible to say which particular nucleus will decay next, but given that there are so many of them, it is possible to say that a certain number will decay in a certain time.
It also means that the process of radioactive decay cannot be speeded up or slowed down by any physical changes such as a change of temperature or pressure.
A random process means that scientists cannot tell when a particular nucleus will decay, but they can use statistical methods to tell when half the unstable nuclei in a sample will have decayed.
This is called the half-life.
The illustration below shows how a radioactive sample is decaying over time.
From the start of timing it takes two days for the count to halve from 80 Bq down to 40 Bq.
It takes another two days for the count rate to halve again, this time from 40 Bq to 20 Bq.
The half-life of this source is 2 days.
Note that this second two days does not see the count drop to zero, only that it halves again.
A third, two-day period from four days to six days sees the count rate halving again from 20 Bq down to 10 Bq.
This process continues and although the count rate might get very small, it does not drop to zero completely.
The half-life of radioactive carbon-14 is 5,730 years.
If a sample of a tree (for example) contains 64 grams (g) of radioactive carbon after 5,730 years it will contain 32 g, after another 5,730 years that will have halved again to 16 g.