Half-life

Half-life is how much time it takes exactly one half of a radioactive sample to decay. Say you have 100 grams of a certain element radioactive element. The half life of that element is the time it takes for half of it to decay, leaving you with 50 grams of the original sample.

For example, Radon-222 has a half life of 3.83 days. If we had 100 grams of radon-222, it would take 3.83 days for half the sample to decay, leaving us with 50g of radon-222. If we waited for another 3.83 days for the 50g sample to decay them we would be left with 25g. If we waited another 3.83 days then there would be 12,5 grams of radon-222 left.. and so on.