Half-life Notes

In chapter 19 we studied half-life, which is the time it takes for half of the radioactive nuclei to undergo radioactive decay. For example, if you have a 100mg sample with a half-life of 1 hour, after 1 hour, you will have 50 mg of that sample remaining. However, this does not mean that half of the sample disappeared; it merely means that only half of the original 100 mg is still radioactive. This rule originates from Dalton's Atomic theory, which states that atoms can not be created or destroyed.
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/halfli.html
http://study.com/academy/lesson/radioactive-decay-definition-formula-types.html