# An original sample of the radioisotope fluorine-21 had a mass of 80.0 milligrams. Only 20.0 milligrams of this original sample remain unchanged after 8.32 seconds. What is the half life of fluorine-21?

##### 1 Answer

Mar 6, 2016

The half-life of fluorine-21 is 4.16 s.

#### Explanation:

You are starting with 80.0 mg of

After 1 half-life, you will have 40.0 mg of

After 2 half-lives, you will have 20.0 mg of

Hence,

∴