Half-Life

Front

What is the definition of half-life?

Back

Half-life is the time required for half of the radioactive atoms in a sample to decay.

Front

If a substance has a half-life of 10 years, how much of a 100g sample remains after 20 years?

Back

25g (after 20 years, which is two half-lives, 100g -> 50g -> 25g).

Front

What is the half-life of uranium-238?

Back

4.5 billion years.

Front

How many half-lives are needed for a sample to reduce to 1/16 of its original amount?

Back

4 half-lives.

Front

If you start with 80g of a substance with a half-life of 5 years, how much will remain after 15 years?

Back

10g (after 15 years, which is three half-lives: 80g -> 40g -> 20g -> 10g).

Front

What is the half-life of iodine-131?

Back

8 days.

Front

How much of a 40mg sample of iodine-131 remains after 32 days?

Back

2.5mg (after 32 days, which is four half-lives: 40mg -> 20mg -> 10mg -> 5mg -> 2.5mg).

Front

What is the relationship between half-life and the stability of a radioactive isotope?

Back

Generally, isotopes with longer half-lives are more stable than those with shorter half-lives.

Front

If a sample has 16 parent isotopes and 16 daughter isotopes, how many half-lives have occurred?

Back

1 half-life (since half of the original parent isotopes have decayed into daughter isotopes).

Front

After how many half-lives will only 1/8 of the original sample remain?

Back

3 half-lives.