Half-Life

Thorium-234 has a half-life of 24 days. If a sample that originally contained 200 grams of thorium-234 now contains only 25 grams, the age of the sample is...

• 24 days

• 48 days

• 72 days

• 96 days

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NGSS.HS-PS1-8

A rock formed with 1000 atoms of a radioactive parent element, but only contains 250 radioactive parent atoms today. If the half-life of the parent element is one million years, how old is the rock?

• 250,000 years old

• 500,000 years old

• 1,000,000 years old

• 2,000,000 years old

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NGSS.HS-PS1-8

Why are isotopes with short half-lives not useful for dating very old rocks?

• because not enough of the parent isotope remains to measure accurately

• because not enough of the daughter product has formed to be detectable

• because neither the parent isotope nor the daughter product will be detectable

• because very old rocks would never have contained these isotopes

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NGSS.HS-PS1-8

The table shows the radioactive decay of carbon-14. Part of the table has been left blank. 
After 22,800 years, approximately what percentage of the original carbon-14 remains? 

• 15%

• 12.5%

• 6.25%

• 3.125%

Tags

NGSS.HS-PS1-8

If the half-life of a radioactive group of atoms is 100 years, then how old is the rock if there have been ten half-lives?

• 10 years

• 100 years

• 1000 years

• 10000 years

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NGSS.HS-PS1-8

The half-life of potassium-40 is 1.25 billion years. If you begin with 12 grams of potassium-40, how much is left after two half-lives?

• 12 grams

• 6 grams

• 3 grams

• 1.5 grams

After four half-lives, what fraction of the original parent isotopes remain?

• 1/4

• 1/16

• 1/8

Tags

NGSS.HS-PS1-8

You find a rock sample with radioactive atoms present.  If 12.5% of the original radioactive atoms were present, how many half-lives has the rock been around for? (Hint, at Half-Life = 0, you start with 100%)

• zero

• 1

• 2

• 3

Tags

NGSS.HS-PS1-8

If the half-life of a radioactive group of atoms is 100 years, then how old is the rock if there have been two half-lives?

• 25 years

• 200 years

• 400 years

• 800 years

Tags

NGSS.HS-PS1-8

What percent of carbon-14 would be left after 11,460 years?

• 100%

• 75%

• 50%

• 25%

Tags

NGSS.HS-PS1-8