New York State High School Regents Exam Prep
Earth Science

What is radioactive dating and how can it tell us the age of an object?

What is an absolute age? It is the age of a rock unit, fossil or geologic event expressed in units of time, such as years. A good example is your birthday. You were born at a specific time on a specific day of the year.

How does radioactive decay work? If an isotope (forms of chemical elements that differ in the number of neutrons in their atomic nuclei) is radioacitve, it will break down naturally into a lighter element called a decay product. This process occurs at a predictable rate and can be used to determine how old an object is.

What is a Half-life? A half-life is the time required for half of an element's atoms in a sample to change to the decay product. In each half-life only half of the remaining radioactive atoms decay, no matter how large the sample is. Look at the diagram below which represents the radioactive decay of uranium-238. The shaded area represents the decay product which is lead-206. The half-life of uranuim-238 is 4.5 billion years, since this object has gone through two half-lives it is 9 billion years old.

How can we determine the number of half-lives that have occured? Look at the decay-product ratio, which is the ratio between the mass of a radioactive element and its decay product. This will help calculate the number of half-lives that have occured since the sample was formed. Once you know that you can find the age of the sample.

Decay Product Examples- If an object is made up of 50 % decay product then the object has been through 1 half-life. 75% decay product equals 2 half-lives, 87.5% decay product equals 3 half-lives, 93.76% decay product equals 4 half-lives, and so on. Once you know the number of half-lives you can determine the age by using the half-life in years listed on the back of the Reference Tables.

Make a chart to help clear things up. The chart below gives information about the radioactive decay of carbon-14. You could make a chart like this one for any radioactive decay question. It will show how the half-lives, radioactive element (or decay product) and the age of the sample are related. Try doing this to see if it will help you.

Test Your Knowledge

Use the Radioactive Decay Data on the back page of your Reference Tables to answer the questions below.
1. Could you fill in the carbon-14 chart? Check your answers below.

2. If 100 grams of pure carbon-14 starts to decay, how much nitrogen-14 (N-14) will be created in 11,400 years?

3. A 200 gram rock sample is found to contain 25 grams of Potassium-40 (K-40), what is the age of this rock?

New York State High School Regents Exam Prep Center: Earth Science