# Fermilab

## Inquiring Minds

How many neutrons?

Dear Mrs. Pordes,

Hello. My name is Andrew Schmidt. I am writing to you concerning a question I have. I am in your daughters science class and it would be greatly appreciated if you could help me. My question is: What happens when you vary he number of neutrons in an element? Any assistence you could give me to help answer this question would be great. Thanks.

andrew s.

Dear Mr. Schmidt,

The simple answer to your question is to say that you make "isotopes" of the element when you vary the number of neutrons in the element. The element for which you may already know about its isotopes is carbon--the most common isotope is C(12), with 6 protons and 6 neutrons. C(14) has two extra neutrons, and this is the stuff most commonly used for "carbon dating."

Note that the number of protons determines which element you have. So if you add a proton to C(12) you get nitrogen, N(13). The number of neutrons determines, mostly, the stability of the substance. C(12) makes up 98.89% of naturally-occuring carbon, and C(13) makes up the rest (1.11%), and C(14) exists in trace amounts. C(14), when it breaks up, changes into nitrogen (N(14), a neutron in C(14) changes into a proton by ejecting an electron). So, if you look at the carbon of an old substance and see a nitrogen atom where you would expect to see a carbon atom, then you infer that there was a carbon there once. If you have a sample of C(14), then half of it will decay into N(14) in 5730 years--this is the "half life".

I also found an interesting and simple description of how to use unstable isotopes to determine the age of rocks at: