Question

hey, i just joined the MIT open course "Our Earth: Its Climate, History, and Processes" as an earth science and i don't understand what they're talking about when they talk about isotopes

Answer 1

Good question!

So, everything is made of kinds of atoms. Let's think about carbon, C. You probably know to be carbon, instead of something else, each carbon atom has 6 protons in the center (and the center is called the nucleus).
You want to be carbon and you only have 5 protons? Sorry, you are a boron atom. You say you have 7 protons? Not carbon either, you are nitrogen.
To balance those 6 positive charges, there are 6 electrons hanging out in the shells of the atom. The electrons determine how the material behaves chemically.

Now, except for fans of quantum physics or advanced chemistry, most people forget that there are two kinds of particles in the center of atoms. The protons, mentioned above, do all the work of attracting electrons and giving the atom its basic identity, but there are also _neutrons_ in the center. Neutrons are more subtle. They weigh about the same as protons. Apparently they help keep all the protons together, and make the atome stable.

The number of neutrons in the nucleus can be different from atoom to atom of the same subsatnce. For instance, most carbon atoms have 6 neutrons, and the average atomic weight of Carbon is 12 a.m.u. - the weight of 6 protons plust the weight of 6 neutrons.

But I said average weight, and _most_ atoms, didn't I? A few carbon atoms have 7 neutrons in the nucleus, and a few even have 8 neutrons. Still carbon, because the 6 protrons attract the 6 electons and define most of the atom's behavior.

Each of these different forms of a carbon atom is called an isotope: C-12, C-13, and C-14.

Answer 2

For the most part, these slight differences in weight don't have any useful properties. But sometimes, they do. For one thing, having 'extra' neutrons make atoms less stable, and 'less stable' can mean 'radioactive'.
Hydrogen isotopes help with fusion reactions.
C-14 breaks down into C-12 at a very slow, controlled rate, so the change can be used to tell how old something prehistoric is.
Uranium U-235 is stable, but U-238 can be used in a fission reaction.

I looked up a few facts here:
http://www.esrl.noaa.gov/gmd/outreach/isotopes/chemistry.html

Does this help? (tag me @mjdennis in your post if you have more questions, I don't visit Earth Sciences often!)

Answer 3

@mjdennis i never took high school chemistry (long, complicated, and frustrating story) so i have no idea what ANY of that means, but you get a gold star or whatever for trying, thank you

Answer 4

Hey, well, you are very daring to jump into an MIT earth science course cold like that! How about this for the non-chemistry version:
'Isotope' is the generic name for a alternate form of an element. An isotope behaves 99% the same as the 'parent' form, except it is heavier. Some are radioactive.
I don't think the fact they are heavier makes a big difference, but the radioactivity is useful for science, for energy, and unfortunately for weapons.

I'm only guessing, but I assume the Open Courseware class uses isotopes of carbon either to figure the dates of things, or to determine how much CO2 comes from fossil fuels?

Carbon isotopes are used for both.

Most carbon (98.9%) is called C-12, which means that something called the "atomic weight" of ordinary carbon is 12. 1.1%) of carbon atoms are C-13, one unit heavier. On in every trillion or so carbon atoms is C-14. C-14 heavy enough that it occasionally breaks the atom itself, and turns into nitrogen.

If you can compare the amount of C-14 in a material with how much C-12 and C-13 there is, there is a way to calculate the age. Also, fossil fuels are so, so, so old that all of their C-14 decayed into nitrogen long ago, so CO2 from burning fossil fuels shows up different than CO2 from wood or nature

Answer 5

@mjdennis that last reply was more helpful, thank you.
i don't know why they used isotopes, i got too confused in that video so i skipped over it, but now that i understand it more, i'll go back to it.