Log in!
Stay Signed In
Do you want to access your site more quickly on this computer? Check this box, and your username and password will be remembered for two weeks. Click logout to turn this off.

Stay Safe
Do not check this box if you are using a public computer. You don't want anyone seeing your personal info or messing with your site.
Ok, I got it


what is an isotope?

Elements can have different atoms with a different number of neutrons in their nucleus. The different possible combinations of the number of neutrons are called isotopes.

For example, on the periodic table, it says that the element Carbon has an atomic mass of 12 and an atomic number of 6. This means it has 6 neutrons in it's nucleus (REMEMBER: atomic mass = atomic number + number of neutrons). Therefore we can call it 12Carbon or Carbon-12.

Diagram 1

But, there are also other isotopes of carbon. There are isotopes such as Carbon-13, which has 7 neutrons in it's nucleus, and Carbon-14, which has 8 neutrons in it's nucleus.

Diagram 2

Carbon-12 is on the periodic table because it is the most abundant in our natural environment when averaged with the percentage of abundance of other isotopes. Nearly 1% of carbon is Carbon-13, 0.01% is Carbon-14, and 99% is Carbon-12.

Diagram 3

Here are some examples with other elements (the protons are in red):

Diagram 4

what's the difference between an isotope and an element?

Isotopes are considered different FORMS of elements. It's almost like an alter-ego for each element. Carbon could be Peter Parker, Carbon-12 being him in his regular human form and Carbon-13 could be Spiderman. They're still both Peter Parker, but in different forms or variations.

Diagram 5

However, isotopes really don't affect any chemical properties of elements such as their reaction rates (or in this case, Peter Parker's personality), but they do affect physical properties, such as melting points and density (or how Peter Parker looks). This is because the number of protons and the number of electrons in the element still remain the same, but only the uncharged neutrons change.

how do you find out the average isotope?

To find out the isotope that will be on the periodic table, you calculate the average of all the isotopes in the given environment. This is called the relative atomic mass that goes on the periodic table.

For example, you've just been given a sample of 100 atoms of chlorine. 75 chlorine atoms are the isotope Chlorine-35, and 25 of those atoms are the isotope Chlorine-37. First of all you'd have to calculate the atomic mass of all of those isotopes!

You would then consider the number of atoms that the isotope has in the sample of atoms given (in this case, 100 atoms were given). So the number of atoms Chlorine-35 had in that sample was 75, and Chlorine-37 had 25 atoms in that sample.

Then you would multiply that with the atomic mass of that isotope. So the atomic mass of Chlorine-35, would be of course, 35, and Chlorine-37 would have an atomic mass of 37.

Multiplying it would give you the TOTAL mass of all those atoms.

So the mass of Chlorine-35 in the sample would be: 35 x 75 = 2,625
So the mass of Chlorine-37 would be: 37x25 = 925

Then add the two masses together to get the total mass of all of the isotopes in the sample of atoms.
So 925+2625 = 3550. The total mass would be 3,550.

Finally, you would divide it Diagram 6 by the TOTAL NUMBER of atoms in the sample. Remember you were given 100 atoms of chlorine in the first place. So you would divide the total (3550) by 100.
Now 3550/100 = 35.5.

So in this sample of atoms, the relative atomic mass of chlorine is 35.5. The actual relative atomic mass is 35.45, which is pretty close.

step-by-step overview

1. Multiply the atomic mass of each isotope (i.e. the number at the end of the element) with the number of atoms it has in the sample. This will give you the total mass of that particular isotope.

2. Add all the masses you calculated together.

3. Divide it by the total number of atoms in that sample.


Note: If you get percentages, multiply each percentage by that mass, and divide by 100.

practice quiz!

You'll need a calculator for this one - be prepared for a little outside-the-box final question. After you've answered each calculator question, click "View Answer" at the top for an explanation of the correct answer.
Isotopes Practice Quiz ยป online testing

images cited

Diagram 1: Description of Carbon on the Periodic Table. N.d. SparkNotes.com. N.p., n.d. Web. 5 Nov. 2011 <http://www.sparknotes.com/chemistry/fundamentals/periodictable/section2.rhtml>.
Diagram 2: Carbon-12 and carbon-14. N.d. Chem4Kids. N.p., 2011. Web. 5 Nov. 2011. <http://www.chem4kids.com/files/atom_isotopes.html>.
Diagram 3: The atomic mass. N.d. Chem4Kids. N.p., 2011. Web. 5 Nov. 2011. <http://www.chem4kids.com/files/atom_isotopes.html>.
Diagram 4: Isotopes of hydrogen, helium, and lithium. N.d. Kaffee. N.p., 31 Dec. 2007. Web. 5 Nov. 2011. <http://kaffee.50webs.com/Science/activities/Chem/Activity.Isotopes.Table.htm>.
Diagram 5: Spiderman Cartoon Wallpaper. N.d. Best Gallery Cartoon. N.p., 20 May 2011. Web. 5 Nov. 2011 <http://bestgallerycartoon.blogspot.com/2011/05/spiderman-cartoon-wallpaper.html>.
Diagram 6: Chlorine on the periodic table. N.d. CDC.gov. N.p., 7 May 2010. Web. 5 Nov. 2011 <http://www.cdc.gov/healthywater/swimming/pools/chlorine-disinfection-timetable.html>.

Diagram 1: Carbon on the Periodic Table
Diagram 2: Carbon-12 and Carbon-14 (Green 0's = Neutrons)
Diagram 3: The Atomic Mass is an Averaged Number
put your HTML here
Diagram 4: Isotopes of Other Elements
Diagram 5: Spiderman - Peter Parker's Isotope
Diagram 6: Chlorine on the Periodic Table