Isotopes and Atomic Mass
Learning Objectives: After this lesson, you should be able to…
- calculate average atomic weights from isotopic weights and natural abundances
- solve for isotope mass or abundance given average atomic weight
An atom's identity is determined by the number of protons in its nucleus. As an example, let's look at carbon (an organic chemist's favorite!). Carbon has an atomic number of 6 and will have 6 protons. However, not all carbon atoms are identical. The number of neutrons in the nucleus can vary; carbon typically has 6, 7, or 8 neutrons. Atoms that have the same number of protons, but a different number of neutrons are known as isotopes. For carbon, 98.93% of carbon atoms have 6 neutrons (known as carbon-12 or C for having 12 particles in the nucleus) and only 1.07% of carbon atoms have 7 neutrons (carbon-13 or C). (Carbon-14 has an abundance that is effectively 0%.) The number of isotopes and their abundance for a given element varies. Like the proton, a neutron has mass, so the mass of each isotope will be different and is known as the isotopic mass. The average atomic mass that you see on the periodic table is calculated by using the abundance and isotopic mass. Since there is so much more carbon-12 than carbon-13, one would expect the mass to be much closer to 12 than 13. In fact, it is 12.01 amu.
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