Isotopes

Two atoms may be called the same name, like Carbon, but they differ in atom mass because they have different number of neutrons.

Carbon-12 has 6 neutrons, Carbon-13 has 7 neutrons and Carbon-14 has 8 neutrons.

Isotopes differ in stability. It is more stable for Carbon, which has 6 protons, to have 6 neutrons.

The instability of Carbon-14 is what makes it interesting. It’s concentration in a fossil can be used to estimate the age of the fossil.

Isotopes will have the same chemical properties. This is helpful in NMR, Nuclear Magnetic Resonance, because a chemist might “pull a trick” by using a molecule with containing a different isotope, and then looking at the NMR spectrograph to see if a product of the reaction has that isotope in it.

Appendix A

You will notice that the value for the mass of an atom is reported as a number that is not an integer. If all the carbon in the world was Carbon-12, the mass for carbon would be 12.000 but we see it is 12.011.

This number relates to the relative masses of the various isotopes of carbon.

• Carbon 12 – 98.89%
• Carbon 13 – 1.11%
• Carbon 14 – 0.000 000 000 1% (too small to affect the calculation’s significant digits)

Keep in mind that 50% is 0.5 and then do the math shown below:

12*(0.9889) + 13*(0.0111) = 12.0111