How does the atomic mass of carbon show that it is an isotopes?

Answer 1

If your asking "how does the mass show there are isotopes", then the answer is this: Isotopes are found by using a mass spectrometer, and the abundance (percent amount) of each isotope is determined. Using this percent of abundance, the AVERAGE mass is reported on the Periodic Table. For example, if carbon 12 had mass of 12 and carbon 13 had mass of 13, but the percent abundance of 12 was 99% and the abundance of 13 was 1%, then the atomac mass would be

12*.99+13*.01=12.01

You can tell it has isotopes by seeing that the mass is a decimal and not a whole number. It is harder to tell with carbon, because it's only off by .01, but looking at chlorine it is more clear. Most isotope weights are very close to being whole numbers, like Cl35 mass is 34.98 and Cl37 is 36.97. But looking at Cl atomic weight, it is 35.45; the .45 tells you it is a average because the value lies between the whole numbers as a decimal. Cl is 35.45, which is an average of 35 and 37 that's 76% and 24% abundance respectively.

Answer 2

The Atomic Mass of carbon represents the weighted average of the masses of its isotopes. Carbon has multiple isotopes, such as carbon-12 and carbon-13, which vary in the number of neutrons in their nuclei. By taking into account the abundance of each isotope, the atomic mass reflects this mixture and shows that carbon is an element with isotopes.