The top number is the Mass Number for each isotope.The Mass Number for any isotope is the addition of all the protons and neutrons in the nucleus.Looking at the first isotope in the chart, Carbon 9 has 9 (protons + neutrons).

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The rest of the carbon isotopes are only of laboratory interest.

To the left side of each C (C is the symbol for Carbon) are two numbers, the bottom number indicates the Atomic Number or the number of protons in the nucleus.

Since all the atoms are carbon, they should all have an Atomic Number of 6.

To understand this process we must first understand a little bit about the atoms themselves and how they get their names.

Most carbon atoms have six positively charged protons and six uncharged neutrons.

Since protons and neutrons weigh about the same, the atomic mass of ordinary carbon is 6 + 6 = 12.It is called "Carbon-12," which is abbreviated "C." The fact that the atom has six protons is what makes it carbon.Most nitrogen atoms have seven protons and seven neutrons, so their atomic mass is 7 + 7 = 14.The fact that it has seven protons is what makes it nitrogen.Other atoms are also named based on the number of protons they carry.Notice in the diagram that eight different isotopes of Carbon are illustrated.