Atomic mass can be defined as the total mass of one atom of any given element. The unit of atomic mass is called the unified atomic mass unit (denoted by ‘u’). Most of the atomic mass of a substance is made up of protons and neutrons. Therefore, it is almost equal to its mass number.
Atomic mass of Carbon
Carbon is present in the 14th group of elements in the Periodic Table. The atomic number of carbon is 6 and the atomic mass is 12.01gmol-1. It is represented by the symbol C.
Isotopes of Carbon
Carbon has three isotopes namely Carbon-12, Carbon-13, and Carbon-14. The numbers 12, 13, and 14 are the isotopes’ atomic masses. Here, Carbon-12 is a stable isotope whereas carbon-14 is mostly a radioactive isotope.
So now will calculate the atomic masses of carbon by the following method:
When the sample is a mixture of isotopes of the given element in varying percentages, the following method can be used:
Step 1: Multiply the atomic mass of the isotope with its abundance percentage and divide the result by 100.
Step 2: Add the values gained from step 1 for each given isotope in the sample.
Carbon consists of 98.93%
12 C (12 u) and 1.07% 13 C(13.003 u).
Let us assume that there are 10 000 atoms of carbon. Then you have 9893 atoms of 12 C and 107 atoms of 13C.
Mass of 12 C = 9893 atoms × 12 u / 1 atom=118 716 u
Mass of 13 C = 107 atoms × 13.003 u / 1 atom=118 716 u
Total mass= 120 110 u
Average mass = 120 110 u / 10 000 atoms = 12.011 u
The average atomic mass of carbon is 12.011 amu.