Use the concepts of relative abundance and relative weight to explain why carbon has an atomic mass of 12.011 amu when there are three isotopes of carbon weighing 12 amu, 13 amu and 14 amu. Why is the atomic mass not 13?

Atomic mass is based on a relative scale and the mass of 12C

(carbon twelve) is defined as 12 amu; so, this is an exact number.

Why do we specify 12C? We do not simply state that the mass of a C atom is

12 AMU because elements exist as a variety of isotopes.

Carbon exists as two major isotopes, 12C, and 13C ( 14C exists and has

a half life of 5730 y, 10C and 11C also exist and their half lives are

19.45 min and 20.3 days respectively). Each carbon atom has the

same number of protons and electrons, 6. 12C has 6 neutrons, 13C has

7 neutrons, and 14C has 8 neutrons and so on. So, we must specify

which C atom defines the scale.

All the masses of the elements are determined relative to 12C.

Average Atomic Mass

Since many elements have a number of isotopes, chemists use average

atomic mass. On the periodic table the mass of carbon is reported as

12.011 amu. No single carbon atom has a mass of 12.011, but in a handful

of C atoms the average mass of a carbon atom is 12.011.

Why 12.011?

If a sample of carbon was placed in a mass spectrometer the

spectrometer would detect two different C atoms, 12C and 13C.

The natural abundance of 14C, 10C and 11C in geologic (i.e. old) samples is so low that we cannot

detect the effect these isotopes have on the average mass.

From the information collected from the mass spectrometer the average

mass of a carbon atom is calculated.

The mass of 12C is, of course, 12 amu.

13C is 1.0836129 times heavier than 12C; so, the mass of 13C is 13.003355

Explanation: