Atomic and nuclear masses
The mass of nuclei is very small when it expressed in SI units (about 10-25 kg or less). Therefore, it is more convenient to express it in terms of another unit namely, the atomic mass unit (u). One atomic mass unit (u) is defined as the 1/12th of the mass of the isotope of carbon 126C , the most abundant naturally occurring isotope of carbon.
In other words
In terms of this atomic mass unit, the mass of the neutron = 1.008665 u, the mass of the proton =1.007276 u, the mass of the hydrogen atom = 1.007825 u and the mass of 126C = 12u. Note that usually mass specified is the mass of the atoms, not mass of the nucleus. To get the nuclear mass of particular nucleus, the mass of electrons has to be subtracted from the corresponding atomic mass. Experimentally the atomic mass is determined by the instrument called Bainbridge mass spectrometer. If we determine the atomic mass of the element without considering the effect of its isotopes, we get the mass averaged over different isotopes weighted by their abundances.
Calculate the average atomic mass of chlorine if no distinction is made between its different isotopes?
The element chlorine is a mixture of 75.77% of 3517Cl and 24.23% of 3717Cl . So the average atomic mass will be
(75.77/100) √ó 34.96885u + (24.23/100) √ó 36.96593u
In fact, the chemist uses the average atomic mass or simply called chemical atomic weight (35.453 u for chlorine) of an element. So it must be remembered that the atomic mass which is mentioned in the periodic table is basically averaged atomic mass.