First, it must be noted that isotopes are atoms of the same element with varying numbers of neutrons in the nucleus. Chemists can determine the atomic weight of an element by processing a sample in a Gas Chromatograph-Mass Spectrometer (or GCMS). The GCMS instrument will separate and measure the mass of the sample and document the results in a graph like form for the scientist to read. Remember, both protons and neutrons have a mass around 1 amu, and electrons have a mass of 9.11x13^-31 amu (or zero). Since the sample will contain all forms of the isotope, there will be multiple mass peaks on the graph. So, more or less neutrons in an atom will affect the mass and will be noticeable on the GCMS graph readout. Each isotope will have its own peak on the graph, and based upon the size of the peak, scientists are able to determine each percentage of isotopes that occur for that element as well as the number of neutrons each isotope has.
Isotopes are atoms that have same atomic number but differ in mass number wherein the sum of their % isotopic abundance is equal to 100% (or 1 in decimal form). The atomic weight of an element is the weighted mass of isotopes and their relative abundance. Formula: atomic weight =∑ (mass of isotope* %/100).
One way to determine the isotopic abundance experimentally is through the use GS-MS (Gas chromatography–mass spectrometry) analysis wherein relative isotope abundance is reflected in pattern through the mass spectrum that indicates how many elements exist. For instance the highest peak has value of 100% while the rest of the peaks are proportionally assigned that are all above 3%.