Why aren’t the masses on the Periodic Table whole numbers? : Why aren’t the masses on the Periodic Table whole numbers? The atomic masses on the Periodic Table are average masses for all of the isotopes of that element.
Isotopes – atoms of the same element that differ only in their mass
The abundance of each isotope is determined with an instrument known as a mass spectrometer.
Slide 3: In a given sample of chlorine… 75.78% is Chlorine-35
24.22% is Chlorine-37 To calculate the average atomic mass, multiply each isotope’s mass by its relative abundance. Then add these up. 0.7578 x 35 u =
0.2422 x 37 u = 26.52 u 8.961 u 35.481u = 35.48u