Let’s Talk Numbers
By the way, mass number doesn’t appear on the periodic table. Why not?
Because for any element, there’s no such thing as one mass number. As we’ve
just learned, different isotopes of the same element have different mass numbers.
However, chemists have figured out, roughly, the degree to which each isotope
of each element tends to occur on Earth. So, for each element, they’ve figured
out an average mass number, which for each element represents the average of
the mass numbers of all isotopes as they occur on Earth. This average number is
called the atomic weight of the element. The atomic weight of each element
appears on the periodic table, just below the element’s symbol. When we want to
know the mass of an atom of a particular element, for practical purposes, we use
the atomic weight that appears on the periodic table.