mass of the elements as seen on the periodic table is a weighted
average of the masses of all naturally occurring
isotopes for the element. Most elements have several naturally occurring
isotopes in varying abundance. Some elements like hydrogen and carbon have 2
naturally occurring isotopes. Others like Magnesium has 3 naturally occurring
isotopes, and Strontium has 4 naturally occurring isotopes.
Atomic masses that are on the periodic table are calculated based
on knowledge of the following information:
the mass of all the naturally occurring isotopes
for the element
the natural abundance of each of these naturally
occurring isotopes (usually expressed as a percentage)
The formula that you need is to calculate the atomic mass
of an element is:
atomic mass =
(the fraction of isotope 1)(mass
of isotope 1) +
(the fraction of isotope 2)(mass
of isotope 2) +
(the fraction of isotope 3)(mass
of isotope 3) + ...
the fractions of the isotopes
is the natural abundance of the isotope written as a decimal.
the masses of the isotopes are determined
accurately to as many as 4 decimal places.