Periodic Table and Atomic Mass

The atomic mass of an element is the sum total of the mass of that element’s protons, neutrons, and electrons.  Atomic mass is measured in units of amu.  On the periodic table, each element has its average atomic mass listed at the bottom of its box.  The average atomic mass is calculated as the weighted average of the atomic mass of all isotopes of that element with the percent abundance of that element in nature (see isotopes subset).  

The atomic mass of a molecule is simply the sum total of the atomic masses of every atom that comprise that molecule.  For example, water is H₂O and is comprised of 2 hydrogen atoms and 1 oxygen atom.  Thus, the atomic mass of 1 water molecule would be about 18 amu (2 amu from the 2 hydrogen atoms, and 16 amu from the 1 oxygen atom). 

The periodic table includes the average atomic mass of all the elements.