Atomic mass units

The Atomic Mass Unit is defined as the twelfth part of the mass of a C$_{12}$ atom. Its value (in SI units, CODATA value) is

$1.66054\times 10^{-27}$ kilograms

Hence, it is approximately equal to the mass of a proton. Elements are asigned masses in these units which are not integer numbers. This is because neutrons and protons have slightly different masses, because nuclear binding forces cause the total mass of a nucleum to be less than that of the sum of its constituents, and, mainly, because of the appearance of natural isotopes. Thus, Potassium has a mass of 39.10 mainly because of natural K$^{40}$ isotopes.

Here is a list of masses of elements and compounds in amus:

Compound/element	mass in amu
Ar	39.95
Xe	131.29
CO2	44.01
H2O, water	18.02
N2	28.01