Atomic mass units

The atomic mass unit (AMU) 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 assigned 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 nucleus 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 atomic mass units:

Compound/element	mass in amu
Ar	39.948
C	12.0107
=CH	13.01864
-CH2-	14.02658
-CH3	15.03452
Xe	131.293
CO2	44.0095
H	1.00794
H2O (water)	18.02
N2	28.01

Note that sometimes the atomic mass unit is also known as the Dalton (Da), especially when dealing with macromolecules such as proteins.

References

1. IUPAC Periodic Table