Atomic Mass UnitDetermining the Mass of an Atom

How big is an atom?Atoms are infinitesimally small. Smaller than

any thing we can imagine. One hydrogen atom, the smallest atom

known, is approximately 5 x 10-8 mm in diameter.

To put that in perspective, it would take almost 20 million hydrogen atoms to make a line as long as this dash ' - '.

How can you measure an atom?The extremely small sizes of atoms are

exceedingly impractical to work with; special measuring units have been devised to determine the dimension of atoms in terms of mass.

The mass or weight of an atom can be computed in the following ways...

Measuring the mass of an atomAn indirect method to determine the absolute

mass of an atom has been devised. The mass of a particular atom is taken as a

standard unit and the masses of other atoms are related to this standard.

Hydrogen being the lightest element and being the smallest atom was chosen and assumed to have a mass of 1.

An atom of hydrogen was assigned an atomic mass equal to one atomic mass unit (a.m.u).

Measuring Atomic MassThe number does not signify the mass of an atom in grams.

It is just a pure number.

The masses of atoms of other elements were compared to that of hydrogen, in order to find their atomic mass relative to it.

If one atom of sulfur weighs as much as 32 atoms of hydrogen, then the relative atomic mass of sulfur is 32 amu.

Atomic Mass Unit (amu)One atomic mass unit (amu) is defined as

one-twelfth of the mass of a single atom of the isotope carbon-12.

It is the standard unit in which the masses of atoms and molecules are measured.

It is equal to 1.66 x 10-24 g.

Relative Atomic Mass (RAM)The atomic mass of an element is a relative

quantity and is the mass of one atom of the element relative to the mass of one carbon-12 atom. Thus, relative atomic mass (RAM) of an element is the number of times one atom of an element is heavier than 1/12 the mass of an atom of carbon [C-12].

For example, if 1 atom of Na weighs as much as 23 parts of isotopes, then the atomic mass of sodium is 23 a.m.u.

Atomic Mass UnitAtomic masses are not expressed as whole

numbers because natural elements are mixtures of two or more isotopes.

The atomic weight of chlorine is 35.43 amu. Chlorine exists as two isotopes in the ratio

3:1 

Therefore average atomic masses are not whole numbers.