You can learn a lot about the physical properties of an element
by reviewing the periodic table.  Let’s take a look at carbon
as an example.
The number that appears above the element’s symbol is called the atomic number.
The atomic number tells you the number of protons that are in the nucleus
of an atom.  So in the case of carbon, you can see that it
has 6 protons in its nucleus.
In an atom of carbon that is neutral, there must also be 6 electrons.
This is because the positive charge of carbon’s protons must 
be balanced by the negative charge of carbon’s electrons.
Because electrons and protons have an equal and opposite charge, 
there must be an equal number of both protons and electrons
in a neutral atom of an element.  The number that
appears below the element’s symbol is called the atomic mass.
The mass of an atom depends on the number of protons, neutrons and 
electrons it contains.  However, since electrons
are so small, their mass is assumed to be zero in most calculations.
Therefore,
the atomic mass of an atomic is equal to the sum of its protons
and neutrons.  However, you’ll notice that carbon has an atomic
mass of 12.01.  This is because the number
associated with the atomic mass on the periodic table, is 
equal to the average of its naturally occurring isotopes.
All of the isotopes of carbon vary slightly in their atomic mass.
When we average these together,
we get 12.01.  If you know the
atomic number and atomic mass, you can determine the number of neutrons
in an element.  Simply subtract the atomic number,
or the number of protons, from the atomic mass,
the number of protons plus neutrons, and take the closest whole
number.  So 12.01 
minus 6 equals 6.01, which
rounds to 6.  So the number of neutrons in a neutral atom 
of carbon, is 6.
