Group 18 - The Nobel Gases

Because all of the elements in this group
have a full outer shell they are very
stable (they don't react with other
elements).  They are
all gases and
found in small amounts in the earth's
atmosphere.  The fact that we
discovered most elements by breaking
down compounds, and this group doesn't
form compounds, most of these elements
were not discovered until after most of
the others.  (Think about that when you
look at how Mendeleev put together the
periodic table- there was an entire group
of elements that he didn't even know
about!)