question on serial key

This may be better discussed in the pgsql-sql forum.  Please let me
know if so.

I am looking for criteria on deciding whether or not to use a serial
(auto-incrementing) key for rows in a table.  For example, if I have a
table of, say, the elements on the periodic table I could use the
atomic number as a unique key which would not be a serial type.
Another example is where I'm inserting lots of rows in a given period
of time and I need to go back and select the max id of one of these
rows; I would need a serial key in this case.

Intuitively, it's pretty clear to me when a serial index is called
for.  Is there a succinct set of guidelines that one could go by?

Thanks.

--
Brandon