Re: When constructing a battery, why are copper pennies preferred over nickles?

Good observation - 

Yes pennies do work  better than nickels.  But I'm afraid that you may not 
be working with copper, but with zinc... which still works better than 
nickels.

This first thing is to remember that american coins are no longer made of 
their traditional metals (pennies - copper,  nickels - nickel and dimes - 
silver) After 1981 pennies became largely zinc, and longer ago than that,  
nickels and dimes went to baser (less valuable) metals as well.

But whether your pennies are primarily pre-1981 copper or 1982 and later 
zinc,  they are still both very "active" metals.  And therefore do wonderful 
things as batteries.

What is this "active?" It has to do with how willing a metal is to part with 
its valence electrons.  The closer to the edges, and the closer to the top 
of the periodic table a metal is,  the more active it usually is.  Further,  
two metals near each other will often (but not always) have similar 
properties.

If you will go look at this periodic table:
 http://www.webelements.com/index.html

you can see zinc (Zn) and copper (Cu) are right next to each other,  and 
therefore have similar properties.  They are also near the top and edge of 
their section,  and are therefore quite willing to play fast and loose with 
their electrons (let them move around).  Nickel (Ni), and iron (Fe) (another 
metal you may have played with) are closer to the middle of their section, 
and therefore their electrons are a little more closely guarded.

The ability for metals to conduct electricity and/or contribute to the power 
of batteries also has to do with the way the electrons are stacked around 
the nucleus.  Copper and zinc are stacked in such a way that they have very 
little or no "forbidden zone" so the electrons can go anywhere they want.
the middle metals have a little bit bigger forbidden zone, so thier action 
in batteries is not so great.  Finally,  the non-metals have an enormous 
forbidden zone, so things like oxygen and sulfur don't conduct electricity 
at all,  and make horrible batteries.

To sum it all up - there are lots of reasons why some metals are better in 
batteries than others.  I  have introduced you the a few of the best known 
and understood.  Yes some of this stuff is still under very sophisticated 
debate! From here I  would reccomend that you get your hands on a good 
chemistry book  and look up and read about the following:

"activity" or "activity series"
"Forbidden zone"
"electron orbitals"
"electrochemistry"
and "standard electrode potential"

That should give you a more thorough idea... with pictures!

Good Luck,

Greta Hardin