The Measure of A Molecule

    Kids, is there an easy way to compare the sizes of gas molecules?  Yes there is, and all you need are two regular balloons and some helium.  Have one of the balloons inflated with helium (you can go to a store and ask them to inflate a regular balloon for you).  Then inflate the second balloon with air.  Try to make the second balloon as identical to the first balloon as possible in size and shape.  Leave the balloons next to each other for a couple of days.  Observe and compare the size of each balloon as time goes by.  What happens? 

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Even though a balloon may look like it has a solid surface, it really has very small holes in it.  These pores, as small as they may be, are big enough to allow gas molecules out.  In this experiment, we had two balloons with two different types of gases in them: air and helium.  Air is mostly oxygen and nitrogen.  Since the helium balloon deflated faster (it should have, anyway!), the helium gas molecules must have been smaller than either the nitrogen or the oxygen molecules.  Therefore, we can use a balloon to compare the sizes of gas molecules.

    Nitrogen and oxygen are in a group of gases that are called “diatomic”.  This means that their molecules exist only in pairs.  You don’t find, for example, oxygen by itself as O in nature.  Instead, you find O2. Helium, however, is not part of this group and exists purely as He. In terms of atomic size, O and He are fairly similar. But in terms of molecular size, O2 and He are different enough to measure just by using the balloon test. Since nitrogen and oxygen are bigger than helium, they have less chance of escaping through pores such as those found in the balloons. Therefore, the balloon inflated with air should deflate more slowly than the one with helium.

    Other diatomic gases include hydrogen (H2), fluorine (F2), nitrogen (N2), and chlorine (Cl2). In addition, bromine (Br2), a liquid, and iodine (I2), a solid, also appear in pairs. Try to devise your own acronym to help memorize these special elements based on their chemical symbols (like HOFBrINCl?).

    Note:  don’t use Mylar balloons; that’s a different ChemShorts article (April 1996 “LeakBusters”); also check June 2003 for a twist on helium vs. air balloons).


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    May 2005


    Reference (found through the National Science Foundation website):