On Sun, 27 Sep 2009 19:04:25 +0100, Bill Wright wrote:

That's why they weigh more.

Yes and possibly they are a little slower than others, since rates of
reactions for molecules with atoms of heavier isotopes are very slightly
slower.

This effect is best observed with heavy water, since the difference in
mass of the hydrogen is double, and why it is toxic and drinking heavy
water is not a good idea.

http://www.straightdope.COM/columns/read/2135/is-heavy-water-dangerous