Yes. The sum of the rest mass of the reaction products is microscopically less
than the sum of the rest masses of the reactants. (Since typical atoms have
rest masses on the order of `A' GeV, where `A' is the "molecular weight,"
whereas typical chemical energies are on the order of an eV or so, the
change in rest mass during a chemical reaction is typically on the order
of a part per billion or so --- which is very small, but theoretically
measurable.)


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

So let me make sure I got this straight:

The rest mass of a molecule is:

The sum of the rest mass of the individual electrons.

PLUS

The sum of the rest mass of the individual nuclei

PLUS

The kinetic energy of the electrons

So whenever an exothermic chemical reaction occurs, the total energy
of the electrons in the reactants is more than in the products. Thus,
since the rest mass of the individual particles remain the same, the
rest mass of the products is less than the reactants.