  A simple model of spacetime foam, made by $N$ Reissner-Nordstr\"{o}m
wormholes with a magnetic and electric charge in a semiclassical approximation,
is taken under examination. The Casimir-like energy of the quantum fluctuation
of such a model is computed and compared with that one obtained with a foamy
space modeled by $N$ Schwarzschild wormholes. The comparison leads to the
conclusion that a foamy spacetime cannot be considered as a collection of $N$
Reissner-Nordstr\"{o}m wormholes but that such a collection can be taken as an
excited state of the foam.
