Electrical conductivity σ of a metallic solid is directly proportional to the number density n of delocalized electrons. Aluminum contributes three valence electrons per atom, whereas sodium contributes one. If two samples—one of Al and one of Na—contain the same number density of atoms and all other factors are equal, what is the predicted ratio σAl/σNa ?
a) 1
b) 9
c) 3
d) 31