= Hall carrier count versus valence
{c}
In a one-carrier isotropic <Drude model>, $R_H=1/(nq)$ and an inferred <carrier density> is $n_H=1/(e|R_H|)$. Dividing by the atomic <number density> gives an effective Hall count per atom. A noninteger result need not mean a literal fractional count of <valence electrons>: multiple <energy bands>, anisotropic <Fermi surfaces> and different <charge carrier mobilities> can change the <Hall coefficient>. In a two-carrier isotropic approximation,
$$
R_H=\frac{p\mu_h^2-n\mu_e^2}{e(p\mu_h+n\mu_e)^2}.
$$
Its sign identifies the dominant Hall response rather than necessarily the most numerous <charge carrier>.
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