Explanation

(c) When donor impurity (+5 valence) is added to a pure silicon (+4 valence), the +5 valence donor atom sits in the place of +4 valence silicon atom. So it has a net additional +1 electronic charge. The four valence electrons form covalent bond and get fixed in the lattice. The fifth electron (with net $- 1$ electronic charge) can be approximated to revolve around +1 additional charge. The situation is like the hydrogen atom for which energy is given by $E = - \frac{13.6}{n^{2}}eV$. For the case of hydrogen, the permittivity was taken as $\varepsilon_{0}$. However, if the medium has a permittivity $\varepsilon_{r},$ relative to $\varepsilon_{0},$ then $E = - \frac{13.6}{\varepsilon_{r}^{2}n^{2}}eV$
For $Si,\ \varepsilon_{r} = 12$ and for $n = 1,\ E \simeq 0.1\ eV$