1. The problem statement, all variables and given/known data Here is an excerpt from a work 2. Relevant equations 3. The attempt at a solution What I don't understand is the excerpt. Okay, silicon is #14, which has 3s2 3p2, total of 4 valence electrons. But I do not know (1) first, which and where is this surplus electron referring to / from? from the substitution (group V), or four covalent-bonded silicon? (2) second, so what about the substitution with an element of group V? Any comment is appreciated.