(a) Why do the inner-core electrons in a silicon atom not contribute to determining its electronic properties? (b) Why are the atoms in a silicon crystal arranged in such a way that each silicon atom has four nearest neighbors? (c) Why are the 3p and 3s quantum states in an isolated silicon atom transformed into the valence and conduction bands in a silicon crystal? (d) Why does the vacuum energy or vacuum level represent? (e) Why does the energy of a hole increase in the opposite direction to that of an electron in an energy-band diagram'?