Actually, an atom of silicon only has 4 electrons in its valence shell. Because of the way a pure silicon crystal is put together, it makes a lousy conductor, that is, each electron is shared by a neighbouring atom.
Good conductors have 1 free electron in its valence shell, and good insulators have 8. Good Semiconductor materials are typically said to have 4, but generally they are made to semiconduct properly by doping.
By adding either arsenic or phosphorus to silicon, an electron is either added making it negatively charged (pentavalent), or taken away making it positively charged (trivalent). After doping the silicon atom, it then has an extra, less than stable electron, or it has an empty space for one, and it will semi-conduct because the electons are no longer all shared/bonded to other silicon atoms.
Consider if you had a suit of ringmail.
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Each inside ring is looped to 4 other rings on 4 different sides. But the rings at one of the openings/edges are not looped to 4, and it would be easier to pull one of those rings from the suit instead of one of the inside rings. The inside rings are like silicon before doping it, all the electrons are being shared. the outside rings are like silicon, that is they don't have all 4 electrons holding on. Not the best analogy, but its kinda close.