Silicon has 14 electrons, of which two orbit the nucleus in the first shell shell and eight fill the second shell. The four remaining electrons are in its outermost third "valence" shell leaving it four electrons short, in which case it is a cation with four positive charges. The four outer electrons are easily borrowed by other elements. Oxygen has eight electrons, leaving it two short of a full second shell. Its hunger for electrons is what makes oxygen such a strong oxidizer, and it makes a very good match with silicon.
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