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I can understand that in Silicon Crystal a central Silicon atom shares its 4 valance electrons with 4 surrounding silicon atoms. And the chain continues. But then What happens to the outermost such Silicon atoms (in the drawing). They each will have incomplete so called octate.

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In a Silicon crystal, outermost Silicon atoms are probably oxidized yielding a monoatomic layer of Oxygen atoms, chemically bound by covalences like in $\ce{SiO_2}$, by analogy with Aluminium. Silicon and Aluminium are neighbors in the periodic table : they often have similar properties. And it is well known that an extremely thin layer of Aluminium Oxide $\ce{Al_2O_3}$ does cover Aluminium foils and pieces. It must be the same for Silicon.

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    $\begingroup$ Is there any reference for the claim? $\endgroup$ – Oscar Lanzi May 11 at 9:52
  • $\begingroup$ Something like that is at least mentioned here on page 4 (15 in pdf) on the bottom. pure.tue.nl/ws/files/3477173/148292.pdf $\endgroup$ – Justanotherchemist May 11 at 12:45
  • $\begingroup$ I did upvoted because beside references would be indeed beneficial the answer has the merit to point OP in the right direction. Every non molecular covalent lattice has boundaries at higher potential than the interior, so in many cases the atoms at the surface saturate their valence shell reacting with the surrounding. Just perfect vacuum would prevent reaction or adsorption, the latter doesn't fix the question but lower the potential as well. For Silicon, a layer of SiOx forms in instants in normal atmosphere. Part of the so called dampling bonds are saturated by OH group from moisture, too. $\endgroup$ – Alchimista May 11 at 13:11

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