Silicon(IV) Oxide

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Why does silicon(IV) oxide have the formula SiO2 but a structure in which each silicon atom is bonded to four oxygen atoms?

In: Chemistry

2 Answers

Anonymous 0 Comments

Each oxygen atom is also bonded to two silicon atoms. The formula SiO2 only really tells us the proportions of the atoms and not the actual number connected together.

Anonymous 0 Comments

Although every silicon atom has bonds to four different oxygen atoms there are still only twice as many Si molecules as there are O molecules in the big picture.

When we zoom in on one individual “unit cell” we’re trying to be as clever as we can when drawing the boundaries of that map so that we don’t exclude too much information, and so that we don’t have any overlap.

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Consider this imaginary 2-D chain example:
> …O–W–O–W–O–W–O–W…

Clearly, every O molecule is bonded to two W molecules, but if we try to draw our map like that:
> …O-[-W–O–W-]-O-[-W–O–W-]…

Then our map undercounts by leaving half the X molecules out (either that or we’d have a lot of W overlapping and therefore overcounting). If we want to draw things without any undercounting or overcounting, then we have to either let our map edge cut some Ws in half:
> …O–V][V–O–V][V–O–V][V–O–V][V…

so that no bonds get cut by a map edge, or we can have one of the O-W bonds get ‘cut’ by the map edge instead:
> …[-O–W-][-O–W-][-O–W-][-O–W-]…

And usually this is the more straightforward way to go. We can still see that every O has a bond to the W on its right, but we also see that there’s another bond going off-map to the closest W on the left.

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The SiO2 situation is the same sort of map-edge problem, only in 3-D. Instead of drawing all four oxygens and cutting them each in half along a map edge, we shift the frame a bit so that two oxygens are completely in the map and the bonds are what gets chopped off at the map edge (the bonds still totally connect to their neighboring unit cells’ oxygens, though).