Lesson 3, Part6, Slide 7 (Question#6)

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BusyBeeh
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Lesson 3, Part6, Slide 7 (Question#6)

Postby BusyBeeh » Sat Apr 29, 2017 4:38 pm

I really thought the question:

Which of the following is closest to the O-Si-O bond angle in silica?
a) 90
b) 109
c) 120
d) 180


... was referring to the molecular/electronic geometry of O = Si = O (just like, O = C = O) hence, I was tempted to choose d), but the correct answer (b) is actually referencing the molecular geometry of SiO4 because this molecule was explicitly discussed in the passage. Shouldn't the question then be, "Which of the following is closest to the SiO4 bond angle in silica?
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NS_Tutor_Andrew
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Re: Lesson 3, Part6, Slide 7 (Question#6)

Postby NS_Tutor_Andrew » Sun Apr 30, 2017 10:51 pm

Hi BusyBeeh,

I see where you're coming from here! The short answer to your question is to go with what the passage tells you -- if it tells you that silica (SiO2) forms a network solid in which each Si atom bonds with 4 O atoms, run with it to answer the question.

The fuller answer to your question has to do with the fact that silica forms a network solid. It may initially seem counterintuitive to say simultaneously (1) that the molecular formula is SiO2 and (2) that each Si bonds with 4 oxygen atoms. The reason why these statements are consistent is that each oxygen atom deep within the crystal is shared between two silicon atoms. Here's a cool image of the structure of silica that might help to make this clearer: http://i.imgur.com/1uueQ.png. Your point would be right if each silicon-oxygen unit occurred in isolation, but in reality in a crystal they're networked together.

Hope this helps and feel free to ask any follow-up questions!
Andrew D.
Content Manager, Next Step Test Prep.

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