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### Book Questions 4.1&4.2

Posted: Sun Nov 12, 2017 11:28 pm
I'm not sure how we would go about these two questions in the book which basically show two shapes and give a bond angle and ask whether there must be, may be, or cannot be one of more lone pairs of electrons on the central atom. If we don't know what the molecule is, how can we determine how many electrons there could/must be around the central atom?

### Re: Book Questions 4.1&4.2

Posted: Sun Nov 12, 2017 11:38 pm
I think it depends on the bond angles?
For example, for part a on the first question, the molecule has a bond angle of 120, which should mean a trigonal planar, or basically 3 electron domains around the central atom. The diagram only shows 2 bonded pairs (which would otherwise be bond angle 180) so there must be a lone pair that makes up the third electron domain.

### Re: Book Questions 4.1&4.2

Posted: Sun Nov 12, 2017 11:41 pm
Rucha Kulkarni 1A wrote:I think it depends on the bond angles?
For example, for part a on the first question, the molecule has a bond angle of 120, which should mean a trigonal planar, or basically 3 electron domains around the central atom. The diagram only shows 2 bonded pairs (which would otherwise be bond angle 180) so there must be a lone pair that makes up the third electron domain.

How would you determine if it was "can have" or "must have"?

### Re: Book Questions 4.1&4.2

Posted: Mon Nov 13, 2017 12:33 am
I'm not sure if I can explain this clearly but I think the textbook wants you to be able to differentiate between electronic arrangement, which treats lone pairs and bonding pairs the same in determining shape, and molecular structure, which is a specification of electronic arrangement that takes into account whether the region of electron density is a lone or bonding pair. Figure a of 4.1 must have lone pairs because it has a trigonal planar electronic arrangement and a bent shape with three atoms bound together- in the case that it had no lone pairs it must have a linear structure, which it does not have. Figure b of 4.1 may have lone pairs because the linear molecular shape can be the result of an electronic arrangement that is linear with no lone pairs and as a result of, for example, a molecule with 5 regions of electron density (electronic arrangement: trigonal bipyramidal) with 3 of them being bonding pairs, resulting in a linear molecular shape.

### Re: Book Questions 4.1&4.2

Posted: Mon Nov 13, 2017 4:10 pm
Page 118 of the textbook provides good graphics for this concept. For the linear model, the molecule can either have the formula AX2 or AX2E3, so it could have lone pairs around the central atom.