Lewis Structures of acids and bases

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Lewis Structures of acids and bases

Postby JulieAljamal1E » Sat Nov 03, 2018 1:12 pm

In the 7th edition 2C.3 the question asks, draw the lewis structure including typical contributions to resonance structure for (b) hydrogen phosphate ion, (c) chloric acid. I am confused as to why in part (b) the solution shows the hydrogen connected/bonded to an oxygen as opposed to bonding to the phosphate since phosphate is allowed to have an expanded octet. I'm also confused on part (c) why the hydrogen bonds to the oxygen instead of to chlorine. Thanks.

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Joined: Fri Sep 28, 2018 12:24 am

Re: Lewis Structures of acids and bases

Postby ian_haliburton_1f » Sat Nov 03, 2018 11:51 pm

The reasons for both cases have to do with formal charges. For b), Phosphate has 5 valence electrons, so you're trying to get the formal charge to equal 5 minus 5 in some way. In the structure of the Hydrogen phosphate ion, Phosphate achieves a formal charge of zero by making 5 bonds. If it were to make any more bonds or have any more electrons in general, it would have a negative formal charge, which is less ideal. Any negative formal charges should instead be carried by the Oxygen atoms as they are more electronegative.
Again for c), Chlorine has seven valence electrons, so we strive for a formal charge of 7-7. This is achieved through chlorine's 5 bonds and 2 unbonded electrons (7-(2+10/2)). If chlorine were also to bond with the hydrogen, it would have a charge of positive 1 instead.

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