## Sapling Weeks 5-6 #9

$FC=V-(L+\frac{S}{2})$

JaylinWangDis1L
Posts: 61
Joined: Wed Sep 30, 2020 10:09 pm

### Sapling Weeks 5-6 #9

The resonance hybrid of the perchlorate ion, ClO4-, is described by 16 resonance structures, a representative subset of which is shown.

Which of the Lewis structures of the perchlorate ion are most plausible based on formal charge?

The chlorine-oxygen bond length in the perchlorate ion is found experimentally to be 144pm for all 4 bonds. The average length of a Cl-O bond is 172 pm and the average length of a Cl=O bond is 140 pm. Which of the Lewis structures of the perchlorate ion are most plausible based on the experimental data?

What is the oxidation number of chlorine in the perchlorate ion?

Which of the Lewis structures of the perchlorate ion is most plausible based on the oxidation number? Assume that lone pairs belong to the atom to which they are attached but that all electrons shared in a bond belong to the more electronegative atom.

Which of these three methods for determining the most plausible Lewis structure of the perchlorate ion is not consistent with the other two methods?
-using oxidation number to determine the most plausible structure
-all three methods are consistent with one another
-none of the methods are consistent with each other
-using bond length to determine the most plausible structure
-using formal charge to determine the most plausible structure

I got everything right eventually by guessing but please help explain how you know which structure is most plausible and how to determine the oxidation number. Thank you!

Megan Singer 3D
Posts: 96
Joined: Wed Sep 30, 2020 9:33 pm
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### Re: Sapling Weeks 5-6 #9

You pretty much just have to memorize the rules for oxidation numbers. Oxygen always has an oxidation number of -2, except in peroxides and F2O. If you add up the -2 charges of all 4 oxygens, you get -8. In order for the charge of the overall molecule to be -1, Cl must have an oxidation number of +7. Based on oxidation numbers, option E is the most plausible because it gives the most negative charges to O and the most positive charge to Cl. However, this option is realistically unlikely because it overestimates the ionic character of the bonds. It is also not consistent with the plausible models for formal charge and the experimental data. For these, C and D were the most consistent. You find plausible models for formal charge by carrying out the formal charge equation for each atom in the molecule, and choosing the model with the least extreme formal charge values. You find plausible models using the experimental data by realizing that 144 is closest to 140, so the molecule should consist mostly of double bonds.

AmyHo2K
Posts: 64
Joined: Wed Sep 30, 2020 9:39 pm

### Re: Sapling Weeks 5-6 #9

The oxidation number is 7. 2 is from the O, and 5 is from Cl. The structure that's most plausible is determined by which structure do Cl have the largest formal charge in.

Tobie Jessup 2E
Posts: 59
Joined: Wed Sep 30, 2020 10:02 pm

### Re: Sapling Weeks 5-6 #9

Megan Singer 1F wrote:You pretty much just have to memorize the rules for oxidation numbers. Oxygen always has an oxidation number of -2, except in peroxides and F2O. If you add up the -2 charges of all 4 oxygens, you get -8. In order for the charge of the overall molecule to be -1, Cl must have an oxidation number of +7. Based on oxidation numbers, option E is the most plausible because it gives the most negative charges to O and the most positive charge to Cl. However, this option is realistically unlikely because it overestimates the ionic character of the bonds. It is also not consistent with the plausible models for formal charge and the experimental data. For these, C and D were the most consistent. You find plausible models for formal charge by carrying out the formal charge equation for each atom in the molecule, and choosing the model with the least extreme formal charge values. You find plausible models using the experimental data by realizing that 144 is closest to 140, so the molecule should consist mostly of double bonds.

I also was having trouble with this homework problem and this helped me break it dow, thanks!