Ozone in the upper atmosphere absorbs much of the ultraviolet light of the Sun…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Ozone in the upper atmosphere absorbs much of the ultraviolet light of the Sun. A student draws one Lewis structure of ozone, , and one of the nitrate ion, , as shown in the figure. In structure I the oxygen atoms are numbered 1, 2 and 3. In structure II the oxygen atoms are labelled a, b and c. Every lone pair is drawn as two dots. No formal charges are written yet.
Measurements tell us more. Both O-O bonds of ozone are long. This lies between the O-O single bond () and the O=O double bond (). All three N-O bonds of the nitrate ion are also equal. So each real species is a resonance hybrid, and one Lewis structure alone does not describe it.
- How many valence electrons does structure I show in all?
- What is the formal charge on oxygen atom 2 of structure I?
- What is the formal charge on oxygen atom 3 of structure I?
- What is the formal charge on the nitrogen atom of structure II?
- What is the O-O bond order in the resonance hybrid of ozone?
- What is the N-O bond order in the resonance hybrid of the nitrate ion?
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Two Lewis structures with all lone pairs drawn. Oxygen has 6 valence electrons and nitrogen 5.
Plan. Count the electrons. Use for each atom. Then spread the bonds of one structure over the equal positions of the hybrid.
Step 1. (a) Electrons in ozone. Three oxygen atoms bring electrons. The figure agrees: lone pairs hold 12 electrons and the three bonding pairs hold 6.
Step 2. (b) Atom 2. It has one lone pair () and shares a double and a single bond (). So .
Step 3. (c) Atom 3. It has three lone pairs () and one single bond (). So . Atom 1 has .
Step 4. (d) Nitrogen. It has no lone pair and four bonding pairs (). So . Oxygen a gets , and oxygens b and c get each.
Step 5. (e) and (f) Bond orders. Ozone has 3 bonds over 2 equal O-O positions: . Nitrate has 4 bonds over 3 equal N-O positions: .
Answer: (a) electrons. (b) . (c) . (d) . (e) . (f) .
Check. The formal charges must add up to the charge of the species. Ozone: . Nitrate: . Both are right. A bond order of also fits a bond length between those of the single and the double bond.
Watch out. A formal charge is not the real charge on an atom. It is a book-keeping number that helps us choose between Lewis structures.
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