Formal charge in ozone: why does one oxygen get +1 and another -1? I keep getting the formula wrong

7 answers & replies

• [+][–]Arjun Nair12thSample· 1mo ago

  Formula is missing a piece. Formal charge:

  $FC = V - N - \frac{B}{2}$

  where $V$ = valence electrons of the free atom, $N$ = non-bonding (lone pair) electrons on the atom, $B$ = bonding electrons shared by it.

  Central O in O3: $6 - 2 - 6/2 = +1$. Terminal O with the double bond: $6 - 4 - 4/2 = 0$. Terminal O with the single bond: $6 - 6 - 2/2 = -1$. Sum: $+1 + 0 - 1 = 0$, which matches the neutral molecule. Always do this sum check, it catches mistakes instantly.

  You forgot to subtract the lone pair electrons and halve the bonding ones.

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  • [+][–]Yash Tiwari11thSample· 1mo agoAuthor

    bhai ab samajh aaya. maine lone pair electrons subtract hi nahi kiye the aur bonding ka half bhi nahi kiya. sum check wala trick pakka yaad rakhunga 🙏

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    • [+][–]Arjun Nair12thSample· 1mo ago

      Good. Also in NCERT questions "which of the following has an expanded octet" is about $PCl_5$ and $SF_6$, not $SO_2$ drawn with charges. Don't mix those two ideas.

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• [+][–]Tanvi Deshmukh12thSample· 1mo ago

  Ozone has two resonance structures!! In one the double bond is on the left O–O, in the other on the right. The −1 and the 0 just swap sides, so both O–O bonds are identical (bond order 1.5, same bond length, about 128 pm) and the real molecule is a resonance hybrid. No actual single and double bond exists in ozone, the picture is just a book-keeping device.

  Draw both structures with arrows in between, that is all the examiner wants!

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• [+][–]Ishita GoyalDropperSample· 1mo ago

  Octet exceptions, I keep this list in my planner:

  • Incomplete octet: $BeH_2$ (4 e−), $BF_3$ and $AlCl_3$ (6 e− on the central atom)
  • Expanded octet: $PCl_5$ (10 e−), $SF_6$ (12 e−), possible because 3rd-period atoms have d orbitals available
  • Odd electron: NO (11 valence e−) and $NO_2$ (17 e−)

  Rest of the molecules like $CH_4$, $H_2O$, $NH_3$ follow octet. One-line reason for NCERT-type questions: octet rule is based on the idea of filling the valence shell with 8 electrons, but it fails when central atom is small with few electrons, big with d orbitals, or odd-electron.

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• [+][–]Vivek Mishra12thSample· 1mo ago

  My simple way: formal charge = group number − (number of lone pair electrons + number of bonds). Count each bond as 1, a double bond as 2.

  Test with $NH_4^+$: N has group number 5, 0 lone electrons, 4 bonds. $5 - (0 + 4) = +1$. Correct. Test with $NO_3^-$: N is $5 - (0 + 4) = +1$, doubly bonded O is $6 - (4 + 2) = 0$, each single-bonded O is $6 - (6 + 1) = -1$. Total is $+1 + 0 - 1 - 1 = -1$ which is the ion's charge. Works.

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• [+][–]Aman YadavDropperSample· 1mo ago

  Best Lewis structure rules. One: lowest formal charges. Two: negative charge on the more electronegative atom. Three: no like charges next to each other. Formal charge is bookkeeping, not real charge. Don't draw five structures. Draw one, count, check the sum. Done.

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