Noesis
2.3.2Chemical Bonding

Formal charge calculation — best resonance structure

1,872 words9 min readdifficulty · medium3 backlinks
🇮🇳 Yeh note Hinglish mein padho →

WHAT is formal charge?

HOW to derive it from first principles

A useful equivalent form (bonds n=B/2n = B/2): FC=V−L−n\text{FC} = V - L - n

Rules for the BEST resonance structure

Figure — Formal charge calculation — best resonance structure

Worked Example 1 — CO₂ (O=C=O)

Atom-by-atom. Valence: C = 4, O = 6.

Carbon (2 double bonds → B=8B=8, L=0L=0): FCC=4−0−12(8)=0\text{FC}_C = 4 - 0 - \tfrac12(8) = 0 Why this step? C has 4 bonds = 8 bonding electrons; it owns half (4), equal to its valence → neutral.

Each Oxygen (1 double bond → B=4B=4, plus 2 lone pairs → L=4L=4): FCO=6−4−12(4)=6−4−2=0\text{FC}_O = 6 - 4 - \tfrac12(4) = 6 - 4 - 2 = 0 Why this step? Owns 4 lone-pair + 2 shared = 6 = its valence → neutral.

Checksum: 0+0+0=0=0+0+0 = 0 = charge of CO₂. ✔ This all-zero structure is the best.

Worked Example 2 — CO (best structure has a triple bond)

Try structure :C≡O: (triple bond, each atom one lone pair).

Carbon (B=6B=6, L=2L=2): FCC=4−2−12(6)=4−2−3=−1\text{FC}_C = 4 - 2 - \tfrac12(6) = 4 - 2 - 3 = -1

Oxygen (B=6B=6, L=2L=2): FCO=6−2−12(6)=6−2−3=+1\text{FC}_O = 6 - 2 - \tfrac12(6) = 6 - 2 - 3 = +1

Why prefer the triple bond? A double-bonded form C=O gives FCC=−2\text{FC}_C=-2, FCO=0\text{FC}_O=0 — magnitude 2 is worse (Rule 2). Triple bond keeps magnitudes at ±1. The odd part: negative sits on less electronegative C — nature accepts this to avoid a larger charge, and it's why CO is a weird, reactive molecule.

Checksum: −1+1=0-1 + 1 = 0. ✔

Worked Example 3 — Thiocyanate SCN⁻ (which resonance form dominates?)

Overall charge =−1=-1. Valence: S=6, C=4, N=5.

Form A: [S=C=N]−[\text{S}=\text{C}=\text{N}]^- (S: 1 double bond + 2 LP; C: 2 double bonds; N: 1 double bond + 2 LP)

  • S: 6−4−12(4)=06 - 4 - \tfrac12(4) = 0
  • C: 4−0−12(8)=04 - 0 - \tfrac12(8) = 0
  • N: 5−4−12(4)=−15 - 4 - \tfrac12(4) = -1

Form B: [S ⁣− ⁣C ⁣≡ ⁣N]−[\text{S}\!-\!\text{C}\!\equiv\!\text{N}]^- (S: 1 single + 3 LP; C: 4 bonds; N: 1 triple + 1 LP)

  • S: 6−6−12(2)=−16 - 6 - \tfrac12(2) = -1
  • C: 4−0−12(8)=04 - 0 - \tfrac12(8) = 0
  • N: 5−2−12(6)=05 - 2 - \tfrac12(6) = 0

Both have one atom at −1-1 (checksum −1-1 ✔ each). Rule 3 decides: the −1-1 should sit on the more electronegative atom. N (3.0) > S (2.6), so Form A (charge on N) is the major contributor.

Why this step? Both pass checks equally, so we go to the tie-breaker: electronegativity placement of the negative charge.

Common mistakes

Recall (test yourself)

Recall Fill in the formula and reasoning
  • FC =V−L−12B= V - L - \tfrac12 B, derived from electrons started with − electrons now owned.
  • Owns all lone pairs, half of each bond.
  • Best structure: most zeros → smallest magnitudes → negative on most electronegative atom.
Recall Feynman: explain to a 12-year-old

Imagine every atom brought some toys (electrons) to a party. When two atoms hold hands (a bond), they must share those held toys — each keeps half. Toys an atom sits on alone (lone pairs) stay all theirs. Now compare: does each kid have as many toys as they came with? If a kid has fewer, they're "++" (gave some away); if more, they're "−-". The fairest, happiest party — where almost everyone has exactly what they came with — is the "best" picture of the molecule.

Connections

  • Lewis Structures — FC is applied on top of a Lewis diagram.
  • Resonance Structures — FC ranks which resonance form matters most.
  • Electronegativity — the tie-breaker for placing charges.
  • Oxidation Number — contrast: unequal (all-to-the-electronegative) sharing vs FC's equal sharing.
  • Octet Rule — best structures usually satisfy octets AND minimise FC together.
  • VSEPR Theory — chosen structure feeds geometry prediction.

Flashcards

Formula for formal charge (in symbols)
FC=V−L−12B\text{FC} = V - L - \tfrac12 B (valence − lone-pair e⁻ − half of bonding e⁻)
In the FC formula, why do we take half the bonding electrons?
Bonds are shared equally in this model, so an atom "owns" only half of each bond's electrons.
FC of C in CO₂ (O=C=O)
4−0−12(8)=04 - 0 - \tfrac12(8) = 0
FC of O in CO₂
6−4−12(4)=06 - 4 - \tfrac12(4) = 0
Rule 1 for best resonance structure
Minimise the number of atoms bearing a formal charge (maximise zeros).
Where should a negative formal charge sit?
On the most electronegative atom.
Where should a positive formal charge sit?
On the least electronegative atom.
Checksum rule for formal charges
Sum of all formal charges must equal the overall charge of the species.
Is formal charge the real charge on the atom?
No — it assumes perfectly equal sharing (ignores electronegativity); it's a bookkeeping tool.
In SCN⁻, why is the negative charge preferred on N over S?
N is more electronegative than S, so it better stabilises the negative formal charge.
FC of C in the CO structure :C≡O:
4−2−12(6)=−14 - 2 - \tfrac12(6) = -1
Between ±1 and ±2 formal charge magnitudes, which structure is better?
The one with smaller magnitudes (±1 beats ±2).

Concept Map

often yields

need to rank

referees

derives

defines

input to

input to

input to

minimise nonzero FC

neg on electronegative atom

validates

applied to

Lewis structures

Resonance structures

Best structure

Formal Charge FC

Started minus kept

FC = V - L - half B

Valence electrons V

Lone-pair electrons L

Bonding electrons B

Ranking rules

Sum FC = overall charge

CO2 and CO examples

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, formal charge ek accounting trick hai. Har atom molecule me apne valence electrons leke aata hai. Ab rule simple hai: jo lone pair electrons hain wo poore uske apne, lekin jo bond me hain wo aadhe-aadhe baant lo (kyunki bond me sharing hoti hai). Formula: FC=V−L−12B\text{FC} = V - L - \tfrac12 B — yaani "jitne electrons leke aaya − jitne ab uske paas hain". Agar kam ho gaye toh ++, zyada ho gaye toh −-.

Iska sabse bada use hai best resonance structure dhoondhna. Jab ek molecule ki kai valid Lewis structures ban sakti hain, toh hum poochte hain: konsi structure sabse "khush" hai? Rule: (1) zyada se zyada atoms ka FC zero ho, (2) charge ka magnitude chhota ho (±1\pm1 better than ±2\pm2), (3) negative charge zyada electronegative atom pe baithe, aur (4) positive charge kam electronegative pe. Aur ek checksum hamesha lagao — saare FC ka sum molecule ke total charge ke barabar hona chahiye, warna kahin galti hai.

Example samjho — CO₂ me sabke FC = 0, toh wahi best hai, seedha. SCN⁻ me do form hain: ek me −1-1 N pe, ek me −1-1 S pe. Dono valid, dono checksum pass — toh tie-breaker electronegativity: N zyada electronegative hai, isliye N-wali form (Form A) major contributor hai. Bas yaad rakhna: formal charge real charge nahi hai, ye sirf equal-sharing maan ke banaya gaya bookkeeping number hai jo humein sahi picture choose karne me madad karta hai.

Go deeper — visual, from zero

D1FoundationsEvery symbol from zero · 1,568 wordsD2Visual walkthroughThe derivation in pictures · 1,862 wordsD3Worked examplesEvery scenario covered · 3,109 wordsD4ExercisesGraded, full solutions · 2,320 wordsD5Question bankConcept traps, quick reveals · 2,089 words

Test yourself — Chemical Bonding

L1 RecognitionL2 RecallL3 ProductionL4 ApplicationL5 Mastery

Connections

Electronegativity — Pauling, Mulliken, Allred-Rochow scalesChemistry · 2.2.6Oxidation number rules — assigning, changeChemistry · 1.3.6Octet rule — Lewis structures, exceptions (incomplete octet, expanded octet, odd-electron species)Chemistry · 2.3.1VSEPR theory — geometry from electron pairs (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, etcChemistry · 2.3.8Octet rule — Lewis structures, exceptions (incomplete octet, expanded octet, odd-electron species)Chemistry · 2.3.1VSEPR theory — geometry from electron pairs (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, etcChemistry · 2.3.8Resonance — delocalization, resonance energy (benzene, ozone, carbonate)Chemistry · 2.3.15