Formal charge calculation — best resonance structure

2.3.2 · Chemistry › Chemical Bonding

WHAT is formal charge?

HOW to derive it from first principles

Ek useful equivalent form (bonds $n = B /2$ ): $FC = V - L - n$

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 → $B = 8$ , $L = 0$ ): $FC_{C} = 4 - 0 - \frac{1}{2} (8) = 0$ Yeh step kyun? C ke paas 4 bonds hain = 8 bonding electrons; woh aadha own karta hai (4), jo uski valence ke barabar hai → neutral.

Each Oxygen (1 double bond → $B = 4$ , plus 2 lone pairs → $L = 4$ ): $FC_{O} = 6 - 4 - \frac{1}{2} (4) = 6 - 4 - 2 = 0$ Yeh step kyun? 4 lone-pair + 2 shared = 6 own karta hai = uski valence → neutral.

Checksum: $0 + 0 + 0 = 0 =$ charge of CO₂. ✔ Yeh all-zero structure hi best hai.

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

Structure :C≡O: try karo (triple bond, har atom ke paas ek lone pair).

Carbon ( $B = 6$ , $L = 2$ ): $FC_{C} = 4 - 2 - \frac{1}{2} (6) = 4 - 2 - 3 = - 1$

Oxygen ( $B = 6$ , $L = 2$ ): $FC_{O} = 6 - 2 - \frac{1}{2} (6) = 6 - 2 - 3 = + 1$

Triple bond kyun prefer karte hain? Double-bonded form C=O mein $FC_{C} = - 2$ , $FC_{O} = 0$ aata hai — magnitude 2 worse hai (Rule 2). Triple bond magnitude ko ±1 par rakhta hai. Odd baat yeh hai: negative charge kam electronegative C par baithta hai — nature iska accept karta hai zyada bade charge se bachne ke liye, aur yahi wajah hai ki CO ek weird, reactive molecule hai.

Checksum: $- 1 + 1 = 0$ . ✔

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

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

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

S: $6 - 4 - \frac{1}{2} (4) = 0$
C: $4 - 0 - \frac{1}{2} (8) = 0$
N: $5 - 4 - \frac{1}{2} (4) = - 1$

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

S: $6 - 6 - \frac{1}{2} (2) = - 1$
C: $4 - 0 - \frac{1}{2} (8) = 0$
N: $5 - 2 - \frac{1}{2} (6) = 0$

Dono mein ek atom $- 1$ par hai (checksum $- 1$ ✔ dono ke liye). Rule 3 decide karta hai: $- 1$ charge zyada electronegative atom par hona chahiye. N (3.0) > S (2.6), isliye Form A (N par charge) major contributor hai.

Yeh step kyun? Dono checks equally pass karte hain, toh hum tie-breaker par jaate hain: negative charge ki electronegativity placement.

Common mistakes

Recall (test yourself)

Recall Formula aur reasoning fill karo

FC $= V - L - \frac{1}{2} B$ , 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: 12-year-old ko explain karo

Socho har atom kuch toys (electrons) party mein laaya. Jab do atoms haath pakadein (a bond), toh unhe woh pakde hue toys share karne padte hain — har ek aadha rakhta hai. Jinhe atom akela baitha ke rakhta hai (lone pairs) woh saare uske paas rehte hain. Ab compare karo: kya har bachche ke paas utne toys hain jitne woh leke aaya tha? Agar kisi ke paas kam hain, toh woh " $+$ " hai (kuch de diya); agar zyada hain, toh " $-$ " hai. Sabse fair, sabse khush party — jahan almost sabke paas exactly utna hai jitna woh leke aaye the — woh molecule ki "best" tasveer hai.

Connections

Lewis Structures — FC ek Lewis diagram ke upar apply hota hai.
Resonance Structures — FC rank karta hai ki kaun sa resonance form zyada matter karta hai.
Electronegativity — charges place karne ka tie-breaker.
Oxidation Number — contrast: unequal (all-to-the-electronegative) sharing vs FC ki equal sharing.
Octet Rule — best structures usually octets satisfy karte hain AUR FC minimise bhi karte hain.
VSEPR Theory — chosen structure geometry prediction mein jaata hai.

Flashcards

Formula for formal charge (in symbols)

FC = V - L - \frac{1}{2} B

(valence − lone-pair e⁻ − half of bonding e⁻)

FC formula mein hum bonding electrons ka aadha kyun lete hain?

Is model mein bonds equally shared hote hain, isliye atom sirf har bond ke electrons ka aadha hi "own" karta hai.

FC of C in CO₂ (O=C=O)

4 - 0 - \frac{1}{2} (8) = 0

FC of O in CO₂

6 - 4 - \frac{1}{2} (4) = 0

Rule 1 for best resonance structure

Formal charge bear karne wale atoms ki sankhya minimise karo (zeros maximise karo).

Negative formal charge kahan honi chahiye?

Sabse zyada electronegative atom par.

Positive formal charge kahan honi chahiye?

Sabse kam electronegative atom par.

Checksum rule for formal charges

Saare formal charges ka sum species ke overall charge ke barabar hona chahiye.

Kya formal charge atom par real charge hota hai?

Nahi — yeh perfectly equal sharing assume karta hai (electronegativity ignore karta hai); yeh ek bookkeeping tool hai.

SCN⁻ mein negative charge S ke bajaay N par kyun prefer hota hai?

N zyada electronegative hai S se, isliye woh negative formal charge ko better stabilise karta hai.

FC of C in the CO structure :C≡O:

4 - 2 - \frac{1}{2} (6) = - 1

±1 aur ±2 formal charge magnitudes mein kaun si structure better hai?

Woh jis mein chhoti magnitudes hain (±1 beats ±2).