2.3.1
Atoms "stability ke liye greedy" hote hain. Noble gases (He, Ne, Ar…) famously lazy aur unreactive hain kyunki unka outermost shell full hota hai. Har doosra atom nearest noble gas jaisa "dikhna chahta" hai — usually 8 electrons apni valence shell mein rakh ke. Yeh "8 chahna" hi octet rule hai. Bonding (electrons share karna ya transfer karna) bas atoms ka us magic number tak pahunchne ka cooperation hai.
YEH KYUN MATTER KARTA HAI: Lewis structures chemistry ki "sketch language" hain. Octet rule + uske exceptions sahi se samajh lo, aur tum almost kisi bhi molecule ki shape (VSEPR), polarity, reactivity, aur acidity predict kar sakte ho.
Compounds banate waqt, atoms electrons gain, lose, ya share karte hain taaki har atom apni valence shell mein 8 electrons total haasil kare (jaise ek noble gas).
Built-in exception: H aur He 2 ko target karte hain (ek "duet"), helium se match karte hue.
8 KYUN? Valence shell mein ek s p = 8 s 2 p 6
Worked example CO₂ (octet follow karta hai)Step A: C(4) + 2×O(6) = 16 electrons.
Kyun? C group 14 mein hai → 4 valence; har O group 16 mein hai → 6.
Step B: N need = 8 × 3 = 24 Step C: N shared = 24 − 16 = 8 ⇒ 4 Yeh step kyun? 4 bonds 2 C–O connections mein split hone chahiye → har ek double bond hai.
Step D: Har O ko do lone pairs do. Check karo: har O ke paas 2 bonds (4 e⁻ shared) + 2 lone pairs (4 e⁻) = 8 ✓. C ke paas 4 bonds = 8 ✓.
C O 3 2 − A: C(4)+3×O(6)+2(charge) = 24.
+2 kyun? 2– charge matlab 2 extra electrons.
B: 8 × 4 = 32 C: 32 − 24 = 8 → 4 FC check: double-bonded O: 6 − 4 − 2 = 0 6 − 6 − 1 = − 1 4 − 0 − 4 = 0 = − 2 Resonance kyun? Double bond teeno O's mein se kisi par bhi equally baith sakta hai → real structure average hai (bond order = 4/3
incomplete octet exception
A: B(3)+3×F(7)=24. B: 32. C: 32 − 24 = 8 → 4 Trap ko steel-man karo: formula suggest karta hai ek B=F double bond, B ko octet deta hai. Lekin B par FC hoga 3 − 0 − 4 = − 1 7 − 4 − 2 = + 1 Fix: B sirf 6 electrons ke saath rehta hai (incomplete octet). B electron-deficient hai → Lewis acid ki tarah act karta hai.
expanded octet
S(6)+6×F(7)=48. Chhe S–F bonds → S ke around 12 electrons hain.
Kyun allowed hai? S period 3+ mein hai; uske paas vacant 3 d 8 se zyada hold kar sakta hai (hypervalency). Sirf period-3-aur-usse-neeche ke elements expand kar sakte hain. N, O, F kabhi bhi expand nahi karte (koi low-energy d nahi, aur size bhi choti hai).
odd-electron (radical)
N(5)+O(6)=11 electrons — ek odd number! Har atom ko pura even octet dena impossible hai. Ek atom (usually N) ek single unpaired electron rakhta hai. Aisi species paramagnetic aur reactive hoti hain (free radicals jaise NO, NO₂, ClO₂).
Type
Central atom e⁻
Examples
Reason
Incomplete octet
<8
BeCl₂ (4), BF₃, BCl₃ (6) bahut kam electrons available hain
Expanded octet
>8
PCl₅ (10), SF₆ (12), XeF₄ vacant d-orbitals, period ≥3
Odd-electron
unpaired
NO, NO₂, ClO₂ total valence electrons odd hain
Common mistake "Har atom ko exactly 8 hone chahiye, hamesha."Kyun sahi lagta hai: teachers "octet" baar baar bolte hain. Reality: yeh ek tendency hai, law nahi. H 2 chahta hai; B/Be kam se settle karte hain; P/S/Xe zyada hold kar sakte hain; radicals fully pair nahi kar sakte. Fix: 8 ko default maano, phir teen exception categories check karo.
Common mistake Electron count mein charge bhool jaana.S O 4 2 − Fix: Step A mein anion charge ke liye hamesha + −
Common mistake N ya O ka octet expand karna.Students N ko 5 bonds ke saath draw karte hain (jaise HNO₃ mein N=O double aur extra bond). Kyun tempting lagta hai: yeh formal charges "cancel" karta hai. Fix: N max = 4 bonds; + / −
Octet rule kitne valence electrons target karta hai? 8 (ek full s 2 p 6
"Duet" exception kya hai? H aur He 2 electrons target karte hain, helium se match karte hue.
Lewis structure mein total shared electrons ka formula? N s ha r e d = N n ee d − N t o t a l = N s ha r e d /2
Formal charge formula? F C = V − L − 2 1 B
Best Lewis structure criterion? Sabse chhote formal charges; negative FC most electronegative atom par.
Ek incomplete-octet example aur uske central-atom electron count do. BF₃ — boron ke paas sirf 6 electrons hain (electron-deficient Lewis acid).
S apna octet expand kar sakta hai lekin N nahi kar sakta, kyun? S (period 3) ke paas accessible vacant d-orbitals hain; N (period 2) ke paas koi nahi aur size bhi choti hai.
Do expanded-octet molecules batao. PCl₅ (10 e⁻), SF₆ (12 e⁻).
NO ke electron count mein kya special hai? 11 valence electrons — odd → ek unpaired electron → paramagnetic radical.
−n charge ke anion ke liye electron count kaise adjust karte hain? Total mein n electrons add karo.
BF₃ arithmetic ke bawajood B=F double bond kyun NAHI banata? Isse fluorine (most electronegative) par +1 formal charge aata — energetically absurd hai.
Recall Feynman: ek 12-saal ke bachche ko explain karoSocho har atom ek bachcha hai jo tab safe feel karta hai jab uski pocket mein 8 marbles hoon (sirf chhota hydrogen 2 se khush hai). 8 tak pahunchne ke liye, bachche marbles share karte hain haath pakad ke (bonds) ya marbles dete/lete hain. Zyadatar bachche exactly 8 tak pahunch jaate hain aur khush rehte hain. Lekin kuch weird hote hain: boron ek chill bachcha hai jo sirf 6 se khush hai, sulfur greedy hai aur 12 pakad leta hai, aur kuch bachche jaise NO ek odd number ke saath end hote hain toh ek marble ka koi partner nahi hota — woh lonely marble unhe restless banata hai aur react karne ko eager.
Mnemonic Teen exceptions yaad karo:"I Expand Oddly"
I = I ncomplete (Be, B — bahut kam), E = E xpanded (P, S, Xe — bahut zyada, d-orbitals chahiye), O = O dd-electron (NO, NO₂, ClO₂ — radicals). Aur: "Fewer, More, Lone-odd."
VSEPR theory — Lewis structure → electron domains → molecular shape.Formal charge & resonance — valid structures choose karna/average karna.Electronegativity — decide karta hai negative formal charge kahan belong karta hai.Lewis acids and bases — incomplete-octet species (BF₃) electron-pair acceptors hain.Hypervalency & d-orbital participation — expanded octets.Free radicals — odd-electron species aur unki reactivity.Periodic trends — kyun sirf period ≥3 elements octets expand karte hain.
s2 p6 fills valence shell
Predict shape polarity reactivity
