1.3.6
Intuition Badi picture (YEH kyun exist karta hai)Molecules mein atoms electrons share karte hain, lekin sharing kabhi fair nahi hoti — jo atom zyada greedy hota hai (zyada electronegative), woh shared electrons ko apne paas rakh leta hai. Oxidation number ek bookkeeping trick hai: hum pretend karte hain ki greedy atom shared electrons ko poori tarah le leta hai, phir us pretend duniya mein har atom ka charge count karte hain.
YEH invent kyun kiya? Taaki hum track kar sakein ki kisne electrons lose/gain kiye ek reaction (redox) mein — bina kuch measure kiye, sirf counting se.Yeh ek hypothetical charge hai, real charge nahi. Bas yahi poora game hai.
Definition Oxidation number (oxidation state)Woh charge jo ek atom ke paas hota agar har bond ko ionic treat kiya jaaye , yaani saare shared electrons poori tarah zyada electronegative atom ko de diye jaayein.
Intuition "Pretend fully ionic" rule kyun kaam karta haiReal H 2 O − 2 thoda khiche hote hain. Lekin electron transfer count karne ke liye, hum exaggerate karte hain: O ko dono bonding pairs de do. Tab O extra electrons "own" karta hai ⇒ negative; H apna electron "khota" hai ⇒ positive. Yeh exaggeration consistently cancel ho jaati hai, isliye changes sahi track hote hain.
Intuition Sum rule master key kyun haiOxidation numbers pretend charges hain; pretend charges ka sum real total charge ke barabar hona chahiye , kyunki electrons sirf redistribute hote hain, create nahi. Yeh akela rule tumhe unknown atom solve karne deta hai.
∑ i n i ⋅ ( ox no ) i = Q species
jahan n i i Q
Worked example 1. Sulfur inH 2 S O 4 = x = 0 2 ( + 1 ) + x + 4 ( − 2 ) = 0 Yeh step kyun? H + 1 − 2 = 0 2 + x − 8 = 0 ⇒ x = + 6 + 6
Worked example 2. Chromium in dichromateC r 2 O 7 2 − 2 x + 7 ( − 2 ) = − 2 Yeh step kyun? Net charge − 2 0 − 2 2 x − 14 = − 2 ⇒ 2 x = 12 ⇒ x = + 6 + 6
Worked example 3. Oxygen inH 2 O 2 2 ( + 1 ) + 2 x = 0 ⇒ x = − 1 Yeh step kyun? Yahaan O = − 2 − 1
Worked example 4. "Average" oxidation number: S inN a 2 S 4 O 6 2 ( + 1 ) + 4 x + 6 ( − 2 ) = 0 ⇒ 4 x = 10 ⇒ x = + 2.5 Fractional kyun? 4 sulfurs saare equivalent nahi hain; + 2.5 average hai. Oxidation number fractional ho sakta hai kyunki yeh bookkeeping average hai, real per-atom charge nahi.
Definition Oxidation vs reduction
Oxidation = oxidation number badh jaata hai (electrons lose hote hain).Reduction = oxidation number ghad jaata hai (electrons gain hote hain).Neeche mnemonic. Jis atom ka number upar jaata hai woh oxidised hota hai; woh reducing agent hai.
Worked example Change tracking:Z n + C u 2 + → Z n 2 + + C u
Atom
Pehle
Baad
Change
Zn
0 + 2 + 2
Cu
+ 2 0 − 2
Balance kyun? Zn ke electrons lost (2 e − 2 e − zero mein sum hona chahiye — electron conservation.
Common mistake Steel-manned common errorsA. "O hamesha − 2
Kyun sahi lagta hai: ~95% cases mein sach hai.
Fix: Peroxides (− 1 − 2 1 O F 2 + 2
B. "Oxidation number = atom par real charge."
Kyun sahi lagta hai: Monatomic ions (N a + hote hain .
Fix: Covalent molecules mein yeh pretend charge hai. S H 2 S O 4 + 6 + 6
C. Ion ke liye net charge = 0
Kyun sahi lagta hai: Neutral compounds practice mein zyada common hain.
Fix: Ions ke liye, sum = C r 2 O 7 2 − − 2
D. "Oxidation numbers whole hone chahiye."
Kyun sahi lagta hai: Charges integers hote hain.
Fix: Averages fractional ho sakte hain (S 4 O 6 2 − + 2.5 F e 3 O 4 + 3 8
Recall Feynman: ek 12-saal ke bachche ko explain karoSocho bachche candy share kar rahe hain lekin sabse bada baccha hamesha poora piece le leta hai. Oxidation number ek scoreboard hai: hum pretend karte hain ki bade bachche ne saari shared candy le li, phir count karte hain ki har bachche ke paas kitna extra (+ − upar jaata hai, unhone candy de di (woh "oxidised" ho gaye); agar neeche jaata hai, unhone kuch grab kiya. Scoreboard changes cancel hone chahiye — candy kabhi destroy nahi hoti, sirf move hoti hai.
Mnemonic Direction yaad rakhoOIL RIG — O xidation I s L oss (of electrons), R eduction I s G ain.
Aur agent flip: jo cheez oxidised hoti hai woh electrons deti hai ⇒ woh reducing agent hai.
Recall Active recall — answers cover karo
Free element ka oxidation number? → 0
N a 2 O 2 − 1 Unknowns solve karne ki master equation? → ox. nos. ka sum = net charge.
Oxidation = ? → oxidation number mein increase / electrons ka loss.
Oxidation number kya hota hai? Woh charge jo atom ke paas hota agar saare shared bonding electrons zyada electronegative atom ko assign kar diye jaayein (pretend fully ionic).
Free element (jaise O₂, Na) mein atom ka oxidation number? 0
Monatomic ion ka oxidation number? Uske ionic charge ke barabar.
Oxygen ka usual oxidation number, aur uske exceptions? −2 usually; −1 peroxides mein, −½ superoxides mein, +2 OF₂ mein.
Metal hydrides jaise NaH mein H ka oxidation number? −1.
Oxidation numbers ki sum rule batao. Saare atoms ke oxidation numbers ka sum = species ka net charge.
H₂SO₄ mein S find karo. +6 (from 2(+1)+x+4(−2)=0).
Cr₂O₇²⁻ mein Cr find karo. +6 (from 2x+7(−2)=−2).
Oxidation number ke terms mein oxidation define karo. Oxidation number mein increase (electrons ka loss).
Oxidation number ke terms mein reduction define karo. Oxidation number mein decrease (electrons ka gain).
Oxidation numbers fractional kyun ho sakte hain (jaise +2.5)? Yeh non-equivalent atoms par ek average hai; yeh bookkeeping hai, real per-atom charge nahi.
Zn + Cu²⁺ → Zn²⁺ + Cu mein kaun oxidised hota hai? Zn (0 → +2), woh reducing agent hai.
OIL RIG ka kya matlab hai? Oxidation Is Loss, Reduction Is Gain (of electrons).
Redox reactions — balancing (half-reaction & ion-electron) Electronegativity & periodic trends Oxidising and reducing agents Stoichiometry — mole conservation in reactions Electrochemistry — cell potentials
greedy atom hogs electrons
treats bonds as fully ionic
Electronegativity difference
Hypothetical charge, not real
