2.2.5
Intuition Ek line mein pictureEk atom ek positive nucleus hai jo electrons se ghira hua hai. Jab hum ek extra electron ko ek neutral gaseous atom pe push karte hain, to atom ya toh use hug karta hai (energy release hoti hai) ya reject karta hai (energy chahiye hoti hai). Electron gain enthalpy measure karta hai ki yeh process mein kitni energy release hoti hai (usually).
Definition Electron gain enthalpy (Δ e g H Enthalpy change jab one mole of electrons ko one mole of gaseous atoms (ya ions) mein add kiya jaata hai taaki gaseous negative ions ban sakein:
X ( g ) + e − ⟶ X − ( g ) Δ e g H
Agar energy release hoti hai → Δ e g H negative hai (exothermic, atom ko electron pasand hai).
Agar energy supply karni padti hai → Δ e g H positive hai (endothermic, atom resist karta hai).
Definition Electron affinity (A e Energy released same process ke liye, opposite sign convention ke saath define ki gayi hai:
A e = − Δ e g H ( at 0 K , ignoring the small 2 5 R T term ) bada positive electron affinity = ek bada negative electron gain enthalpy = atom strongly electron chahta hai. Yeh same idea hai, opposite sign ke saath.
DO quantities kyun? Chemists historically "affinity" ki baat karte the (atom electrons ko kitna pasand karta hai → positive = achha). Thermodynamics enthalpy prefer karta hai jahan exothermic = negative hota hai. Signs mat mix karo.
Intuition Physics ko feel karoAane wala electron door se aata hai aur essentially zero energy ke saath hota hai. Jab yeh partly-empty orbital mein girta hai, nucleus ki positive charge use attract karti hai → system lower energy pe aa jaata hai → woh energy heat ke roop mein chali jaati hai. Effective nuclear pull jitna strong hoga aur orbital mein jitna zyada room hoga, utni zyada energy release hogi.
Do competing effects magnitude decide karte hain:
Effect
Δ e g H Reason
Strong effective nuclear charge Z e f f
zyada negative
naye electron ke liye tighter attraction
Chhota atom / compact orbital
zyada negative ...
nucleus ke zyada paas
...lekin bahut chhota → electron–electron repulsion
zyada positive
naya electron crowded, tiny orbital mein thunsa jaata hai
Woh tension (attraction vs. crowding) hi poori kahani hai anomalies ke peeche.
Intuition Full aur half-full shells electrons kodoor dhakelte hain
Noble gases (Ne, Ar): shell complete hai → naya electron next shell mein jaana padta hai → strongly positive Δ e g H Group 2 (Be, Mg) aur Group 15 (N, P) : filled n s 2 n p 3 resist karte hain → Δ e g H
Intuition Tiny F atom Cl se kyun haarta haiAap expect karte ki F (chhota, higher Z e f f 2p orbital tiny aur already packed hai electrons se. Ek aur add karna use cramped space mein force karta hai → huge electron–electron repulsion nuclear attraction ko partly cancel kar deta hai. Cl ki 3p orbital larger aur roomier hai , isliye naya electron comfortably fit ho jaata hai → net energy released zyada hoti hai.
Δ e g H ( F ) ≈ − 328 kJ/mol , Δ e g H ( Cl ) ≈ − 349 kJ/mol
Cl zyada negative hai → Cl ki higher electron affinity hai. Same reason: O < S aur N < P second-row-being-smaller anomaly ke liye.
"Chlorine ke paas bada ghar hai, isliye woh guest ka swagat karta hai." Chhota F = cramped room, guest ke stay ka kuch part reject karta hai. Compact 2p ⇒ repulsion ⇒ anomaly.
Worked example 1. Neon ke liye sign predict karoQ: Kya Δ e g H ( Ne ) Step 1 — config: Ne hai 1 s 2 2 s 2 2 p 6 complete shell. Kyun? Yeh determine karta hai ki room hai ya nahi.
Step 2 — naya electron kahan jaayega? 3 s Kyun? n=2 mein space nahi hai.
Step 3 — verdict: weak attraction + high-energy shell occupy karni padegi → energy supply karni padegi → Δ e g H positive hai (≈ +116 kJ/mol). ✅
Δ e g H Step 1 — configs: C 2 p 2 2 p 3 2 p 4 2 p 5 Step 2 — trend apply karo: period mein across → zyada negative, except N ki half-filled stability. Kyun? Stable configs electrons resist karte hain.
Step 3 — result: value mein F < O < C < N F most negative , N least (even positive-ish). Order of magnitude: F ≈ −328, O ≈ −141, C ≈ −122, N ≈ +0 to +7 . ✅
N rank kyun todta hai? Half-filled 2 p 3
Worked example 3. Explain karo kyunΔ e g H 2 doosra electron add karna) hamesha positive hota hai
Q: O − ( g ) + e − → O 2 − ( g ) Step 1: species O − pehle se negative hai. Kyun matters? Like charges repel karte hain.
Step 2: ek aur electron ek negative ion pe force karna matlab Coulomb repulsion se ladna → energy supply karni padti hai.
Step 3: Δ e g H 2 hamesha positive hota hai (endothermic), jaise O: +744 kJ/mol. Isliye oxide formation overall ke liye lattice energy ki zaroorat padti hai taaki yeh favourable ho. ✅
Common mistake "Fluorine ki highest electron affinity honi chahiye kyunki yeh smallest aur most electronegative hai."Kyun sahi lagta hai: F hai most electronegative element aur smallest halogen; chhota size usually = strong attraction. Flaw: electronegativity (bonding tendency) ≠ electron gain enthalpy (isolated-atom energetics). F ki 2p itni compact hai ki electron–electron repulsion attraction ka kuch part cancel kar deti hai. Fix: Cl > F. Yaad rakho size too small bhi ho sakta hai.
Δ e g H Kyun sahi lagta hai: "positive = bada/achha" everyday language mein. Flaw: sign conventions opposite hain. Positive Δ e g H Fix: A e = − Δ e g H Δ e g H
Common mistake Doosre electron gain ko exothermic report karna.Kyun sahi lagta hai: pehle wale ne energy release ki, toh zaroor agla bhi karega. Flaw: ion ab negative hai, electron ko repel kar raha hai. Fix: Δ e g H 2 > 0
Recall Feynman: 12-year-old ko explain karoSocho ek atom ek ghar hai aur electrons mehmaan hain. Zyaadatar atoms ek aur mehmaan ko khushi se andar aane dete hain aur tumhe thodi "shukriya" energy bhi dete hain. Chlorine ek bada aramadayak ghar hai — kaafi jagah hai — isliye yeh extra mehmaan ka warmly swagat karta hai aur bahut energy release karta hai. Fluorine ek tiny cramped ghar hai: yeh mehmaan chahta toh hai lekin uske kamre itne packed hain ki mehmaan sabse takraata hai, isliye yeh sirf thodi energy wapas de pata hai — roomy chlorine se kam! Aur ek full house (neon) simply bina ladai ke kisi ko andar nahi aane deta.
Δ e g H electron pasand karta hai ? ::: Negative (exothermic).Cl ka Δ e g H
Define electron gain enthalpy Δ e g H Enthalpy change jab 1 mole electrons ko 1 mole gaseous atoms mein add kiya jaata hai: X ( g ) + e − → X − ( g )
Electron affinity A e Δ e g H A e = − Δ e g H
Δ e g H Negative (exothermic).
Ek period mein Δ e g H Zyada negative hota jaata hai (rising Z e f f
Group mein neeche Δ e g H Generally less negative hota jaata hai (larger atom, weaker attraction).
Cl ka Δ e g H F ki compact 2p orbital large e⁻–e⁻ repulsion cause karti hai, nuclear attraction ka kuch part cancel kar deti hai; Cl ki larger 3p electron ko better accommodate karti hai.
Noble gases ke liye Δ e g H Positive (complete shell; electron ko next shell mein jaana padta hai).
N aur P ka Δ e g H Half-filled n p 3
Doosre electron gain enthalpy Δ e g H 2 Hamesha positive (already negative ion pe e⁻ add karna → repulsion).
Kaun se pairs "second-period-too-small" anomaly dikhate hain? F<Cl, O<S, N<P (period-3 period-2 se zyada negative).
F aur Cl ka approx Δ e g H F ≈ −328, Cl ≈ −349.
attraction, more negative
Electron gain enthalpy ΔegH
Energy released, exothermic
Energy needed, endothermic
Effective nuclear charge Zeff
Electron-electron repulsion
