2.2.4
Ek atom mein nucleus electrons ko kheench raha hai, aur baaki electrons shield aur repel kar rahe hain. Ionization energy (IE) basically yeh pooch rahi hai: "Ek electron churaana kitna mushkil hai?" Iska jawaab nuclear attraction aur electron shielding/repulsion ke beech ki tug-of-war par depend karta hai. Har trend aur har "ajeeb" anomaly bas yahi tug-of-war hai jo play out ho rahi hai.
Definition Ionization energyFirst ionization energy (I E 1 ek mole se, unke ground state mein, sabse loosely held electron ko remove karne ke liye chahiye.
X ( g ) → X + ( g ) + e − Δ H = I E 1 > 0 Second ionization energy (I E 2 X + ( g ) → X 2 + ( g ) + e − Δ H = I E 2
Hamesha positive (endothermic) hoti hai — electron bahar nikaalte waqt nucleus se ladna padta hai.
Gaseous atoms ke liye measure ki jaati hai taaki koi bonding/lattice energy interfere na kare.Units: usually kJ mol − 1 1 eV = 96.49 kJ mol − 1
Intuition Coulomb hi poori kahani haiEk rough hydrogen-like model shell n E n = − n 2 Z eff 2 × 13.6 eV + ∣ E n ∣ I E ≈ 13.6 eV × n 2 Z eff 2 Isse machine ki tarah padho:
Z eff ↑⇒ I E ↑ n ↑ ⇒ I E ↓
Effective nuclear charge: Z eff = Z − S S S
Yeh akela formula dono periodic trends explain karta hai 👇
Yeh period ke across smooth increase ko "todti" hain. Do causes hain.
Mnemonic Do "dips" yaad rakho"Be Bored, N O!"
Be > B : full s p N > O : half-filled p 3 p 4 pehle waala unusually stable hai, na ki anomalous element unstable hai.
Worked example IE jumps se electron configuration padhnaMg ke successive IEs (kJ/mol): 738 , 1451 , 7733 , 10540 , …
I E 2 I E 3 KYUN? Mg hai [ Ne ] 3 s 2 noble-gas core [ Ne ] Rule: g sudden large jump batata hai ki atom ke g Group 2 .
Common mistake Common errors ko steel-man karnaGalti A: "Zyada protons hamesha higher IE dete hain, koi exception nahi."
Kyun sahi lagta hai: Z eff default hai.
Fix: IE yeh bhi depend karta hai ki electron kis orbital se nikala ja raha hai aur pairing repulsion kaisi hai. Jab woh effects extra proton par bhaari pad jaate hain (B<Be, O<N), trend dip karta hai. Orbital compare karo, sirf Z
Galti B: "Na ka I E 2 I E 1
Kyun sahi lagta hai: successive IEs hamesha gradually badhte hain... yeh assumption hai.
Fix: Na hai [ Ne ] 3 s 1 I E 2 enormous jump (~9× bada). Core-breaking jumps dhyaan se dekho.
Galti C: "IE aur electron affinity same magnitude/sign ke hain."
Fix: IE woh energy hai jo electron remove karne mein absorb hoti hai (endothermic, +). Yeh EA (ek add karna) se alag process hai. Inhein mix mat karo.
Worked example Coulomb model se Li kaI E 1 = 1 s 2 2 s 1 2 s n = 2 Z = 3 1 s
Step: S ≈ 2 × 0.85 = 1.70 KYUN? Inner electrons nuclear charge ka zyaadatar hissa block karte hain.Z eff = 3 − 1.70 = 1.30 I E ≈ 13.6 × 2 2 1.3 0 2 = 13.6 × 4 1.69 ≈ 5.75 eV Check karo: experimental I E 1 ( Li ) = 5.39 eV Chhoti error kyun? Slater's rules approximate hain aur real orbital perfectly hydrogen-like nahi hai.
Recall Quick self-test (answers chupaao)
I E 2 > I E 1 Z Z eff Group mein neeche IE kyun girta hai? → naya shell, bada n
I E ( B ) < I E ( Be ) 2 p I E ( O ) < I E ( N ) 2 p 4 p 3 3 electrons remove karne ke baad jump ka matlab? → 3 valence electrons ⇒ Group 13.
Recall Feynman: ek 12-saal ke bachche ko samjhaoSocho har electron ek bachcha hai jo ek magnet (nucleus) se haath mila ke khada hai. Ionization energy yeh hai ki us bachche ko tod ke alag karne ke liye kitni mehnat lagti hai. Agar magnet zyada strong hai (zyada protons), toh todna mushkil hai. Agar bachcha door outer ring mein khada hai, ya baaki bachche magnet ko block kar rahe hain, toh todna aasaan hai. Kabhi kabhi ek bachcha ek bheed wali jagah khada hota hai jahan do bachche ek doosre ko dhakka de rahe hain — woh pehle se ek doosre ko hata rahe hain, toh ek ko kheenchna kam mehnat maangta hai. Isliye oxygen nitrogen se zyada aasaani se electron chodta hai!
First ionization energy ki definition Ek mole gaseous ground-state atoms se sabse loosely held electron remove karne ki minimum energy: X ( g ) → X + ( g ) + e −
IE hamesha positive (endothermic) kyun hoti hai Nuclear attraction overcome karke negative electron kheenchne ke liye energy supply karni padti hai.
Same element ke liye I E 2 > I E 1 Kam electrons lekin same nuclear charge → higher Z e f f
Period ke across trend aur kyun IE badhti hai; Z Z e f f ↑ ↓
Group mein neeche trend aur kyun IE ghatti hai; naye shells n
I E 1 ( B ) < I E 1 ( B e ) B ka outer electron higher-energy 2 p 2 s 2 2 s 2
I E 1 ( O ) < I E 1 ( N ) N ka stable half-filled 2 p 3 2 p 4
Hydrogen-like IE formula I E ≈ 13.6 eV × Z e f f 2 / n 2 Z e f f = Z − S
Successive IEs mein sudden large jump kya indicate karta hai Atom noble-gas core tak pahunch gaya; aasaani se remove hone wale electrons ki sankhya = valence electrons = group.
Z e f f Actual nuclear charge Z S
Gaseous ground-state atoms
Effective nuclear charge Zeff
Shielding and repulsion S
Across period IE increases
