Group 17 (Halogens) — properties, oxidizing power; HX strengths; interhalogens; pseudohalogens

3.2.9 · Chemistry › p-Block

Members: F, Cl, Br, I, At (At radioactive hai). Valence config $n s^{2} n p^{5}$ — noble gas se ek electron kam, isliye ye family table mein sabse zyada non-metallic, oxidizing hai.

1. Halogens aisa behave kyun karte hain

General trends (upar → neeche: F → I)

Property	Trend	WHY
Atomic/ionic radius	increases	naye shells add hote hain
Ionization enthalpy	decreases	electron nucleus se door
Electronegativity	decreases (F sabse zyada EN)	bonding e⁻ pe pull kamzor
Electron gain enthalpy	Cl pe sabse zyada negative, F pe nahi	F itna chhota hai ki uske e⁻–e⁻ repulsion gain ko partly cancel kar deti hai
Oxidizing power	decreases	$X^{-}$ mein reduce hona mushkil hota hai
Bond enthalpy $X - X$	Cl > Br > F > I (F anomalous hai)	chhota F atom → lone-pair repulsion $F - F$ ko kamzor karti hai

2. Oxidizing Power — yaad mat karo, derive karo

F₂ sabse strong oxidizer kyun hai (thermodynamic cycle)

Reduction ko ek Born–Haber-style cycle mein todenge (per mole of $X_{2}$ , aqueous):

$\frac{1}{2} X_{2} atomize, \frac{1}{2} Δ_{d i ss} H X (g) Δ_{e g} H X^{-} (g) Δ_{h y d} H X^{-} (a q)$

Release hone wali total energy $E^{\circ}$ govern karti hai. F ke liye:

$Δ_{d i ss} H (F_{2})$ low hai (weak F–F bond, lone-pair repulsion) → todna aasaan.
$Δ_{h y d} H (F^{-})$ bahut high hai (chhota ion → strong hydration).

Yeh do terms F ki thodi si kharab electron gain enthalpy ko overpower kar dete hain, jisse F₂ sabse strong oxidizing agent ban jaata hai.

Figure — Group 17 (Halogens) — properties, oxidizing power; HX strengths; interhalogens; pseudohalogens

3. HX Acid Strength — counter-intuitive winner

$H X$ ki stability (thermal) bond strength ke saath chalti hai: $H F > H C l > H B r > H I$ (HI aasaani se decompose ho jaata hai).

4. Interhalogen Compounds $X X_{n}^{'}$

Type	Example	Hybridisation	Shape (VSEPR)
$X X^{'}$	$C l F, B r F, I C l$	—	linear
$X X_{3}^{'}$	$C l F_{3}, B r F_{3}$	$s p^{3} d$	bent T-shape
$X X_{5}^{'}$	$I F_{5}, B r F_{5}$	$s p^{3} d^{2}$	square pyramidal
$X X_{7}^{'}$	$I F_{7}$	$s p^{3} d^{3}$	pentagonal bipyramidal

Key facts (WHY/HOW):

Interhalogen bonds $X - X^{'}$ , $X - X$ se kamzor hote hain → interhalogens, halogens se zyada reactive hote hain (F₂ ko chhodkar).
Ye covalent, diamagnetic, volatile hote hain; hydrolyse hokar halide + oxohalide dete hain.
Fluorinating agents ki tarah use hote hain (e.g. $C l F_{3}$ ).

5. Pseudohalogens & Pseudohalides

7. Active Recall

Recall Quick self-test (answers chhupao)

Kis halogen ka $Δ_{e g} H$ sabse zyada negative hai? → Cl (F nahi)
Sabse strong oxidizing halogen? → F₂
Sabse strong HX acid? → HI
HF weak acid kyun hai? → strong short H–F bond + H-bonding
$I F_{7}$ ki shape? → pentagonal bipyramidal
Interhalogen ka general formula? → $X X_{n}^{'}$ , bada X central

Recall Feynman: ek 12-saal ke bacche ko explain karo

Socho har halogen ek aisa bachcha hai jise apni book complete karne ke liye exactly ek aur sticker chahiye. Fluorine sabse chhota, sabse bhookha bachcha hai — woh kisi se bhi sticker jhapat lega (strongest oxidizer). Jab ye hydrogen ke saath haath pakdte hain (HX), toh bade bacche dheelay pakdte hain, isliye paani mein hydrogen aasaani se chhhod dete hain — isliye HI strong acid hai lekin chhota tight-gripping HF nahi chhodta. Interhalogens bas do alag sizes ke bacche hain jo haath pakde hain, aur bada bachcha kaafi saare chhote bacchon ko ek saath pakad sakta hai (jaise IF₇).

8. Connections

Periodic Trends — Electronegativity & Ionization Enthalpy
Electrochemistry — Standard Reduction Potentials
Born–Haber Cycle & Hydration Enthalpy
VSEPR Theory & Hybridisation
Group 16 (Oxygen family)
Oxoacids of Halogens

Konsa halogen sabse strong oxidizing agent hai aur kyun?

F₂; low F–F bond dissociation enthalpy + F⁻ ki bahut high hydration enthalpy uski modest electron gain enthalpy ko outweigh kar deti hai.

Cl ka, F ka nahi, electron gain enthalpy sabse zyada negative kyun hai?

F itna chhota hai ki electron add karne se compact 2p shell mein strong e⁻–e⁻ repulsion hoti hai, jisse ΔegH, Cl se kam negative hoti hai.

HX acid strength ka order aur governing factor?

HF < HCl < HBr < HI; H–X bond dissociation enthalpy se govern hota hai (kamzor bond → stronger acid).

HF weak acid kyun hai jabki F sabse zyada electronegative hai?

Bahut strong short H–F bond plus strong hydrogen bonding proton ko pakde rehte hain, isliye HF zyada ionize nahi hota.

Interhalogens ka general formula aur central atom rule?

XX'ₙ (n=1,3,5,7); bada, kam electronegative halogen central hota hai.

ClF₃ ki shape aur hybridisation?

Bent T-shape; sp³d (3 bond pairs + 2 lone pairs equatorial).

IF₇ ki shape?

Pentagonal bipyramidal, sp³d³.

Interhalogens, halogens se zyada reactive kyun hote hain?

X–X' bond, X–X se kamzor (aur zyada polar) hota hai, isliye aasaani se toot jaata hai (F₂ ko chhodkar).

Pseudohalogens ko define karo ek example ke saath.

Polyatomic species jo chemically X₂ jaisi resemblance rakhte hain, e.g. cyanogen (CN)₂; inke anions (CN⁻, SCN⁻, OCN⁻, N₃⁻) pseudohalides hain.

Kya Cl₂, Br⁻ ko displace karega? E° se justify karo.

Haan; E°(Cl₂/Cl⁻)=+1.36 > E°(Br₂/Br⁻)=+1.07, ΔE°>0 isliye Cl₂+2Br⁻→2Cl⁻+Br₂ spontaneous hai.

X–X bond enthalpy ka trend aur anomaly?

Cl > Br > F > I; F–F anomalously low hai kyunki chhote F atoms mein lone-pair repulsion hoti hai.