Phosphorus allotropes (white, red, black); P₄O₆, P₄O₁₀; oxoacids of P (H₃PO₃ vs H₃PO₄ basicity)

3.2.6 · Chemistry › p-Block

1. Allotropes of Phosphorus

WHY allotropes itne alag hote hain? Kyunki bonding alag hoti hai. White P strained, isolated molecules se bana hai (high energy, reactive); red aur black P giant covalent networks hain (low energy, stable).

Property	White P	Red P	Black P
Unit	discrete $P_{4}$ tetrahedra	chains of $P_{4}$ units (polymer)	layered sheets (like graphite)
Bond angle	$60°$ (strained!)	normal	normal
Stability	least (most reactive)	intermediate	most stable
Glow in dark	yes (phosphorescence)	no	no
Toxic	very	non-toxic	non-toxic
Ignition	air at ~ $35° C$	needs ~ $260° C$	very stable
Storage	under water	open	—

2. The Oxides: P₄O₆ and P₄O₁₀

HOW inhe build karte hain? $P_{4}$ tetrahedron se shuru karo (6 edges, 4 corners). Oxygen insert karo.

$P_{4} + 6 O (bridges) P_{4} O_{6} + 4 O (terminal) P_{4} O_{10}$

	$P_{4} O_{6}$	$P_{4} O_{10}$
O atoms	6 bridging	6 bridging + 4 terminal
Oxidation state of P	$+ 3$	$+ 5$
Each P bonds to	3 O	3 bridging O + 1 terminal O
Acid on hydrolysis	$H_{3} P O_{3}$ (phosphorous)	$H_{3} P O_{4}$ (phosphoric)

3. Oxoacids of P — the basicity rule

Structures compare karo

$H_{3} P O_{4}$ (phosphoric acid), P = $+ 5$ : P bonded hai: ek $= O$ se, teen $- O H$ se. → 3 OH groups → basicity = 3 (tribasic).

$H_{3} P O_{3}$ (phosphorous acid), P = $+ 3$ : P bonded hai: ek $= O$ se, ek P–H se, do $- O H$ se. → sirf 2 OH groups → basicity = 2 (dibasic).

Acid	Formula	Ox. state P	P–H bonds	Basicity
Hypophosphorous	$H_{3} P O_{2}$	$+ 1$	2	1
Phosphorous	$H_{3} P O_{3}$	$+ 3$	1	2
Phosphoric	$H_{3} P O_{4}$	$+ 5$	0	3

4. Common Mistakes (Steel-man + Fix)

Recall Feynman: 12-saal ke bacche ko samjhao

Socho LEGO men (P atoms) jiske paas 3 haath hain. White phosphorus mein, chaar men ek chote triangle-pyramid mein haath thamte hain, lekin unke haath uncomfortably mude hue hain — toh woh itche kar rahe hain chhod dene ke liye (super reactive, aag pakad leta hai). Red/black phosphorus mein woh men lambi comfortable chains/sheets banate hain — chill aur stable. Jab oxygen aati hai, ek O atom do thame hue haathon ke beech slide karta hai (ek bridge); agar bahut saari oxygen ho, toh har man apne free haath mein bhi ek extra O pakad leta hai → yahi farq hai $P_{4} O_{6}$ aur $P_{4} O_{10}$ mein. Acids ke liye: sirf woh haath jo O–H thaame ho, hydrogen ( $H^{+}$ ) "drop" kar sakta hai. Jo haath seedha H thaame ho (P–H) woh chhodne se mana karta hai. Toh O–H wale haath count karo — utne hi $H^{+}$ acid deta hai.

Connections

p-Block Group 15 (Nitrogen family) trends
Oxidation states and disproportionation
Oxides — acidic vs basic character
Hydrolysis reactions
Reducing agents and P–H bonds
Allotropy — Carbon, Sulfur, Oxygen comparison

White P, red P se zyada reactive kyun hai?

White P discrete

P_{4}

tetrahedra se bana hai jisme strained

60°

bond angles hain (stored angular strain); red P ek relaxed polymer hai normal angles ke saath.

White P ko kaise store karte hain aur kyun?

Paani ke neeche, kyunki yeh air mein ~

35° C

par jalata hai.

White P se red P kaise prepare karte hain?

White P ko ~

573 K

par inert atmosphere mein (no air) heat karo.

Sabse stable P allotrope kaun sa hai?

Black phosphorus (layered, graphite-jaisi sheets).

P_{4} O_{10}

mein kitne bridging aur terminal O atoms hain?

6 bridging (P–O–P) + 4 terminal (P=O).

P_{4} O_{6}

aur

P_{4} O_{10}

mein P ki oxidation state?

Krama se

+ 3

aur

+ 5

P_{4}

ke limited vs excess oxygen combustion ki equation?

P_{4} + 3 O_{2} \to P_{4} O_{6}

;

P_{4} + 5 O_{2} \to P_{4} O_{10}

P_{4} O_{6}

aur

P_{4} O_{10}

ki hydrolysis ke products?

4 H_{3} P O_{3}

aur

4 H_{3} P O_{4}

P_{4} O_{10}

ko drying agent ke roop mein kyun use karte hain?

Yeh strongly dehydrating hai; greedily paani absorb/remove karta hai (yahan tak ki

H N O_{3}

se bhi →

N_{2} O_{5}

Acid ki basicity define karo.

Ionisable (replaceable) H atoms ki sankhya = P–OH groups ki sankhya.

H_{3} P O_{4}

H_{3} P O_{3}

H_{3} P O_{2}

ki basicity?

3, 2, 1.

H_{3} P O_{3}

mein 3 H hone ke bawajood dibasic kyun hai?

Iska ek P–H bond hai (non-ionisable); sirf 2 P–OH hydrogens acidic hain.

Sabse strong reducing agent oxoacid kaun sa hai aur kyun?

H_{3} P O_{2}

— iske do P–H bonds hain; P–H bonds reducing power dete hain.

Kya

H_{3} P O_{4}

reducing agent hai?

Nahi — iska koi P–H bond nahi hai.

Basicity ko structure se jodne wala formula?

basicity = (total H) − (P–H bonds ki sankhya).