CHEMISTRY TEXTBOOK

Table 7.7 Allotropes of sulfur
Rhombic Sulfur (α - Sulfur) Monoclinic Sulfur (b - Sulfur)
Color Pale Yellow Bright yellow solid

Shape Orthorhombic crystals Needle shaped monoclinic crystals
M. P. 385.8 K 393 K
Density 2.069/ cm^3 1.989 / cm^3

Solubility Insoluble in water and soluble in CS
2

Soluble in CS 2

Stability

Stable below 369 K and transforms

to b - Sulphur above this

temperature.

Stable above 369 K and transforms

into α - Sulphur below this

temperature.

Structure S^8 molecules having puckered ring
structure

S 8 molecules with puckered ring

structure

Method of
preparation

It is prepared by evaporation of

roll sulphur in CS 2.

Rhomic sulphur melted in a dish

and cooled till crust is formed. Two

holes are made in the crust and

remaining liquid is poured out to

give needle shaped crystals of

b - Sulphur

Remember...

Several modifications of sulfur

containing 6-20 sulfur atoms per ring, have

been synsthesised. In the S 8 molecule the

ring is puckered and has a crown shape.

In cyclo - S 6 , the ring adopts the chair

form. At elevated temperature (~ 1000 K),

S 2 is the dominant species which like O 2

is paramagnetic.

Structure of S 8 ring in rhombic sulfur

Chair form of S 6

107 °

2.04

S pm S

S

S

S

S

S

S

S

S S

S

S

S

205.7°c

02.2°

7.8 Oxoacids

7.8.1 Oxoacids of sulfur : Sulfur forms

a number of oxoacids. Some of them are

unstable and cannot be isolated. They are

known to exist in aqueous solutions or in the

form of their salts.

Some important oxoacids of sulfur and

their structures are given below.

i. Sulfurous acid, H 2 SO 3

S

HO

HO

O

HO S

HO

O

O

ii. Sulfuric Acid, H 2 SO 4

iii. Di or pyrosulfuric acid, H 2 S 2 O 7

S

HO

O

O

O S

OH

O

O

O