for UG students

1. Dr. Sakina.Z.Bootwala
Wilson College

2.  Allotropy is the property of some chemical
elements to exist in two or more different
forms, having different physical property but
same chemical properties known
as allotropes .
 All the elements of this group exist in more
than one allotropic form.
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3. Oxygen:
 Oxygen has two non-metallic forms, - diatomic O2 &
triatomic O3.
 O2 is paramagnetic in the gaseous, liquid & solid
state as it contains unpaired electrons.
 O3 is a powerful oxidizing agent; blue in colour & is
diamagnetic.
 The bond length O-O in O3 is 0.128 nm & it is
intermediate between that of single bond 0.149 nm
& double bond 0.121 nm. This indicates that O-O
bond in O3 has considerable double bond character.
 Ozone is highly poisonous gas with a strong odour.
 Ozone boils at –111.9 °C, melts at –192.5 °C, and
has a specific gravity of 2.144 g/l.
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4. Sulphur:
 Exists large number of allotropes forms.
 When molten sulphur is slowly cooled, its
physical properties change in accordance with
the temperature, pressure, and method of
crust formation.
 Thus sulphur exists in a variety of forms
called allotropic modifications.
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5.  Rhombic or α suphur:
 Monoclinic or β-sulphur
 Plastic or γ- Sulphur
 Liquids Sulphur: Sλ and Sµ,
 Colloidal or δ Sulphur:
 Engel’s or ε-sulphur
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6. Rhombic or α suphur:
 The most stable allotropy
of the element is rhombic
sulfur, a yellow, crystalline
solid with a density of
2.06 g/cm3 at 20 °C.
 Rhombic sulphur is
slightly soluble in alcohol
and ether, moderately
soluble in oils and
extremely soluble in
carbon disulfide.
 Crystal structure has S8
puckered ring look like
crown
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7.  When rhombic sulphur is heated
at temperatures above 94.5 °C but
below 120 °C the rhombic form
changes into monoclinic sulfur.
 consisting of elongated,
transparent, needle like structures
with a density of 1.96 g/cm3at 20
°C.
 The temperature at which rhombic
and monoclinic sulphur are in
equilibrium, 94.5 °C, is known as
the transition temperature.
 Crystal structure is similar like
rhombic only the packing is
different
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8. Difference between Rhombic and Monoclinic sulphur
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9.  If sulphur is heated almost to its boiling point of
444.6 °C and is then poured rapidly into cold water,
it does not have time to crystallize into the rhombic
or monoclinic state, but forms a transparent, sticky,
elastic substance known as amorphous, or plastic
sulfur,
 Its specific gravity is 1.95. γ- Sulphur is a mixture of
many allotropes of S containing S8 rings and long
helical or zig-zag long chains of S atoms.
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10.  When ordinary rhombic sulphur is melted at
115.21 °C, it forms the mobile, pale yellow
liquid Sl
 which becomes dark and viscous at 160 °C
forming Sµ.
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11.  Formed when hydrogen sulphide is passed
through a saturated and cooled solution of
sulphur dioxide in water
SO2+ 2H2S → 3S + 2H2O
or oxidising solution like HNO3.
2HNO3 + H2S → S+ 2NO2 +2H2O
 Colloidal sulphur is used as a part of
medicines
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12.  It is prepared by pouring sodium thiosulphate in
concentrated HCl and extracting sulphur with toluene .
Na2S2O3 + 2HCl → 2NaCl + S + SO2 +H2O
 This form is metastable and has chair conformation.
 In cyclo-S6, the ring adopts the chair form .At elevated
temperatures (~1000 K), S2 is the dominant species
and is paramagnetic like O2.
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13. 13
6
allotropes
Four are red form
Three red
forms of Se are
non-metallic
and are
composed of
Se8 rings..
fourth
form is
amorpho
us red
form
which
has a
polymeric
chain of
Se atoms
two are grey/black .
black vitreous
form is made up
of large irregular
chains having
thousand of
atoms of Se, this
black form easily
change to grey
form when
heated to 453K.
One grey metallic
form which is
most stable and it
has infinite spiral
chains of Se atoms
with very weak
interactions with
the neighboring
chains.

14. 14
Allotropy:Tellurium.
Tellurium
non-
metallic
Non metallic amorphous
form is obtained by passing
SO2 gas in telluric acid.
H6TeO6 + SO2 → Te +
3H2SO4
metallic
metallic form exists as
grey crystalline solid
obtained during
electrorefining of Cu.
Elemental
tellurium
forms
spiral
chains.

15. Polonium:
 Po exists in α & β metallic forms, α forms is
cubic while β form is rhombohedral .
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