p-BLOCK ELEMENTS - OXYGEN FAMILY

GENERAL CHARACTERISTICS

The elements oxygen, sulphur, selenium, tellurium and polonium belong to group VIA or 16 group of periodic table.

These elements are known as chalcogens i.e. ore forming elements.

ELECTRONIC CONFIGURATION

Elements	At.No.	Electronic confg.	Valence shell electronic confg
Oxygen	8
Sulphur	16
Selenium	34
Tellurium	52
Polonium	84

The oxygen differs from the rest of the elements due to its-

small size
higher electronegativity
absence of d atomic orbitals in valence shell
tendency to form multiple bonding

METALLIC AND NON METALLIC CHARACTER

Metal (Radio active) t1/2 138.4 days

ABUNDANCE

O > S > Se > Te > Po

Oxygen is the most abundant element. It constitutes 46.6% of earth’s crust, 21% of air and 89.1% of ocean by weight. Sulphur forms about 0.052% of earth’s crust.

DENSITY

Increases down the group regularly.

MELTING POINT AND BOILING POINT

Both show a regular increase down the group due to increase in molecular weight and Van der Waal’s forces of attraction.

OXIDATION STATE

O	S	Se	Te	Po
–1,–2	–2 to +6	–2 to +6	–2 to +6	–2 to +6

In OF2 the oxidation state of oxygen is +2

IONISATION ENERGY

They possess a large amount of ionisation energy which decrease gradually from O to Po due to increase in size of atoms and increase in screening effect.

ELECTRON AFFINITY

They have high electron affinity which decrease from O to Po. As the size of the atom increases the extra added electron feels lesser attraction by nucleus and electron affinity decreases.

ELECTRONEGATIVITY

It decreases down the group due to decrease in the effective nuclear charge down the group.

CATENATION

The tendency to form chains of identical atoms is known as catenation. It follows the order

S-S > Se - Se > O - O > Te - Te

226 172 142 126 kJ/mol

The higher the bond strength, the higher is the catenation.

ATOMICITY

Oxygen is diatomic, sulphur and selenium octa atomic with puckered ring structure

Ring S6

ALLOTROPY

All the elements exhibit allotropy

Oxygen - O2 dioxygen and O3 ozone

Sulphur - Rhombic (or

) sulphur S8

Monoclinic (or

) sulphur S8 (most stable)

Plastic (or

) sulphur open chain

Colloidal (or

) sulphur

The SR changes to SM above 95.4ºC.

Selenium Rhombic Se8 , Monoclinic Se8 (Grey)

Grey is the most stable consisting of regularly arranged spirals of Se atoms.

Tellurium Non metallic, Metallic (more stable)

Polonium

and

both metallic

ATOMIC RADII

Increases regularly from O to Po.

IONIC RADII

Increases regularly from O to Po

ATOMIC VOLUME

Increase regularly from O to Po

MULTIPLE BOND FORMATION

The tendency of these elements to form multiple bonds to C and N decreases down the group eg. S = C = S is moderately stable.

Se = C = Se decomposes readily and

Te = C = Te not known

COMPOUNDS OF SIX GROUP ELEMENTS

HYDRIDES

All these elements form stable hydrides of the type H2M either by directly combining with hydrogen or by the action of acids on metal sulphides, Selenides and tellurides

H2O is a liquid due to hydrogen bonding. Others are colourless gases with unpleasant smell.

H2O > H2S > H2Se > H2Te

104.5º 92.5º 91º 90º (all sp3 hybridised)

Stability, volatile character (from H2S to H2Te)

Poisonous nature, acidic character, reducing character.

The weakening of M – H bond with the increase in the size of M (not the electronegativity) explains the acid character of hydrides.

HALIDES

All these elements form a number of halides. The halides of oxygen are not very stable eg OF2, Cl2O7, I2O5 etc.

HEXAHALIDES

These are formed by fluorine only (not by Cl, Br, I) where elements exhibit maximum valency of +6. SF6, SeF6, TeF6 are colourless gases with sp3d2 hybridisation and octahedral structure. These are covalent in nature. Due to bigger size of Cl, Br and I the coordination number of 6 is not achieved.

TETRAHALIDES

With the exception of SBr4, SI4 and SeI4 all tetrahalides are known. SF4 is gaseous, SeF4 is liquid and TeF4 is solid. SCl4 is unstable liquid. These have lewis acid character.

They have trigonal bipyramidal shape with sp3d hybridization.

DIHALIDES

The dihalides eg SCl2, OF2, TeBr2 are sp3, hybridised and have distorted bond angles due to electron pair repulsions

SCl2 OF2 TeBr2

104º 101.5º 98º

DIMERIC MONOHALIDES

The dimeric monohalides are given by sulphur and selenium eg S2F2, S2Cl2, Se2Cl2 S2Br2, Se2Br2. These are slowly hydrolysed and undergo disproportionation.

The monohalides have structure similar to H2O2 with distorted bond angle of sp3 hybridisation

OXIDES

Ozone is considered as oxide of oxygen O. Oxides of other elements are as follows

Element	Mono Oxide	Dioxide	Tri Oxide
S	SO	SO2	SO3
Se	-	SeO2	SeO3
Te	TeO	TeO2	TeO3
Po	PoO	PoO2	-

SO2 is a gas having sp2 hybridisation and V-shape.

SO3 is a gas, sp2 hybridised and planar in nature.

In solid state it exist as a cyclic trimer (SO3)3 α-form or as a linear chain cross linked sheets

-form

- and

- form

SeO2 volatile solid consists of non planar infinite chains

SeO3 has tetrameric cyclic structure

OXY ACIDS

Sulphur forms four series of oxy acids

SULPHUROUS ACID SERIES

Sulphurous acid (H2SO3)

Thiosulphurous acid (H2S2O2)

Hyposulphurous acid (H2S2O4)

Pyrosulphurous acid (H2S2O5)

SULPHURIC ACID SERIES

Sulphuric acid (H2SO4)

Thiosulphuric acid (H2S2O3)

Pyrosulphuric acid (H2S2O7)

THIONIC ACID SERIES

Dithionic acid (H2S2O6)

Polythionic acid (H2SnO6)

(n=3, 4, 5, 6)

PEROXY ACID SERIES

Peroxomonosulphuric acid (H2SO5)

(Caro’s Acid)

Peroxodisulphuric acid (H2S2O8)

(Marshall’s acid)

Oxyacids of Selenium - Selenous acid (H2SeO3), Selenic acid (H2SeO4)

Oxyacids of Tellurium - Tellurous acid (H2TeO3), Telluric acid (H2TeO4)

OZONE (O₃)

Discovered by Van Marum by passing electric discharge through air, named by Schonbein (azo - I smell) and Sorret established the formula O3 as allotrope of oxygen.

It is formed in atmosphere by action of UV rays on O2. It is also formed by-

slow oxidation of phosphorus in air
Reaction of fluorine with water at low temperature
Electrolysis of water
SO2 reacts with H2O2

PREPARATION

Lab method
By passing silent electric discharge through cold, dry oxygen in ozoniser

The ozonisers used are

Siemens ozoniser
Brodie’s ozoniser

MANUFACTURE

For the manufacture of ozonised air Siemen and Halske ozoniser is employed.

Electrolytic method. When acidified water, using high current density and Pt. anode, is electrolysed 95% O3 is obtained at anode rest being O2.

PROPERTIES

Pale blue gas, dark blue liquid and violet black solid with characteristic strong smell, slightly soluble in water but more soluble in turpentine oil, glacial acetic acid and carbon tetrachloride.
Decomposition

Oxidising action

(nascent oxygen hence powerfully oxidising)

Reducing action

Addition reactions

USES

Bleaching ivory, oils, flour etc.
As germicide and disinfectant, for sterilising water
For improving atmosphere in crowded places
Manufacture of KMnO4 and artificial silk.

TESTS

Turns starch iodine paper blue.
Tailing of Hg - Mercury loses its meniscus in contact with O3 and sticks to the surface of glass due to formation of Hg2O
Clean silver foil blackened by O3
It turns benzidine paper brown and tetramethyl base violet

STRUCTURE

Oxidation state of O is +1 and –1

OXIDES

Oxides are the binary compounds of oxygen with metals and non metals. Based on their oxygen content they have been classified as-

NORMAL OXIDES

Oxides containing oxygen according to normal oxidation number of M eg. H2O, MgO, Al2O3.

POLYOXIDES

These contain more oxygen than normal oxidation number of M and M – O and O – O bonds. They are further classified as

PEROXIDES
They contain ion, produce hydrogen peroxide with dil. acids and O2 with concentrated acids eg BaO2, Na2O2

SUPER OXIDES
They contain ion. With water they give hydrogen peroxide and oxygen

DIOXIDES
They give chlorine with conc HCl and oxygen with Conc H2SO4 eg. MnO2, PbO2 etc.

SUBOXIDES
They contain lower percentage of oxygen eg. N2O, C3O2

They have M – M and M – O bonds. eg. O = C = C = C = O (carbon suboxide)

MIXED OXIDES
Formed by the combination of two simple oxides
eg. Red lead, Pb3O4 (PbO2.2PbO), Fe3O4 (FeO+Fe2O3)

CLASSIFICATION ON THE BASIS OF CHEMICALS BEHAVIOUR

ACIDIC OXIDES
Oxides of non metals which give acids when dissolved in water are called acidic oxides eg.

CO2, NO2, P2O5, SO2, SO3, Cl2O7 etc.

carbonic acid

The metallic oxides of high oxidation state eg Mn2O7, V2O5 and CrO3

BASIC OXIDES

Ionic in nature. Oxides of alkali and alkaline earth metals eg Na2O, CaO, BaO.
In water they give basic solutions

Covalent oxides - Oxides of transition metals are covalent in nature eg CuO, FeO. Insoluble in water.

AMPHOTERIC OXIDES

The oxides which react with both acids and alkalis are known as amphoteric oxides eg ZnO, Al2O3, SnO etc.

NEUTRAL OXIDES

Such oxides do not combine with an acid or a base eg NO, N2O, CO, H2O etc.

OXYGEN

Chinese observed the presence of oxygen in air. Priestley & Scheely prepared oxygen by heating suitable oxygen compounds.

OCCURRENCE

About

of the atmosphere is free of elemental oxygen. In the combined state it is present in water 89% by weight, earth’s crust about 50% and plants and animal tissues 50–70%.

PREPARATION

By action of heat on oxygen rich compounds

From oxides

From peroxides and other oxides

From certain compounds

By the action of some chemical reagent on compound rich in O2.

By electrolysis of water either acidified with H2SO4 using platinum electrodes or by making it alkaline with NaOH or Ba(OH)2 using nickel electrodes.

By decomposition of steam by chlorine

MANUFACTURE

By fractional distillation of liquid air

PHYSICAL PROPERTIES

It is colourless, odourless, tasteless slightly heavier than air, sparingly soluble in water, soluble in pyrogallol.

CHEMICAL PROPERTIES

On heating it combines directly with metals and non metals

Combination with O2 is accelerated by using catalyst. Platinum is particularly active.

(Contact process)

USES

For breathing
In welding and cutting – oxy-hydrogen or oxy-acetylene torch is used
In iron and steel industry – to increase the content of blast in the Bessemer and open hearth process
As a fuel in rockets

TESTS

With NO it gives reddish brown fumes of NO2
Absorbed by alkaline pyrogallol
A smouldering wood splinter bursts into flames in a jar of O2

STRUCTURE

It is paramagnetic with following electronic configuration

Atomic oxygen :

Isotopes O16 O17 O18

10000: 1: 8

SULPHUR

OCCURENCE

It occurs free in volcanic regions. In combined state it occurs as

Gypsum CaSO4.2H2O
S-Celesite SrSO4
Galena PbS
Zinc blende ZnS
Copper pyrites Cu2SFe2S3
Iron pyrites FeS2

PROPERTIES

Ordinary sulphur is pale yellow, insoluble in water but dissolves in CS2, C6H6 and turpentine

Allotrophic forms

Crystalline - Rhombic, monoclinic
Amorphous - Plastic, milk of sulphur, colloidal sulphur

SULPHURIC ACID (H₂SO4)

It is also known as oil of vitriol and king of chemicals.

MANUFACTURE

LEAD CHAMBER PROCESS

The various steps involved are

Production of SO2 : By burning S or iron pyrites.

Production of catalyst - Oxides of nitrogen

Reaction in lead chamber

(used again)

Gay-Lussac Tower : The residual gases (mainly air + oxides of nitrogen) from lead chambers + conc. H2SO4 from glover tower give nitrated acid.

Glover Tower : The nitrated acid from Gay-Lussac tower is denitrated in this tower.

Conditions - Temperature 50ºC, excess of steam, lead chamber since lead is not attacked H2SO4.
Purification - The acid obtained contains the impurities of PbSO4, AS2O3, NO and NO2 which are removed as follows

As2O3 is precipitated as As2S3 by passing H2S

PbSO4 is insoluble in water and settles down on dilution and filtered off.

NO and NO2 are removed by heating with (NH4)2SO4

Concentration - The sulphuric acid is concentrated by evaporation

CONTACT PROCESS

The steps involved are

Production of SO2 - It is produced by burning sulphur and iron pyrites and purified by treating with steam to remove dust particles. The arsenic is removed by ferric hydroxide, water vapour removed by conc H2SO4. The gases are filtered through coke filters and purity is tested by Tyndal box.
Conversion of SO2 to SO3 - It is done in contact or catalyst chamber after being preheated to 450ºC.