Oxygen group - group
elements oxygen, sulphur, selenium, tellurium and polonium constitute 16th
group of the periodic table. The first four elements are non metals.
Collectively they are called the 'chalcogens' or ore -forming elements. This is
because a large number of metals are oxides or sulphides.
Oxygen is a very important element in inorganic
chemistry, since it reacts with almost all the other elements. Oxygen is the
most abundant of all elements. It exists in the free form as dioxygen or
molecular oxygen and makes up 20.9% by volume and 23% by weight of the
Importance of molecular oxygen
Haemoglobin is an iron containing coordination
compound in red blood cells responsible for the transport of oxygen from the
lungs to various parts of the body. Myoglobin is a similar substance in muscle
tissue, acting as a reservoir for the storage of oxygen and as a transport of
oxygen within muscle cells.
consists of heme, a complex of Fe(II) bonded to a porphyrin ligand and globin
protein. The sixth position is vacant in free hemoglobin but is occupied by
oxygen in oxyhaemoglobin. Hemoglobin (Hb) and O2 are in equilibrium with
Hb + O2 -- > < -- HbO2
oxyhaemoglobin is formed in the lungs and carried to the cells, where it
gives up its oxygen.
then binds with HCO3- which is formed by the reaction of
CO2 (released by the cell) with water. After reaching the lungs, due
to hydrolysis CO2 is released.
Hb + O2
-- > < -- HbO2
-- > Hb + O2 Cell wall
Myoglobin -- > MbO2 -- > O2 -- >
Most of the
O2 has been produced by photosynthesis. The dioxygen (or) molecular oxygen is
prepared by the green plants. The chlorophyll in the green parts of the plants
uses the solar energy to make carbohydrate and molecular oxygen. Oxygen makes
up 46.6% by weight of the earth's crust, where it is the major constituent of
Practically all the elements react with dioxygen either at room
temperature (or) on heating except Pt, Au, W and Noble gases. Eventhough the
bond energy of oxygen is high (493 kJ mol-1), the reactions are
generally strongly exothermic and once started often continue spontaneously.
Dioxygen is also called as molecular oxygen. The molecular oxygen is
essential for respiration (for the release of energy in the body) by both
animals and plants. It is therefore essential for life. Hence molecular oxygen
acts as a cell fuel.
complex formed between dioxygen and haemoglobin (the red pigment in blood) is
of vital importance. Since it is the method by which higher animals transport
dioxygen around the body to the cells.
Nascent oxygen and molecular oxygen
Oxygen molecule is very stable. It dissociates
only to a small extent when heated to a very high temperature. This reaction is
O2 -- > 2[O] delH=+116.8
when an electric discharge is passed through oxygen at a very low pressure, it
dissociates to the extent of about 20%.
For example when oxygen is passed at about 1 mm pressure through a
discharge tube, the resulting gas is found to be chemically more reactive. Its
line spectrum shows that it consists of the free atoms. Hence atomic oxygen is
formed according the following endothermic reactions.
O2 -- > O + O delH=489.6kJ
1. Formation of molecular
thin platinum wire is placed in atomic oxygen, it quickly gets heated up and
begins to glow due to the recombination of oxygen atoms with liberation of heat
energy. The rise of temperature of platinum wire under standardised conditions
is a measure of the concentration of the atomic oxygen in the gas.
2. Formation of ozone
Atomic oxygen combines with molecular oxygen to
give ozone which may be condensed by means of liquid air
O2 + [O] -- > O3
Atomic oxygen is an extremely powerful oxidizing agent and oxidises
aliphatic and aromatic hydrocarbons and methyl alcohol with emission of heat
and light. With nitric oxide, a characteristic greenish - white luminescence is
produced. H2S and CS2 react with it and burst into
greyish blue coloured flame.
Generally all the elements react with dioxygen to form oxides. Oxides
are binary compounds of oxygen. Oxides may be classified depending on their
structure (or) their chemical properties.
i) Acidic oxides
The oxides of non-metals are usually covalent and acidic. They have low
melting and boiling points, though some B2O3 and SiO2 form infinite "giant
molecules" and have high melting points. They are all acidic. Some oxides
dissolve in water and thus forming acids. Hence they are called as acid
B2O3 + 3H2O 2H3BO3
N2O5 + H2O 2HNO3
P4O10 + 6H2O 4H3PO4
SO3 + H2O H2SO4
which do not react with water such as SiO2 reacts with NaOH and shows acidic
ii) Basic oxides
oxides are generally basic. Most metal oxides are ionic and contain the O2-
ion. Some oxides dissolve in water and form alkaline solution.
Na2O + H2O --
BaO + H2O -- > Ba(OH)2
Many metal oxides with formula M2O3 and MO2, though ionic, do not react
Examples : Tl2O3, Bi2O3 and
These oxides react with water to form salts and hence they are bases.
CaO + 2HCl -- > CaCl2
If a metal exists in more than one oxidation state and they form more
than one oxide
eg. CrO, Cr2O3, CrO3, PbO, PbO2
iii) Amphoteric oxides
The oxides which react with both strong acids
and strong bases are called as amphoteric oxides.
ZnO + 2NaOH -- > Na2ZnO2 + H2O
ZnO + 2HCl -- > ZnCl2 + H2O
These oxides contain more oxygen than would be
expelled from the oxidation number of M. Some are ionic and contains the
peroxide ion O22-. The metal belonging to the group I and
II (Na2O2, BaO2) contain O22-
ion. Others are covalently bound and contain -O-O- in the structure.
Oxides such as PbO2 react with acids liberate Cl2
PbO2 + 4HCl -- > PbCl2
+ 2H2O + Cl2
v) Compound oxides
Some oxides behave as if they are compounds of the two oxides.
Ex. Ferrous-ferric oxide (Fe3O4). This is
considered to be the mixture of FeO and Fe2O3.
with acids and forms a mixture of ferrous and ferric salts.
Fe3O4 + 8HCl -- > FeCl2 + 2FeCl3
vi) Neutral oxides
A few covalent oxides have no acidic (or) basic
properties (N2O, NO, CO).
They also contain higher proportion of O2
than expected. But they do not liberate H2O2 with acid.
Ex. NO2, SO2