The elements Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te) and Polonium (Po) belong to group 16 of the Periodic Table. These elements are known as chalcogens, i.e., ore forming elements.
The name sulphur has been derived from sanskrit word ‘Sulvezi’ meaning ‘killer of copper’.
General Physical Properties of Group 16 Elements:
(i) Electronic configuration- Their valence shell electronic configuration is ns2, np4.
[He] 2s2 2p4
[Ne] 3s2 3p4
[Ar] 3d10, 4s2 4p4
[Kr] 4f10, 5s2 5p4
[Xe] 4f14,5d106s2 6p4
(ii) Metallic and non-metallic character-
(iii) Abundance- O > S > Se > Te > Po
(iv) Density- It increases down the group regularly,
(v) Melting point and boiling point- Both show a regular increase down the group due to increase in molecular weight and Van der Waals’ forces of attraction.
(vi) Oxidation state-
-2 to +6
-2 to +6
-2 to +6
-2 to +6
In OF2, the oxidation state of oxygen is +2.
(vii) Ionisation energy- They possess a large amount of ionisation energy which decreases gradually from O to Po due to increase in size of atoms and increase in screening effect.
(viii) 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 hence, electron affinity decreases.
(ix) Electronegativity- It decreases down the group due to decrease in effective nuclear charge down the group.
(x) Catenation- 16 group elements follow the order as shown below-
S-S > Se-Se > O-O > Te-Te
(xi) Atomicity- Oxygen is diatomic, sulphur and selenium are octatomic with puckered ring structure.
Oxygen – Dioxygen (O2) and ozone (O3)
Sulphur – Rhombic (01′ a) sulphur S8
Monoclinic (or β) sulphur, S8(most stable), plastic sulphur
(xiii) Atomic radii and ionic radii They increase regularly from O to Po.
Chemical Properties of 16 Group Elements-
(i) Hydrides: All these elements form stable hydrides of the type H2E. (Where. E = O, S, Se, Te and Po).
2H2 + O2 ⇔ 2H2O
FeS + H2SO4 → H2S + FeSO4
H2O is a liquid due to hydrogen bonding, while others are colourless gases with unpleasant smell.
[Down the group acidic character increases from H2O to H2Se. All the hydrides except water possess reducing property and this character increases from H2 S to H2Te].
(ii) Halides: The stability of the halides decreases in the order-
F– > Cl– > Br– > I–
Amongst hexahalides, hexafluorides are the only stable halides. All hexafluorides are gaseous in nature. SF6 is exceptionally stable for steric reasons.
SF4 is a gas, SeF4 is a liquid and TeF4 is a solid. These fluoride have sp3 d hybridization and see-saw geometry. They behave Lewis acid as well as Lewis base e.g.,
SF4 + BF3 → SF4 → BF3
SeF4 + 2F– → [SeF6]2-
The well known mono halides are dimeric in nature. Example are S2F2, S2Cl2, S2Br2, Se2Cl2 and Se2Br2. These dimeric halides undergo disproportion as given below:
2 SeCl2 → SeCl4 + Se
(ill) Oxides They form AO2 and AO3 type oxides. Their acidic nature follow the order
SO2 > SeO2 > TeO2 > PoO2 and SO3 > SeO3 > TeO3
Ozone is considered as oxides of oxygen.
SO2 is a gas having sps -hybridisation and V-shape.
SO3 is a gas which is sp2-hybridised and planar in nature.
SeO2 is a volatile solid consists of non-planar infinite chains.
SeO3 has tetrameric cyclic structure in solid state. SO2 and SO3 are the anhydrides of sulphurous (H2SO3) and sulphuric acid (H2SO4) respectively.
Note: In photocopying (xerox) machines, Se acts as photoconductor.
Oxygen and its Compounds:
Priestley and Scheele prepared oxygen by heating suitable oxygen compounds.
Preparation By action of heat on oxygen rich compounds.
(i) From oxides-
(ii) From peroxides and other oxides-
(iii) From certain compounds-
Physical properties: It is colourless, odourless, tasteless, slightly heavier than air and sparingly soluble in water.
Chemical properties: On heating, it combines directly with metals and non-metals, e.g.,
2Mg + O2 → 2MgO
4Na + O2 → 2 Na2O → Na2O2
Combination with O2 is accelerated by using catalyst. Platinum is particularly an active catalyst.
Uses: It is used in welding and cutting oxy-hydrogen or oxy-acetylene torch and in iron and steel industry to increase the content of blast in the Bessemer and open hearth process. It is also used for life support systems e.g., in hospitals, for divers, miners and mountaineers.
1. With NO it gives reddish brown fumes of NO2.
2. It is adsorbed by alkaline pyrogallol.
2. Ozone (O3)
Preparation- By passing silent electric discharge through cold, dry oxygen in ozoniser. (Lab method)–
3O2 ⇔ 2O3; + 284.3 kJ
Physical properties- It is pale blue gas with characteristic strong smell. It is slightly soluble in water.
2. Oxidising action:
3. It acts as a powerful oxidizing agent. It liberates iodine from neutral KI solution and the liberated I2 turns starch paper blue.
2KI + H2 + O3 → 2KOH + I2 + O2
I2 + Starch → Blue colour
It is used
1. as a germicide and disinfectant for sterilizing water.
2. as a bleaching agent for oils, ivory wax and delicate fibres.
3. for detecting ‘the position of double bond in unsaturated compounds.
4. in destroying odours coming from cold storage room, slaughter houses and kitchen of hotels.
Compounds of Sulphur:
1.Sulphur Dioxide (SO2)
Method of preparation:
(i) By heating sulphur in air
(ii) Roasting iron pyrites in excess of air
(iii) Lab method
Physical Properties: SO2 is a colourless gas with pungent smell and is highly soluble in water.
Chemical reactions: It turns lime water milky due to the formation of calcium bisulphite. However, in excess of SO2 milkiness disappears due to the formation of calcium bisulphite.
Ca(OH)2 + S02 --- > CaS03 + H20
CaSO3 + S02 + H20 --> Ca(HSO3)2
2NaOH + S02 --- > Na2SO3 + H20
NajSO3 + H20 + S02 --- >2NaHSO3
S02(g) + CL2(g) --- >S02Cl2(l)
2S02(g) + 02(g) --- >2SO3(g)
2Fe3+ + S02 + 2H20 --- > 2Fe2+ + SO42- + 4H+
5S02 + 2MnO42- + 2H20 --- > 5SO42- + 4H+ + 2Mn2+
when H2S gas is passed through a saturated solution of SO2 till its smell disappears, it turns in a milky solution, the Wacken roder’s liquid. When H2S is passed through H2SO4 the reaction is called Wacken roder’s reaction.
Oxoacids of Sulphur:
2. Sulphuric Acid (H2SO4)
Sulphuric acid is one of the most important industrial chemicals world wide. It is called the king of chemicals. It is manufactured by lead chamber process or contact process. Contact process involves three steps:
(i) Burning of sulphur or sulphur ores in air to generate SO2.
(ii) Conversion of SO2 to SO3 by the reaction with oxygen in the presence of a catalyst (V2O5).
(iii) Absorption of SO3 in H2SO4 to give oleum (H2S2O7) which upon hydrolysis gives H2SO4.
1. Sulphuric acid is a colourless, dense, oily liquid.
MX + H2SO4 → 2HX + M2SO4
2. Concentrated sulphuric acid is a strong dehydrating agent.
The burning sensation of concentrated H2SO4 on skin.
3. Hot concentrated sulphuric acid is a moderately strong oxidising agent. In this respect, it is intermediate between phosphoric acid and nitric acid.
Uses It is used in petroleum refining, in pigments paints and in detergents manufacturing.
It is chemically sodium thiosulphate pentahydrate, Na2S2O3*5H2O. The solid is an efflorescent (loses water readily) crystalline substance that dissolves well in water. It is also called sodium hyposulfite or "hypo".
Preparation 1. It is prepared by boiling sodium sulphite solution with flowers of sulphur and stirring till the alkaline reaction has disappeared.
Na2SO3 + S → Na2S2O3
2. It is also prepared by spring’s reaction.
Na2S + Na2SO3 + I2 → Na2S2O3 +2NaI
1. It is a colourless, crystalline and efflorescent substance.
2. It gives white ppt with a dilute solution of AgNO3 which quickly changes into black due to the formation of Ag2S.
1. Due to its property of dissolving silver halide, it is used in photography for fixing under the name hypo.
2 Na2S2O3 + AgBr → Na3 [ Ag(S2O3)2] + NaBr
2. During bleaching, it is used as antichlor.
Na2S2O3 + CI2 + H2O → Na2SO4 + S + 2HCI
3. It is used to remove iodine stain, for volumetric estimation of iodine and in medicines.