Group-16 Elements: Oxygen Family | Chemistry Class 12 - NEET PDF Download

What are Group 16 Elements?

The group 16 elements of the modern periodic table consist of 5 elements oxygen, sulphur, selenium, tellurium and polonium. The elements in this group are also known as the chalcogens or the ore-forming elements because many elements can be extracted from sulphide or oxide ores.

The name sulphur has been derived from Sanskrit word ‘Sulvezi’ meaning ‘killer of copper’.

General Physical Properties of Group 16 Elements

1. Electronic configuration: 

   Their valence shell electronic configuration is ns²np⁴.
   

2. Metallic and non-metallic character:

   

3. Abundance: 

   O > S > Se > Te > Po

4. Density: 

   It increases down the group regularly,

5. Melting point and boiling point: 

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

6. Oxidation state:

   
   In OF₂, the oxidation state of oxygen is +2.

7. Ionisation energy: 

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

8. Electron affinity: 

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

9. Electronegativity: 

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

10. Catenation: 

    Group 16 elements follow the order as shown below: S-S > Se-Se > O-O > Te-Te

11. Atomicity: 

    Oxygen is diatomic, sulphur and selenium are octatomic with a puckered ring structure.
    

12. Allotropy: 

    Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements.
    - Oxygen – Dioxygen (O₂) and ozone (O₃)
    - Sulphur – Rhombic (01′ a) sulphur S₈
    - Monoclinic (or β) sulphur, S₈(most stable), plastic sulphur

13. Atomic radii and ionic radii: 

    They increase regularly from O to Po.

Chemical Properties of Group 16 Elements:

1. 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 an 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].

2. 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 solid. These fluorides 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

3. 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 that is sp²-hybridised and planar in nature.
   SeO₂ is a volatile solid consist of non-planar infinite chains.
   SeO₃ has a tetrameric cyclic structure in solid-state. SO₂ and SO₃ are the anhydrides of sulphurous (H₂SO₃) and sulphuric acid (H₂SO₄) respectively.
   
   Question for Group-16 Elements: Oxygen Family

   Try yourself:Which of the following can classified as an amphoteric oxide?

   • A.

     Iron (III) oxide
   • B.

     Zinc oxide
   • C.

     Mercury (II) oxide
   • D.

     Lime

   Explanation

   Zinc oxide is the only amphoteric oxide out of these four. On dissolution in acid like HCl, it acts as a base and forms ZnCl₂ salt. On dissolving in base like NaOH, it acts an acid to form sodium zincate, Na₂ZnO₂.

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Note: In photocopying (xerox) machines, Se acts as a photoconductor.

Oxygen and its Compounds:

1. Dioxygen:

   
   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 the 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.
   Tests:
   1. With NO it gives reddish-brown fumes of NO₂.
   2. It is adsorbed by alkaline pyrogallol.
   
   Question for Group-16 Elements: Oxygen Family

   Try yourself:Which of the following in air, forms compounds readily?

   • A.

     Nitrogen
   • B.

     Oxygen
   • C.

     Sulfur
   • D.

     Carbon

   Explanation

   Oxygen, being a highly electronegative element and reactive tends to react with several substances. These include hydrogen and different occurring metals which form compounds. E.g. iron, magnesium, calcium. Nitrogen is highly unreactive since it bears a triple bond and reacts only at high temperatures. Carbon and sulfur are not elements present in air since they are not gases.

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2. Ozone (O₃)

   
   Preparation: Bypassing silent electric discharge through cold, dry oxygen in ozonised.
   Lab method: 3O₂ ⇔ 2O₃; + 284.3 kJ
   
   Physical properties: It is pale blue gas with a characteristic strong smell. It is slightly soluble in water.
   
   Chemical reactions:
   1.Decomposition:
   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
   
   Uses: 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 the double bond in unsaturated compounds.
   4. in destroying odours coming from the cold storage room, slaughterhouses and kitchen of hotels.

Compounds of Sulphur

1. Sulphur Dioxide (SO₂)

   
   Method of preparation:
   (i) By heating sulphur in the air
   (ii) Roasting iron pyrites in excess of air
   (iii) Lab method
   
   Physical Properties: SO₂ is a colourless gas with a 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 (milkiness)
   CaSO₃ + S0₂ + H₂0 → Ca(HSO₃)₂ (soluble)
   2NaOH + S0₂ → Na₂SO₃ + H₂0
   Na₂SO₃ + H₂0 + S0₂ → 2NaHSO₃
   S0₂(g) + CL₂(g) → S0₂Cl₂(l)
   2S0₂(g) + 0₂(g) → 2SO₃(g)
   
   Reducing agent
   2Fe³⁺ + S0₂ + 2H₂0 → 2Fe²⁺ + SO₄^2- + 4H⁺
   5S0₂ + 2MnO₄^2- + 2H₂0 → 5SO₄^2- + 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 worldwide. It is called the king of chemicals. It is manufactured by the lead chamber process or contact process. The 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.
   
   Properties:
   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.

3. Hypo: 

   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
   
   Properties:
   (i). It is a colourless, crystalline and efflorescent substance.
   (ii). It gives white ppt with a dilute solution of AgNO₃ which quickly changes into black due to the formation of Ag₂S.
   
   Uses:
   1. Due to its property of dissolving silver halide, it is used in photography for fixing under the name hypo.
   2Na₂S₂O₃ + AgBr → Na₃ [ Ag(S₂O₃)₂] + NaBr
   2. During bleaching, it is used as an antichlor.
   Na₂S₂O₃ + CI₂ + H₂O → Na₂SO₄ + S + 2HCI
   3. It is used to remove iodine stain, for volumetric estimation of iodine and in medicines.