Chemistry ATOMIC AND PHYSICAL PROPERTIES

Atomic and Physical Properties :

The Elements are `undersettext(Chalcogens)(O, S, Se, Te, Po)`

(i) `text(Atomic radii and Ionic radii :)` Covalent radius : `O < S < Se < Te`

(ii) `text(Ionization Enthalpies :)` `O > S > Se > Te > Po` ( `IE_1` values)

(iii) `text(Melting and Boiling points :)` `M.P.` `Te > Po > Se > S > O`

`B.P.` `Te > Po > Se > S > O`

(iv) `text(Electronegativity :)` `O > S > Se > Te`

(v) `text(Metallic Character:)` `O < S < Se < Te < Po`

(vi) `text(Elemetal State :)` Oxygen exist as diatomic molecular gas in this case there is pn - pn overlap thus two `O` atoms form double bond `O = O`. The intermolecular forces in `O_2` are weak `V.B.` forces. `O_2` exist as gas. On the other hand, other elements of family do not form stable pn - pn bonds and do not exist as `M_2` molecules. Other atoms are linked by single bonds and form poly atomic complex molecules for eg. `S - S_8, Se - Se_8`

(vii) `text(Allotropy :)` All element exhibit allotropy for e.g.

`text(Oxygen-)`

`O_2` and `O_3`

Liquid `O_ 2` - pale base

Solid `O_2` - blue

`text(Sulphur-)`

The main allotropic forms are

(a) Rhombic sulphur (`alpha` sulphur)

(b) Monoclinic (`beta` sulphur)

(c) Plastic sulphur (`delta` sulphur)

(a) `text(Rhombic Sulphur)` `(alpha` `text(sulphur))` : This allotrope is yellow in colour (m.p. `385.8` `K`). It is insoluble in water but readily soluble in `CS_2`.

(b) `text(Monoclinic Sulphur)` `(beta` `text(sulphur))` : It is soluble in `CS_2`.

`S(alpha) overset(> 95.5^oC)underset(<95.5^oC)⇋ S(beta)`

(c) `text(Plastic Sulphur)` `(delta` `text(sulphur))` It is insoluble in `CS_2`.

(viii) `text(Catenation)` :

In this group only `S` has a strong tendency for catenation. Oxygen has this tendency to a limited extent.

`H_2O_2 quad H - O - O - H` (Poly oxides)

`H_2S_2 quad H - S - S - H` (Polysulphides or polysulphones)

`H_2S_3 quad H - S - S - S - H`

`H_2S_4 quad H - S - S - S - S - H`

Sulphur Chemistry :

`text(Allotropes :)`

(i)Rhombic or `alpha`-sulphur.

(ii) Monoclinic or `beta` sulphur.

(iii) `gamma` Sulpher

`underset(text(below) 95.5^oC)(S(alpha)) overset(95.5^oC)⇋ underset(text(above)95.5^oC)(S(beta))`

`text(Amorphous forms are)`

(i) Plastic sulphur

(ii) Milk of sulphur

(iii) Colloidal sulphur

`text(Viscosity of 'S' with tempemture :)`

m.p. of `'S' -> 112.8^oC.`

(i) `> 112.8^oC` to `160^oC =>` slow decreases due to `S_8` rings slip and roll over one another easily. See fig.1.

(ii) `> 160^oC,` increases sharply due to breaking of `S_8` rings into chains and polymerises into large size chain.

(iii) `190^oC,` again large chains are being broken into small chain.

`text(Milk of sulphur :)`

Powdered `'S' + Ca(OH)_2` suspension `->` Solution `oversettext(acidified)-> `Milk of 'S'

`12 S + 3Ca(OH)_2 -> 2CaS_5 + CaS_2O_3 + 3H_2O`

`2CaS_5 + CaS_2O_3 + 6HCl -> 3CaCl_2 + 12 S + 3H_2O`

`text(Colloidal Sulphur :)`

`Na_2S_2O_3 + 2HCl -> 2NaCl + H_2O + SO_2 + S downarrow`

`2H_2S + SO_2 -> 3S downarrow + 2H_2O`

`text(Propperties of) 'S' :`

(a) Thin `Cu`-strip catches fire in sulphur vapour.

`Cu + S -> CuS`

(b) 'S' burns spontaneously in fluorine. `S + 3F_2 -> SF_6`

`Cl_2` passed into liq. sulphur `=> 2 S + Cl_2 -> S_2Cl_2`

(c) `S + 2H_2SO_4 -> 3SO_2 + 2H_2O`

`S + 2HNO_3 -> H_2SO_4 + 2NO`

(d) `4 S + 6 KOH -> 2K_2S + K_2S_2O_3 + 3H_2O`

(e) Burns in air : `S + O_2 -> SO_2`

Properties of Sulphur :

`text(Atomic Number: 16)`

Outer electronic configuration: `3s^2 3p_x^2 3p_y^1 ,3p_z^1,`

Number of unpaired electrons : `2`

Covalency : `2`

Oxidation States : `-2, 0, + 2, +4, +6`

Occurrence : Sulphur is one of the widely occuring element. It constitutes about `0.1` percent of the earth's crust. It is found in free (native) as well as combined state. In the combined state it occurs as sulphides and sulphates.

(i) `text(As sulphides :)` Cinnabar (`HgS`), Galena (`PbS`), Zinc blende (`ZnS`), Copper pyrites (`CuFeS_2`)

(ii) `text(As Sulphates :)` Gypsum (`CaSO_4* 2H_2O`), barytes (`BaSO_4`), epsomite (`MgSO_4*7H_2O`). Organic substances such as eggs, proteins onions, garlic, mustard, hair and wool contain sulphur.

`text(Properties)`

`ast` `text(Physical Properties :)`

(a) Sulphur is a pale yellow, crystalline, brittle solid. It is odourless and tasteless.

(b) It has no physiological action on human beings as it is poisonous to lower organisms.

(c) It is poor conductor of heat and a bad conductor of electricity. Hence sulphur is a very good insulator.

(d) It is insoluble in water but soluble in carbon disulphide, sparingly soluble in alcohol and ether.

(e) Burning: When heated to about `250^oC` sulphur bums in air with a pale blue flame forming mainly

`S + undersettext(air)(O_2) underset(250^oC)overset(Heated)-> SO_2`

`text(Uses :)`

(a) For vulcanization of rubber.

(b) In match industry and fireworks.

(c) As a di sinfectant for houses, for destroying bacteria, fungi , insects, etc.

(d) For the manufacture of `SO_2 , CS_2 , H_2SO_4` etc.

(e) In manufacture of sulphur dyes.

(f) In skin medicines.

(g) For preparing gun powder (a mixture of charcoal, sulphur and potassium nitrate).

(h) `Po` is radioactive element.