Position of sulphur in periodic table and allotropes

Position of sulphur in periodic table

1. Oxygen is the first member of the group VIA and sulphur is the second member of the group VIA.

2. Other member of this group are selenium, tellurium and polonium.

3. Members of group VIA have 6 electrons in their valence shell. They generally show a valency of -2. Oxygen shows a fixed valency of 2 while sulphur shows variable valencies of 2, +4 and +6 also.

4. All the members of this group are non-metals. The metallic character of the group increases down the group. Polonium is radioactive metal.

5. The members of group VIA show allotropy. For example: the allotropes of oxygen are O₂ (dioxygen) and O₃ (ozone). Allotropes of sulphur are rhombic, monoclinic, plastic and colloidal.

6. The first four members of this group are often called chalcogens (meaning ore forming elements) since they form the oxide and sulphide ores of many metals.

Allotropes of sulphur

Rhombic Sulphur (Octahedral or Alpha Sulphur)

Rhombic sulphur is prepared by dissolving roll sulphur in carbon disulphide, and then evaporating the solution slowly, at room temperature. Eight sided crystals of rhombic sulphur crystallise out (Fig.13.22).

Rhombic or octahedral sulphur consists of rings of 8 atoms of sulphur. It is the most stable of all the allotropes of sulphur. It is soluble in carbon disulphide, benzene, chloroform, etc., but is insoluble in water. It is non-conductor of heat and electricity. It is transparent and pale yellow in color.

Monoclinic Sulphur (Prismatic or Beta Sulphur)

This allotrope of sulphur is prepared by melting roll sulphur in a dish. The molten sulphur is allowed to cool slowly. The top layer solidifies first and forms a crust. Two holes are made in the crust with the help of a heated nail. The molten sulphur is poured out through one of the holes. Then with the help of a knife the crust is carefully peeled off (Fig 13.23).

Pale-yellow, transparent needle shaped crystals are seen projecting out form the inner surface of the dish. These are the crystals of monoclinic sulphur. Monoclinic sulphur also consists of 8 atom rings. It is stable only above 96^oC. When it cools down below 96^oC, it changes to rhombic sulphur i.e., 96^oC is the transition temperature of this sulphur.

Remember :

Transition temperature is the temperature at which an allotrope of an element changes to another allotrope of the same element, e.g. at 96^oC or above, rhombic sulphur changes to prismatic sulphur. At 96^oC or below, prismatic sulphur changes to rhombic sulphur.

Such allotropes that can transform form one type to another type, by a change in the temperature are called enantiotropic allotropes.

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Plastic Sulphur

On heating sulphur, till almost the boiling point and suddenly cooling it by pouring into cold water a viscous mass is formed. This sudden cooling does not allow sufficient time to the molecules to rearrange themselves to form monoclinic or rhombic forms of sulphur. Hence the molecules form an interwined mass, consisting of both rhombic and monoclinic varieties of sulphur. This is called plastic sulphur (Fig.13.24).

This type of sulphur is a dark brown or even black, sticky substance. It is elastic. It has no sharp melting point. It does not dissolve in carbon disulphide. On standing, it slowly changes to the rhombic forms, as it gains the eight atom ring structure.

Colloidal Sulphur

This type of sulphur is prepared by passing hydrogen sulphide through a cooled saturated solution of sulphur dioxide in water, or by adding a solution of sulphur and alcohol in water. Colloidal sulphur is soluble in carbon disulphide. It is used in medicine.

Milk of Sulphur

Milk of sulphur is prepared by the action of dilute hydrochloric acid on ammonium sulphide. Milk of sulphur is also prepared by boiling roll sulphur with an aqueous solution of calcium hydroxide. The mixture is then filtered and dilute hydrochloric acid is added to the filtrate to get milk of sulphur.

Milk of sulphur is non-crystalline and white in color. It is soluble in carbon disulphide. When heated it changes to the ordinary yellow variety of sulphur. It is used in medicine.