CHEMISTRY ASSIGNMENT 
ROLL NO. 4940-4955

 ALLOTROPHY

Allotropy  is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements. Allotropes are different structural modifications of an element;^[1] the atoms of the element are bonded together in a different manner. For example, the allotropes of carbon include diamond (where the carbon atoms are bonded together in a tetrahedral lattice arrangement), graphite (where the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (where the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations). The term allotropy is used for elements only, not for compounds

ALLOTROPHS OF CARBON

1. Diamond – an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor.
2. Lonsdaleite – also called hexagonal diamond.
3. Q-carbon – a ferromagnetic, tough, and brilliant crystal structure that is harder and brighter than diamonds.
4. Graphite – a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices (graphene), which are then layered in sheets.
5. Linear acetylenic carbon (Carbyne)
6. Amorphous carbon
7. Fullerenes, including Buckminsterfullerene, a.k.a. "buckyballs", such as C₆₀.
8. Carbon nanotubes – allotropes of carbon with a cylindrical nanostructure

FULLERENE

These are small molecules of carbon in which the giant structure is closed over into spheres of atoms (bucky balls) or tubes (sometimes caled nano-tubes). The smallest fullerene has 60 carbon atoms arranged in pentagons and hexagons like a football. This is called Buckminsterfullerene.
The name 'buckminster fullerene' comes from the inventor of the geodhesic dome (Richard Buckminster Fuller) which has a similar structure to a fullerene. Fullerenes were first isolated from the soot of chimineys and extracted from solvents as red crystals.
The bonding has delocalised pi molecular orbitals extending throughout the structure and the carbon atoms are a mixture of sp2 and sp3 hybridised systems.
Fullerenes are insoluble in water but soluble in methyl benzene. They are non- conductors as the individual molecules are only held to each other by weak van der Waal's forces.

Structure As the molecule is totally symmetrical with all bond lengths and angles being equal, it is likely/inevitable that the hybridisation of the carbon atoms is somewhere between that of sp2 and sp3. Another example of a theory (hybridisation in this case) having to be modified to accomodate the observed experimental data.

Property	Explanation	Fullerene structure
Soft and slippery	Few covalent bonds holding the molecules together but only weak Vander Waals forces between molecules.	Click on the image with the left mouse button and drag to get a different view. (if you can't see the image you have to download chime)
Brittle	Soft weak crystals typical of covalent substances
Electrical insulator	No movement of electrons available from one molecule to the next. The exception could be the formation of nano-tubes that are capable of conducting electricity along their length. These are the subject of some experiments in micro electronics
Insoluble in water.	There are only very weak Van der Waal's attractions between the carbon atoms and the water molecules whereas the carbon atoms are bonded very tightly to one another in the molecules.
Low m.p. solids	Typical of covalent crystals where only Van der Waal's interactions have to be broken for melting.

^{ALLOTROPHY IN SULPHUR}

^{The allotropes of sulfur refers to the many allotropes of the element sulfur. In terms of large number of allotropes, sulfur is second only to carbon.[1] In addition to the allotropes, each allotrope often exists in polymorphs, delineated by Greek prefixes (α, β, etc.).}

1. Rhombic Sulphur:

It is an allotropic form of sulphur which is stable below 96 one molecule of rhombic sulphur contains 8-atoms i-e 8_g. The crystal of rhombic sulphur has octahedral structure.

 Properties:

• It is consist of pale yellow crystals.
• It melts at 110℃.
• It is insoluble in water and soluble in carbon disulphide.
• It is stable at room temperature.
• Its specific gravity is 208g/cm3.

2. Monoclinic Sulphur:

It is the allotropic form of sulphur which is stable between 96 to 119 a molecule of monoclinic sulphur consists of eight sulphur atoms i-e 8g, but is different from rhombic sulphur in the arrangement of atoms.

Properties:

• It is stable from 96℃-119℃.
• Its melting point is 119℃.
• It is soluble in carbon disulphide.
• Its one molecule consist of 8 atoms.
• It is found as pale yellow needle shaped crystals.

3. Plastic Sulphur:

It is a non crystalline allotropic form of sulphur, it can be stretched like a rubber, it is unstable and changes into rhombic sulphur on slight heating even at room temperature it also changes.

4.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.

5.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.

 ALLOTROPHS OF PHOSPHORUS

Elemental phosphorus can exist in several allotropes, the most common of which are white and red solids. Solid violet and black allotropes are also known. Gaseous phosphorus exists as diphosphorus and atomic phosphorus

White phosphorus

White phosphorus, yellow phosphorus or simply tetraphosphorus (P₄) exists as molecules made up of four atoms in a tetrahedral structure. The tetrahedral arrangement results in ring strain and instability. The molecule is described as consisting of six single P–P bonds. Two different crystalline forms are known. The α form, which is stable under standard conditions, has a body-centered cubic crystal structure. It transforms reversibly into the β form at 195.2 K. The β form is believed to have a hexagonal crystal structure.

Red phosphorus

Red phosphorus may be formed by heating white phosphorus to 300 °C (482 °F) in the absence of air or by exposing white phosphorus to sunlight. Red phosphorus exists as an amorphous network. Upon further heating, the amorphous red phosphorus crystallizes. Red phosphorus does not ignite in air at temperatures below 240 °C, whereas pieces of white phosphorus ignite at about 30 °C. Ignition is spontaneous at room temperature with finely divided material. Heating red phosphorus in the presence of moisture creates phosphine gas, which is both highly flammable and toxic

Black phosphorus 

Black phosphorus is the thermodynamically stable form of phosphorus at room temperature and pressure. It is obtained by heating white phosphorus under high pressures (12,000 atmospheres). In appearance, properties, and structure, black phosphorus is very much like graphite with both being black and flaky, a conductor of electricity, and having puckered sheets of linked atoms. Phonons, photons, and electrons in layered black phosphorus structures behave in a highly anisotropic manner within the plane of layers, exhibiting strong potential for applications to thin film electronics and infrared optoelectronics. Light absorption in black phosphorus is sensitive to the polarization of incident light, film thickness, and doping. Black phosphorus photo-transistors exhibit hyper-spectral detection attributes in the infrared and visible spectrum.
Black phosphorus has an orthorhombic structure and is the least reactive allotrope, a result of its lattice of interlinked six-membered rings where each atom is bonded to three other atoms.Black and red phosphorus can also take a cubic crystal lattice structure. A recent synthesis of black phosphorus using metal salts as catalysts has been reported.

Diphosphorus

The diphosphorus allotrope (P₂) can normally be obtained only under extreme conditions (for example, from P₄ at 1100 kelvin). In 2006, the diatomic molecule was generated in homogenous solution under normal conditions with the use of transition metal complexes (for example, tungsten and niobium).^[23]
Diphosphorus is the gaseous form of phosphorus, and the thermodynamically stable form between 1200 °C and 2000 °C.

THANKING YOU 

VIKAS BHATI