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Allotropy and Allotropes of Phosphorus - Definition of Allotropy, Allotropes of Phosphorus, Structures, Physical and Chemical Properties, Uses

Allotropy and Allotropes of Phosphorus - Definition of Allotropy, Allotropes of Phosphorus, Structures, Physical and Chemical Properties, Uses

Have you ever wondered looking at a chameleon (if at all you could recognise one sitting on a branch of a tree or so) how quickly it can change its colour. Indeed this is intriguingly admirable how one single species can exist in multiple physical forms without changing its core properties or values!

Certain elements in our periodic table are known to exhibit such properties wherein a particular element in the same physical state can exhibit more than one physical form. Allotropy originated from the Greek word ‘allottropia’ meaning ‘changeable’.

In the year 1841, Swedish scientist Baron Jöns Jakob Berzelius first proposed the concept of Allotropy. 

Let’s dig deeper into the vivid variations shown by allotropes!

Allotropes are different structural modifications of a chemical element existing mostly in the same physical state; wherein the atoms of the element are bonded together in a different manner.

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TABLE OF CONTENTS

What is Allotropy?

Allotropes are different structural modifications of the same element and can exhibit quite different physical properties and chemical behaviours. The change between allotropic forms is caused by physical forces like pressure, light, and temperature. Hence, the stability of a particular allotrope of an element depends on particular conditions and its structural composition.

Many elements (especially non-metals) from the p-block of the periodic table exhibit allotropy. For example, carbon, oxygen, phosphorus, sulphur, and selenium from the p-block exhibit allotropy. Allotropes of carbon include diamond, graphite, graphene, fullerenes, carbon nanotubes, etc. Phosphorus also has many solid state allotropes and also a gaseous phase allotrope.

Properties of Allotropes

At different temperatures, pressure conditions and atmospheric conditions, an element finds stability in different geometries where atoms are bonded in different ways. Hence, these elements show allotropy.

  • Allotropes have different structural features belonging to the same element and therefore can exhibit different physical and chemical properties.
  • The change between one allotropic form to another is caused by physical forces like pressure, light, and temperature
  • Stability of the different allotropes relies on specific conditions.
  • For example, diamond and graphite (two allotropes of carbon) have different appearances, hardness values, melting points, boiling points, and reactivities.
  • Allotropes of some elements have different molecular formulae or different crystalline structures, as well as they differ in physical phase. For example, two allotropes of oxygen (dioxygen, O2 and ozone, O3) can both exist in the solid, liquid and gaseous states.
  • All elements showing allotropy do not maintain distinct allotropes in different physical phases. For example, phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state. We shall discuss the various allotropes of phosphorus in detail here.

Allotropes of Phosphorus

Allotropes of phosphorus are originally P4 and there are around 12 allotropes of phosphorus. The major ones are white phosphorus, red phosphorus, black phosphorus, diphosphorus (a gaseous allotrope), scarlet and violet phosphorus. 

Phosphorus is a solid non-metallic compound at room temperature. The most common (and reactive) of all its allotropes is white (or yellow) phosphorus which looks like a waxy solid or plastic. The other common form of phosphorus is red phosphorus which is much less reactive and is one of the components of the matchstick head. Red phosphorus can be transformed into white phosphorus by careful heating.

White Phosphorus - Structure and Preparation

White phosphorus comprises of discrete tetrahedral P4 molecules, whereas the crystal structure of red phosphorus has a more complex network like bonding. White phosphorus is very reactive and will spontaneously ignite in the air. So, to avoid natural combustion, it is stored underwater. For red phosphorus, this does not happen.

  • The four sp3 hybridised phosphorus atoms are placed at the four corners of a regular tetrahedron with ∠PPP= 60o. All are P-P covalent bonds.

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Preparation: White or yellow phosphorus is made by heating phosphate rock, silica, and coke in an electric furnace at 1770 K.

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Physical Properties of White Phosphorus

  • It is a translucent, white waxy soft solid.
  • It has a distinct garlic-like odour.
  • It is naturally poisonous. 
  • It is insoluble in water but soluble in carbon disulphide and oils.
  • The molecules of white phosphorus are held together by weak van der Waals forces of attraction and thus its melting point is 317 K and boiling point is 553 K.
  • It has the property to glow in the dark, i.e., it shows chemiluminescence. 

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Chemical Properties of White Phosphorus

  • Due to the angular strain in P4 molecule (wherein the angle is 60∘), it is thermodynamically less stable and hence more reactive than other solid allotropes. 
  • So, in the presence of air, it readily catches fire when freely exposed to air and gives out dense white fumes of P4O10.

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  • In an inert atmosphere of CO2, it dissolves in boiling NaOH to give phosphine.

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  • In the insufficient supply of oxygen, white phosphorus produces P4O6.

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  • White phosphorus readily reacts with several metals forming phosphides.

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  • It reacts with non-metals like halogens or sulphur having more electronegativity than phosphorus.

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  • P4 is a powerful reducing agent as well. It reduces sulphuric acid to sulphur dioxide and produces phosphoric acid alongside.

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Red Phosphorus - Structure and Preparation

Red phosphorus is polymeric, consisting of chains of P4 tetrahedrals linked together, forming a giant molecule. Heating white phosphorus at 573 K in an inert atmosphere for several days produces red phosphorus.

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Red phosphorus has more atoms linked together in a network. So, it is more stable than white phosphorus and is less reactive.

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Physical and Chemical Properties of Red Phosphorus

  • Red phosphorus has an iron-grey lustre and is an odourless crystalline solid.
  • Red phosphorus is non-poisonous.
  • The melting point of red phosphorus is 860 K.
  • It is neither soluble in water, nor in carbon disulphide.
  • Red phosphorus does not glow in the dark. 
  • Unlike white phosphorus, red phosphorus is steady under normal conditions and does not ignite in the air. But red phosphorus reacts with oxygen at 565 K to give P4O10.
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  • It does not react with boiling NaOH, but disintegrates in alcoholic potash.
  • Red phosphorus under high pressure can be converted to black phosphorus under an inert atmosphere.
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  • Because it is less reactive than white phosphorus, it only forms salts with halogens, sulphur, and alkali metals when heated.

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Black Phosphorus - Structure and Preparation

α-Black and β-Black are two forms of black phosphorus. 

  • When red phosphorus is heated in a sealed tube at 803 K, α-black form is formed. It can be sublimed in the air and has either rhombohedral or opaque monoclinic crystals.
  • When white phosphorus is heated under high pressure of around 12,000 atm at 473 K, β-Black is formed.

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General Properties of Black Phosphorus

  • Black phosphorus is the most stable of all allotropes.
  • α-Black and -Black do not oxidise in the air. 
  • They are non-toxic, non-inflammable and chemically inert up to very high temperatures.
  • α-Black is either opaque, monoclinic or rhombohedral crystals.
  • image-Black has corrugated sheets that form flaky crystals. Their structures resemble graphite, it has puckered sheet-like structure and so it is flaky like graphite.
  • image-Black does not burn in the air up to 673 K.
  • image-Black phosphorus conducts electricity due to such structure.
  • α-Black phosphorus does not conduct electricity.

Violet Phosphorus

Violet phosphorus is obtained from heating and crystallising red phosphorus in a certain way. The phosphorus forms pentagonal "tubes".

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Properties of Violet Phosphorus

  • This allotrope does not ignite in the air until heated to 300 °C .
  • It is insoluble in all solvents. 
  • It is not attacked by any alkali and only slowly reacts with halogens. 
  • It can be oxidised by nitric acid to phosphoric acid.

P4(s) + 20HNO3 (aq) → 4H3PO4(aq) + 20NO2(g) + 4H2O(l)

  • If it is heated in an atmosphere of inert gas, for example, nitrogen or carbon dioxide, it sublimes and the vapour condenses as white phosphorus. 
  • If it is heated in a vacuum and the vapour condensed rapidly, violet phosphorus is obtained. 
  • It would appear that violet phosphorus is a polymer of high relative molecular mass, which on heating breaks down into P2 molecules.
  • Diatomic phosphorus on cooling dimerize to give P4 molecules, but in a vacuum, they join up by linking up again to form the polymeric violet allotrope.

Diphosphorus (P2)

Diphosphorus (P2) is the gaseous form of phosphorus that is thermodynamically stable in between 1200 °C and 2000 °C. It can be generated by heating white phosphorus to 1100 K and is highly reactive with a bond-dissociation energy (117 kcal mol-1 or 490 kJ mol-1).

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Uses of Allotropes of Phosphorus

  • White phosphorus is used to manufacture chemical fertilisers, cleaning substances, pesticides, fire-crackers etc.
  • White phosphorus is used to make rat poison.
  • It is also used to make explosives and bombs, in pyro techniques, especially in warfare.
  • Phosphorus allotropes are also used for the synthesis of compounds like phosphorus chlorides, phosphoric acids and hypophosphites used in industry and in medicine.
  • The most common use of red phosphorus is in making matchboxes and match sticks. The striking surface of a matchbox is made up of red phosphorus and powdered glass, which on friction with the stick converts to white phosphorus and ignites a flame in the air.
  • It is also used in combination with iron as additives for gasoline and lubricating oil.
  • Red phosphorus is also used as a flame retardant in many thermoplastics and thermosetting plastics.
  • Red phosphorus mixed with magnesium and a binder can be used as a smoke device that can quickly create a smokescreen.
  • Black phosphorus has several biomedical applications. Its nanosheets are used as nanomedicine agents for photothermal and photodynamic therapy. 

Practice Problems

Q1. Which of the following isotopes of phosphorus glows in the dark?

A. White Phosphorus
B. Red Phosphorus
C. Black Phosphorus
D. Violet Phosphorus

Answer: White phosphorus glows in the dark due to chemiluminescence. It occurs due to slow oxidation on its surface due to the fact that it is thermodynamically less stable. So, it reacts with oxygen on the surface and glows in dark. So, option A) is the correct answer.

Q2. Thermodynamically most stable allotrope of phosphorus is

A. White Phosphorus
B. Black Phosphorus
C. Red Phosphorus
D. Scarlet Phosphorus

Answer: The lattice structure of black phosphorus is an interlinked ring of six P atoms. Here, each atom is bonded to three other atoms. This makes it a puckered sheet-like strongly interlinked structure, which is difficult to break. Hence, it is the most stable allotrope. This allotrope has the maximum amount of interlinking.

Q3. The reaction of P4with which of the following reagents will produce P4O6?

A.  Dry O2
B. A mixture of O2and caustic soda
C. Limited O2
D. None of the above

Answer: 

In presence of excess oxygen supply, P4O10 is formed.

P4(s)+5O2(g) → P4O10(s) occurs. 

Limited supply of oxygen produces P4O6.

P4(s)+5O2(g) → P4O6(l). 

So, option C) is the correct answer. 

Q4. What is the phase of white phosphorus at STP?

A. Solid
B. Liquid
C. Gas
D. None of the above

Answer: White phosphorus (P4) has a covalently bonded tetrahedral shape. Hence, it is a solid at STP. 

Frequently Asked Questions - FAQ

Question 1. Why white phosphorus has the structure of P4 and sulphur can exist as S2?
Answer:
White phosphorus has the structure of P4 as one phosphorus atom can form three bonds at a time. Thus, phosphorus forms a P4 white phosphorus tetrahedron (being sp3hybridised), while sulphur can only form two bonds. Hence, sulphur only forms rings and chains. 

Question 2. Which allotrope of phosphorus is poisonous in nature?
Answer:
The least stable, the most reactive, the most volatile, the least dense, and the most toxic of the allotropes is white phosphorus. It eventually changes to red phosphorus, a light and heat-accelerated transition.

Question 3. Which phosphorus allotrope is used in matchstick?
Answer:
Red phosphorus. The striking surface of a matchbox is made up of red phosphorus and powdered glass, which on friction with the stick converts to white phosphorus and ignites a flame in the air.

Question 4. Why do some of the beaches show chemiluminescence?
Answer:
White phosphorus present in the ocean helps in the production of microbes and tiny marine plants called phytoplankton. When white phosphorus is particularly abundant in the water, phytoplankton produce and store a form of phosphorus called polyphosphate to use later during times of phosphorus scarcity. This is why phytoplankton rich beaches produce chemiluminescence.

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Related Topics

Phosphorus

Oxygen

Phosphorus halides

Potassium

Phosphine

Alkali Metals

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