Call Now
1800-102-2727Have 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.
TABLE OF CONTENTS
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.
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 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 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.
Preparation: White or yellow phosphorus is made by heating phosphate rock, silica, and coke in an electric furnace at 1770 K.
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.
Red phosphorus has more atoms linked together in a network. So, it is more stable than white phosphorus and is less reactive.
α-Black and β-Black are two forms of black phosphorus.
General Properties of Black Phosphorus
Violet phosphorus is obtained from heating and crystallising red phosphorus in a certain way. The phosphorus forms pentagonal "tubes".
Properties of Violet Phosphorus
P4(s) + 20HNO3 (aq) → 4H3PO4(aq) + 20NO2(g) + 4H2O(l)
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).
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.
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.
Related Topics
Phosphorus |
Oxygen |
Phosphorus halides |
Potassium |
Phosphine |
Alkali Metals |