Phosphorus- Allotropes, Properties, Reactions, Compounds, Uses, Practice Problems & FAQs

One fine night in 1669, a German physician Hennig Brand, inspired by the various features of alchemy, tried to create the fabled philosopher’s stone.

Earlier that day, Brand, in his laboratory, had been heating a mixture of sand and charcoal with a tar-like substance produced by boiling down huge gallons of urine over two weeks.

Unbeknownst to him, this was going to lead to a truly world-altering discovery!

Hours later, a white vapour formed and condensed into thick drops that gleamed brightly for hours. This glowy, waxy substance was called phosphorus, by Brand, a Latin term for things that give off light.

TABLE OF CONTENTS

Introduction to Phosphorus
Properties of Phosphorus
Reactions of Phosphorus
Compounds of phosphorus
Isolation of Phosphorus
Uses of Phosphorus
Practice Problems
Frequently Asked Questions-FAQs

Introduction to Phosphorus

Phosphorus, is a highly reactive non-metal, denoted by the symbol P and atomic number is 15. It lies below nitrogen in Group 15 (pnictogens) of the modern periodic table.

Phosphorus is so reactive that it is never found in a free state in Earth’s crust. It is majorly present in phosphates (PO₄^3-such as apatite and fluorapatite. Pure phosphorus is typically mined from phosphates.

Phosphates are also essential substances in the human body because they are part of DNA materials and contribute to the delivery of energy. Phosphates can also be commonly found in plants.

The red matchstick heads actually have red phosphorus on them. When they are rubbed against the surface, friction between the two generates heat and causes red phosphorus to be converted to white phosphorus which spontaneously ignites in the air.

Properties of Phosphorus

Phosphorus belongs to group 15, Period 3, p-block of elements.
Its relative atomic mass is 30.974 u.
It has a melting point of 44.15°C.
It has a boiling point of 280.5°C.
Phosphorous in nature exists in the form of P₄ molecules. The phosphorus atom has an electronic configuration of [Ne]3s²3p³.
Phosphorus is a non-metal which is solid at room temperature, and a poor conductor of heat and electricity.
The most common oxidation states exhibited by Phosphorus are +3, -3 and +5. It exhibits oxidation states of +3 or −3 depending on the electronegativity of the combining atoms.
The presence of vacant d-orbitals in phosphorus allows an expansion of the octet, which leads to the +5 state oxidation state of P.
Phosphorus exists in around ten allotropic forms out of which three major allotropes are white, red, and black phosphorus. White phosphorous is the most important and reactive one which exists as p₄.
The gaseous form of phosphorus is Diphosphorous (p₂) which is thermodynamically stable above 1200°C and until 2000°C.
White phosphorus p₄has a tetrahedral structure. It is soft and waxy, but insoluble in water.

The most stable allotrope of phosphorus is black phosphorus; the atoms are linked together in puckered sheets, like graphite.
p³¹ is the most stable isotope of phosphorus. Out of around 21 isotopes, p³² and p³³ are radioactive.
Elemental phosphorus exhibits phosphorescence.
Elemental phosphorus in all its forms is extremely toxic to human beings, the most toxic form being white phosphorus.

Reactions of Phosphorus

Phosphorus readily undergoes combustion with oxygen, forming dense white fumes.

With a sufficient amount of oxygen, phosphorus forms p₄o₁₀.

With an insufficient amount of oxygen, phosphorus forms p₄o₆.

Phosphorus reacts with all halogens to form phosphorus halides (px₃ and px₅) except with iodine which can only form phosphorus trihalide and not pentahalide. Since the atomic radius of iodine is big, so 5 iodine atoms cannot be accommodated around the phosphorus atom owing to steric crowding. Hence pl₅ does not exist.

Phosphorus does not react with water, so phosphorus is stored underwater to prevent its reaction with air. On boiling, phosphorus reacts with water producing phosphine and phosphorous acid.

Phosphorus reacts with hydrochloric acid to produce phosphine and phosphorous trichloride.

Compounds of Phosphorus

Phosphorus exists in a combined state due to its highly reactive nature. It is majorly present as phosphates. Apart from that it also forms hydrides, oxides and oxoacids of phosphorus are also well known.

Phosphates

Commonly found phosphate compounds are: Calcium phosphate: Ca₃(PO₄)₂ , apatites: Ca₅(PO₄)₂(OH), fluorapatite: Ca₅(PO₄)₂F; and chlorapatite: Ca₅(PO₄)₂Cl. The most popular phosphate fertilizer contains the compound calcium dihydrogen phosphate, or superphosphate:Ca(H₂PO₄)₂

Hydride of phosphorus

Hydride of phosphorus is known as phosphine (PH₃) and is obtained on reacting sodium hydroxide with phosphorus.

Oxides of phosphorus

Phosphorus pentoxide, P₄O₁₀ is formed when white phosphorus is heated in sufficient oxygen supply, and it acts as a dehydrating agent.

Tetraphosphorus hexaoxide, P₄O₆ is an anhydride of phosphorous acid. Phosphorus in this compound is in +3 oxidation state. It is formed when white phosphorus is heated in an insufficient oxygen supply.

Oxoacids of phosphorus

Phosphorus forms oxoacids when these oxides react with water. Phosphorous pentoxide reacts vigorously with water to form orthophosphoric acid where phosphorus is in the +5 oxidation state.

Tetraphosphorus hexa-oxide P₄O₆, reacts with water to form phosphorous acid.

When orthophosphoric acid is heated at about 250⁰Cit forms pyrophosphoric acid.

Meta-phosphoric acid is formed when orthophosphoric acid is heated at 850 K.

Isolation of Phosphorus

Industrially elemental phosphorous is isolated from calcium phosphate, obtained from phosphorous rock [Ca₃(PO₄)₂] by heating it with sand (Sio₂) and coke at very high temperatures (around 1500 K):

Using aluminium in the reaction mixture helps in conducting the same reaction at lower temperatures.

Uses of Phosphorus

Phosphate is a significant ingredient in fertilizers and helps produce high crop yields.
Phosphorus compounds are also used in making pesticides, detergents, baking powder and plasticisers.
Phosphorus plays a major role in the making of steel.
Phosphates are also used in the production of fine chinaware and special glasses.

Practice Problems

Q.1. What is the elemental state of phosphorus?

Solid
Gas
Plasma
Liquid

Answer: Elemental phosphorus is solid and exists as a tetrahedral shaped P₄ molecule.

Q.2. Complete the following reactions:

Q.3. What is the state of phosphorus at STP?

Answer: Elemental phosphorous is solid at STP with the formula P₄.

Q.4. What is the oxidation state of Phosphorus in Na₄p₂o₇?

+5
-5
+3
-3

Answer:

Let the oxidation state of p be x.

So oxidation state of phosphorous is +5 in Na₄p₂O₇.

Frequently Asked Questions- FAQs

Question1. What does phosphorous do in a human body?

Answer: Phosphorus is a major component of bones, teeth, as well as DNA and RNA. It is an integral part of our cells, as phospholipids and cell membranes. Moreover, it is an active energy carrier in our body in the form of adenosine triphosphate.

Question2. Why is phosphorous known as the Devil’s element?

Answer: This was due to its uncanny glow and its tendency to undergo combustion and burst into flames. Also, because it was the 13th element, it was considered related to the Devil.

Question3. Mentions the food sources that are rich in phosphorus?

Answer: Phosphorous is easily found in protein-rich foods such as eggs, poultry, meat, fish, dairy products, beans and nuts. Organic phosphorus obtained from animal sources is more readily absorbed by the body than those obtained from plant sources.

Question4. Is phosphorus harmful to the human body?

Answer: Elemental phosphorous is toxic. Hence, ingestion of elemental phosphorous can typically cause severe vomiting and diarrhoea. Severe poisoning may lead to, coma, hypotension, dysrhythmias, and eventually death.

NCERT Class 12 Chemistry Chapters

Chapter 1 The Solid State	Chapter 6 General Principles and Processes of Isolation of Elements	Chapter 11 Alcohols, Phenols, and Ethers
Chapter 2 Solutions	Chapter 7 The p-Block Elements	Chapter 12 Aldehydes, Ketones, and Carboxylic Acids
Chapter 3 Electrochemistry	Chapter 8 The d & f Block Elements	Chapter 13 Amines
Chapter 4 Chemical Kinetics	Chapter 9 Coordination Compounds	Chapter 14 Biomolecules
Chapter 5 Surface Chemistry	Chapter 10 Haloalkanes and Haloarenes	Chapter 15 Polymers