Phosphorus

What is Phosphorus?

Phosphorus is a nonmetallic chemical element of the nitrogen family or Group 15 of the periodic table with the symbol P and atomic number 15. They are collectively called pnictogen or pnictides from the Greek word choking. Phosphorus is a colourless, semitransparent, soft, waxy solid at room temperature that glows in dark conditions. Nitrogen and phosphorus are essential constituents of the living body. Another heavier member of the nitrogen family, arsenic is extremely toxic and causes water pollution.

Properties of Phosphorus

The phosphorus atom has 15 outer orbitals electrons and the nucleus contains 15 protons and 15 neutrons. The electrons are distributed as 2-8-5. Elemental phosphorus has several allotropic forms like white, red, violet, and black. In the liquid and gaseous state below 800 °C, it constitutes a tetrahedral P₄ molecule. But above 800 °C, it forms a P₂ molecule.

Phosphorus
Symbol	P
Discovery	Hennig Brandt in 1669
Name derived from	The Greek word phosphoros, meaning bringer of light
Allotropes	White P, Red P, Black P
Common isotope	31P
Crystal structure	Body-centered cubic (bcc) crystal lattice
Periodic properties
Atomic number	15
Electron per shell	2, 8, 5
Atomic weight	30.9737
Electronic configuration	1s2 2s2 2p6 3s2 2p3
Group	group-15
Period	period-3
Block	p-block
Physical properties
State at 20 °C	Solid
Melting point	white		red
	44.15 °C		590 °C
Boiling Point	white: 280.5 °C
Density (g/cm3)	white	red	violet	black
	1.823	2.34	2.36	2.39
Chemical properties
Atomic radius (non-bonded)	1.80 Å
Covalent radius	1.09 Å
Oxidation number or states	−3, −2, −1, 0, +3, +4, +4, +5
Ionization energy (kJ/mol)	1st	2nd	3rd
	1011.8	1907	2914.1
Electron affinity	72.037 kJ mol−1
Electronegativity	2.19 (Pauling scale)
Molar heat capacity	For white P: 23.824 J mol−1 K−1
CAS number	7723-14-0 (red) 12185-10-3 (white)

Phosphorus in the Periodic Table

The valence shell electronic configuration of the element is 3s²3p³ with two electrons in the s-orbital and three electrons in three p-orbitals. The electronic configuration suggests that phosphorus is a nonmetal or p-block element placed in group 15 of the periodic table.

Where is Phosphorus Found?

Phosphorus is a highly abundant chemical element in crustal rocks (1120 ppm). It is not found free in nature but is widely distributed in many minerals like fluorapatite, 3Ca₃(PO₄)₂, and CaF₂. It is also found in the plant and animal bodies where it is an essential element for many biological systems.

Over 200 phosphate minerals are distributed in different countries of the world. Morocco, the United States, Australia, China, and Russia are the major depositors of phosphorus minerals.

Isotopes of Phosphorus

It has 23 isotopes with mass numbers ranging from ²⁵P to ⁴⁷P. ³¹P is the only stable isotope of the element. ³²P (beta ray emitter, t_½ = 14 days) is the most important radioactive isotope obtained by (n,p) reaction on ³²S or (n, γ) nuclear reaction on ³¹P in reactors. The radioactive decay of isotopes is extensively used in tracer studies.

Production Process

It is obtained by reducing phosphate rocks with carbon in the presence of sand at 1400 to 1500 °C in an electric furnace. The overall reaction may be,
2 Ca₃(PO₄)₂ + 6 SiO₂ + 10 C → 6 CaSiO₃ + 10 CO + P₄

The vapour is condensed under water molecules to form white phosphorus.

Allotropes of Phosphorus

Elemental phosphorus has several allotropic forms. White, red, violet, and black are the main allotropes of the element.

White Phosphorus

• White phosphorus is the most common allotrope with a melting point of 44 °C and a boiling point of 287 °C. It becomes yellow on exposure to light.
• It is insoluble in water but highly soluble in carbon disulfide, benzene, liquid sulfur dioxide, liquid ammonia, etc.
• Concentrated nitric acid slowly oxidizes the P₄ molecule to form phosphoric acid.
• It is highly poisonous and contacts on skin or inhalation which may cause harmful metabolic disorders.

Red Phosphorus

• Red phosphorus is obtained by getting white-P in the absence of air at 270 °C to 300 °C.
• It is an amorphous solid, melting under pressure at a temperature of 592 °C.
• The nonpoisonous red phosphorus is much denser than the white allotrope.
• The reactivity of the red-P molecule is much less and unattacked by aqueous alkali metals.

Black Phosphorus

• Black phosphorus is the least reactive allotrope of the element. It is formed by heating the P₄ molecule at about 200 °C and at very high pressure.
• An orthorhombic form is obtained at 1200 atmosphere pressure, still higher pressure forms rhombohedral and cubic crystal lattice are formed.
• It does not ignite in the air even at 400 °C and is thermodynamically the most stable allotrope of the element.

Chemical Compounds

The chemistry of nitrogen and phosphorus differs, in the same way as silicon differs from carbon due to the presence of an empty d-orbital. The electronic configuration of phosphorus is closer to the next noble gas than the preceding noble gas.

The formation of a +5 cation by losing five outer electrons is just impossible. A huge amount of energy is necessary for this purpose. The fact suggests that the sum of five ionization energies cannot be compensated by gaining lattice energy for ionic bonding. Therefore most of the compounds in the P(V) are covalent in nature.

Hydrides of Phosphorus

Phosphine is the most common hydride of phosphorus with the chemical formula PH₃. It is a volatile hydride formed by sp³ hybridization. Phosphine can be obtained by hydrolysis of metal phosphide. The thermal stability of the group-15 hydrides decreases rapidly with the increasing atomic weight of the parent element. The hydrides are less stable than the group-14 hydrides.

Phosphorus Halides

Due to the presence of vacant d-orbital, phosphorus formed two types of halides trihalides (PX₃) and pentahalides (PX5). All the covalent trihalides (PX₃) have tetrahedral (sp³) structures with a lone pair of electrons residing in the fourth orbital of the atom.

The pentahalides contain a trigonal bipyramidal structure with sp³d² hybridization and 10 valence shell electrons. The pentahalide like PF₅ is a very strong Lewis acid, forming complex compounds with amines or ethers. The valence shell of the element also expands in octahedral complexes like [PCl₆]⁻ that contain 12 electrons with sp³d² hybridization.

Phosphorus Oxides

The elements of group 15 elements form oxides in their usual +3 and +5 oxidation states. Phosphorus forms two common oxides P₄O₆ ( tetraphosphorus hexaoxide) and P₄O₁₀ (tetraphosphorus decaoxide) in the +3 and +5 oxidation states. It is readily oxidized to form P₄O₁₀ when burnt in excess air.

The structure of P₄O₁₀ is based on PO₄ tetrahedra. P(III) oxide is formed when the element is burnt in a limited supply of oxygen. It is structurally similar to that of the P₄ tetrahedron. It also formed a series of oxides P₄O_x (x = 7, 8, 9).

Oxoacids of Phosphorus

Oxoacids of this element are extensively used and commercially important chemical or biological compounds. It is a structurally complicated chemical. They are acidic protons bound to oxygen atoms, some have nonacidic protons that are bonded directly to phosphorus and some contain P-P chemical bonding.

Only one is commercially important, and three of them are hypophosphorous acid, phosphorous acid, and phosphoric acid. Phosphoric acid (H₃PO₄) contains a tetrahedral skeleton with sp³ hybridization.

Acids	Properties of oxoacids
Phosphonic acid (H3PO2)	It is a monobasic reducing agent known only in the solution.
Phosphorus acid (H3PO3)	It is a dibasic acid crystalline solid.
Hypophosphoric acid (H4P2O6)	It is a tetrabasic acid, although the known salts are Na2H2P2O6.
Phosphoric acid (H3PO4)	It is a tribasic acid with a very low melting point and a variety of salts of phosphoric acid is known.

Uses of Phosphorus

• Phosphorus is the essential constituent of plants or animals and is used mainly for the preparation of concentrated phosphoric acid and phosphate fertilizer.
• It is converted to P₄S₁₀ and P₄S₃ which are used in the production of organo-phosphorus compounds and matches.
• Ferrophosphorus obtained as a byproduct in phosphorus preparation is used in high-density concrete, in radiation shields for nuclear power reactors, and in making special steel and cast iron.
• Phosphates enter the plant and animal kingdom through food chains and play a vital role in the formation of nucleic acids, metabolic functions via adenosine triphosphate (ATP), and in the formation of bone, teeth, etc.
• A huge amount of phosphorus is used in different types of detergent preparation.