Allotropic Form of Phosphorus

Phosphorus is a component that cannot be found naturally in our surroundings. It is highly reactive. Phosphorus is an element with an atomic number of 15, which means it has 15 protons and 15 electrons in its atomic structure. Phosphorus has the chemical symbol P. The phenomenon of the existence of a component in addition to one physical form is named allotropy. Allotropes are chemical elements that exist in two or more forms that may differ in the arrangement of atoms in a crystalline solid or in the presence of molecules containing different numbers of atoms. The various physical forms of an element are called allotropes. Phosphorus exists in several allotropic forms. Among these, the three main forms are white phosphorus, red phosphorus and black phosphorus. Red phosphorus is the most stable allotropic sort of phosphorus.

Black Phosphorus

Black phosphorus is a multilayered allotrope of phosphorus with a two-dimensional structure that is weakly bound together by van der Waals forces. Once separated, the material’s resulting monolayer (or layers) is called phosphorene. Due to its unique properties and potential in electronics and optoelectronics, this material is currently of great interest to the scientific community. Black phosphorus is of two types: alpha black phosphorus and beta black phosphorus.

The crystal shape of alpha black phosphorus is non-transparent monoclinic or rhombohedral. It is made by heating red phosphorus in a sealed tube to 803K. Beta black phosphorus has low reactivity. It is made by heating white phosphorus to 473K under high pressure.

Preparation of Red Phosphorus

Black phosphorus can be made from white phosphorus as follows:

White phosphorus  →  470K (inert atmosphere) Black phosphorus

Among all three allotropes, black phosphorus is the most stable. Black phosphorus has a layered structure and is a highly polymerised form of phosphorus. It comes in two forms, alpha black phosphorus and beta black phosphorus.

Structure of Black Phosphorus

Structure of black phosphorus. Schematic representation of the crystal structure of black phosphorus obtained from structural relaxation using density functional theory calculations . The layered structure consists of plates with phosphorus atoms arranged in a pleated honeycomb lattice. Adjacent layers interact via weak van der Waals forces and stack in an ABA stacking order.

Properties of Red Phosphorus

The general properties of black phosphorus are as follows:

• It exists in three crystalline forms and one amorphous form.

• Alpha-black phosphorus is a relatively good conductor of electricity, but – black phosphorus is a non-conductor.

• β-Black phosphorus has a black metallic lustre.

• It is the significantly less reactive and most stable form of phosphorus.

Phosphonic Acid

Phosphonic acid is a phosphorus oxoacid that consists of single pentavalent phosphorus, covalently bound via single bonds to single hydrogen, and two hydroxyl groups via a covalent bond to an oxygen. The parent of the category of phosphonic acids and a phosphorus oxoacid, it works as a fungicide. It is the conjugate acid of a phosphonate(1-) and a tautomer of hypophosphorous acid.

Phosphinic acid or phosphorous acid, H3PO2

Preparation of Phosphinic Acid or Hypophosphorous Acid

Phosphinic acid is prepared by heating phosphorus with alkalis such as barium hydroxide to form a barium phosphate precipitate. The precipitate is set apart and heated with a

Calculated amount of sulphuric acid.

3Ba(OH)2 + 2P4 + 6H2 O —–> 2PH3 +3Ba(H2 PO2)2

Ba(H2 PO2)2 + H2SO4 → BaSO4+ 2H3 PO2

Barium sulphate is separated by filtration, and therefore, the solution is concentrated when hypophosphorous acid separates.

It possesses one P=0, one P-OH and two P-H bonds. Since there is just one P-OH bond, it has

Only one ionisable hydrogen atom. Thus, it behaves as monobasic acid, ionising as:

H3 PO2 ⇔ H+ + H2 PO2 ¯

The other two hydrogen atoms directly bonded to P are not ionisable. Thanks to the presence of two P–H bonds, the acid behaves as a strong reducing agent, just like its salt.

On heating to 313K, it decomposes to phosphine.

3H3PO2 → PH3 + 2H3 PO3

However, it decomposes to phosphoric acid and phosphine when heated above 333K.

2H3PO2 →H3 PO4 + PH3

Conclusion

Phosphorus is a vital element utilised in chemistry. It is mainly obtained through phosphates, which are molecules containing the phosphate ion PO43–. ATP, DNA, RNA, and phospholipids are found in phosphates, all essential components of cells. Human urine was the earliest elemental phosphorus source, and ash was a crucial early phosphate source. Elemental Phosphorus exists in several allotropic forms; the most common forms are white Phosphorus, red Phosphorus and black Phosphorus. Black phosphorus is obtained by dissolving red phosphorus in molten lead in a sealed tube at 400°C for a long time. As the mixture cooled, black phosphorus crystals formed. Weak nitric acid treatment dissolves lead. It is a highly stable phosphorus allotrope and does not oxidise until heated strongly in the air. It does not conduct electricity.