Allotropes of phosphorus

Elemental phosphorus can exist in several allotropes, which are different structural forms of the same element. The most common forms are white and red solids, but solid violet and black allotropes are also known. In its gaseous state, phosphorus can be found as diphosphorus and atomic phosphorus.

These different forms have unique properties and uses. For example, white phosphorus is highly reactive and glows in the dark, while red phosphorus is more stable and is used in safety matches. Understanding these allotropes helps scientists and engineers develop new materials and technologies.

White phosphorus

Main article: White phosphorus

White phosphorus is a special form of the element phosphorus. It looks like a waxy solid and turns yellow when exposed to light. It is made of tiny molecules shaped like a tetrahedron, with four phosphorus atoms connected together.

This form of phosphorus is very reactive. It can catch fire easily in air, especially when it is very finely divided. It also glows in the dark when exposed to oxygen. Because it is toxic and can cause serious health problems if eaten or breathed in over time, it must be handled with care. It is usually stored under water to keep it from catching fire.

Red phosphorus

Main article: Red phosphorus

Red phosphorus is made by heating white phosphorus to 300 °C without air, or by letting white phosphorus sit in sunlight. It looks different from white phosphorus because it doesn’t catch fire as easily. In fact, red phosphorus needs to be much hotter—over 240 °C—before it will ignite, unlike white phosphorus which can catch fire at around 30 °C.

Red phosphorus was first shown to scientists in 1847 by a person named Anton von Schrötter. It is more stable than white phosphorus but not as stable as black phosphorus, another form of the element.

Violet or Hittorf's phosphorus

Monoclinic phosphorus, also known as violet phosphorus or Hittorf's metallic phosphorus, is a special form of phosphorus that comes from red phosphorus. It was first made in 1865 by a scientist named Johann Wilhelm Hittorf, who heated red phosphorus in a sealed tube. This process created shiny, opaque crystals.

Violet phosphorus is different from other forms because it does not catch fire easily and does not dissolve in solvents. It can be oxidized by nitric acid to make phosphoric acid. When heated in certain conditions, it can change into white phosphorus or remain as violet phosphorus depending on how it is cooled.

Black phosphorus

Black phosphorus is a stable form of phosphorus that exists at normal room temperature and pressure. It was first made in 1914 by heating white phosphorus under very high pressure. Black phosphorus looks and behaves much like graphite—it is black, flaky, and can conduct electricity. It has a special structure made of linked rings, which makes it less reactive than other forms of phosphorus.

One interesting feature of black phosphorus is that it can be made into a very thin material called phosphorene, similar to graphene. Phosphorene has special properties that make it useful for tiny electronic devices and sensors. However, it can break down when exposed to water and air. Scientists are exploring its potential for use in new types of batteries.

Main article: phosphorene

Ring-shaped phosphorus

Scientists first thought about a special form of phosphorus called "ring-shaped phosphorus" in 2007. They made it by putting tiny pieces of phosphorus inside special tubes called carbon nanotubes. One ring they found had a diameter of about 5.3 nanometers and was made up of many phosphorus atoms.

There is also a smaller ring-shaped molecule made of six phosphorus atoms, but it cannot stay stable on its own.

Blue phosphorus

Single-layer blue phosphorus was first made in 2016 using a special method called molecular beam epitaxy, starting from black phosphorus. This new form of phosphorus is interesting to scientists for its unique properties.

Main article: Molecular beam epitaxy

Diphosphorus

Main article: Diphosphorus

Diphosphorus is a special form of the element phosphorus, existing as a gas. It is usually made under very high temperatures, but in 2006, scientists found a way to create it under normal conditions using certain metals. Diphosphorus is stable at very high temperatures, between 1200 °C and 2000 °C, and it forms when tetraphosphorus breaks apart at lower temperatures. At even higher temperatures, above 2000 °C, diphosphorus can break down into single phosphorus atoms.

Phosphorus nanorods

Phosphorus can form tiny, thin shapes called nanorods. Scientists found these nanorods by using special methods at low temperatures. These nanorods are a type of phosphorus that stays stable in air for weeks and look different from regular red phosphorus. Using electron microscopy, they saw that these nanorods are long and straight, with a very small diameter.

Main article: nanorod

Properties

Phosphorus can be found in several different forms, known as allotropes. The most common types are white and red phosphorus, which are solids. There are also solid violet and black forms, as well as gaseous forms such as diphosphorus and atomic phosphorus. Each of these allotropes has its own unique properties and behaviors.