Allotropes of Phosphorus: Forms, Uses & Examples

Instructor: Joelle Mumley
You might have heard of the element phosphorous before. But did you know it exists in different forms that can each be used for a specific purpose? Learn more about these different forms and examples of how they are used in our everyday living.

Forms

When you think of the term 'phosphorous' what first comes to mind? Perhaps you think about the fact that its atomic number is 15 on the periodic table. Or maybe you remember its atomic symbol of P.

Something else well known about phosphorus is that it exists in other forms, or allotropes. An allotrope describes an element that has a variety of different physical properties. The chemical properties of each allotropic form of an element are always comparable.

There are roughly 10 different allotropic forms of phosphorus. The three most common forms include white, red, and black phosphorus. The physical properties are quite different from each other.

White Phosphorus

The white phosphorus allotrope is the most reactive, least stable, and most toxic of the three. It consists of four phosphorus molecules that chemically bond to form a tetrahedral shape. Some of its physical properties include the appearance as a white, waxy substance when a solid. It cannot dissolve in water but is easy to break apart as a solid.

White phosphorus is known for smelling like garlic and is poisonous in nature. It's also highly reactive with both sunlight and air. In sunlight, white phosphorus changes to a yellow phosphorus. In the air it reacts with oxygen, causing the substance to emit a green glow. Out of all the allotropes this one is the least stable, most reactive, and also most poisonous.

We use this last characteristic to our advantage though, since white phosphorus is commonly used to make rat poison and is also used by the military to help generate smoke. However, it's also been used as a chemical weapon, and due to its reactivity and ability to cause massive damage to human tissue, its use is still very controversial, with many calling for its ban.

Red Phosphorus

Red phosphorus is an odorless allotropic form and is certainly more stable than white phosphorus. It's characterized as having a color that resembles a burnt orange, reddish hue. Chemically, when it's heated to around 250°C and in the presence of light, it converts to red phosphorus.

Unlike white phosphorus, red phosphorus is not poisonous. However, it's also unable to dissolve in water just like white phosphorus. It's nonpoisonous, yet flammable nature make it useful in making matches.

Black Phosphorus

When compared to red and white phosphorus, black phosphorus is the most stable form. When white phosphorus is exposed to heat under high-pressurized conditions, it will chemically change to black phosphorus.

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