Question

Why is red phosphorus less reactive than white phosphorus?

Answer 1

The chemical reactivity of any substance depends on the type of bonding present in the substance. If the bonds present are strong bonds, then it is very difficult to break these bonds. So such a substance will be chemically less reactive. On the other hand, if the bonds present are not strong enough, the substance will be chemically reactive.

Complete answer:
White phosphorous and red phosphorus are two of several allotropes of phosphorus. In both allotropes, white phosphorus and red phosphorus, only phosphorus atoms are present. No atoms of other elements are present. Hence, we should expect that these two allotropes have similar chemical reactivity. However, in reality, red phosphorus is less reactive than white phosphorus. This is due to different types of bonding present in these two allotropes.

We can describe the structure of white phosphorus as individual tetrahedral units. Each tetrahedral unit has four phosphorus atoms present at four corners of a regular tetrahedron.

Covalent bonds are present between phosphorus atoms in a particular tetrahedral unit. However, no covalent bond is present between two different tetrahedral units. Physical forces of attraction are present between different tetrahedral units. No chemical force of attraction I present between different tetrahedral units. This gives rise to an unstable arrangement of the tetrahedral units. Due to this, white phosphorus has high reactivity.

The structure of red phosphorus is somewhat similar to the structure of white phosphorus. We can describe the structure of red phosphorus consisting of tetrahedral units. Each tetrahedral unit has four phosphorus atoms present at four corners of a regular tetrahedron.

Covalent bonds are present between phosphorus atoms in a particular tetrahedral unit. However, covalent bonds are also present between two different neighboring tetrahedral units. This leads to the formation of a giant amorphous network of tetrahedral units. It leads to the formation of a stable arrangement. Due to this, red phosphorus has less reactivity. Hence, red phosphorus is less reactive than white phosphorus.

Note: Allotropes are different physical forms of an element, in the same physical state. Allotropes can have different physical and chemical properties. White phosphorus, red phosphorus and black phosphorus are three major allotropes of phosphorus.