Question

why Yellow P4 is less stable and more reactive than other allotropes.

Answer 1

White phosphorus is also called as Yellow phosphorus. Although Phosphorus exists in many allotropic forms and comes in different colours, the yellow phosphorus is the most reactive and least stable form.

This is because of the tetrahedral structure of the yellow or white phosphorus. The tetrahedral structure of P₄ is arranged in such a way that a single atom is connected to the other 3 atoms by single bonds. The angles in between these single bonds are 60° which means that the angular ring strain is high in between the angles. This high angular strain makes the yellow phosphorus to be highly reactive and so it is less stable than the red phosphorus which has lesser angular strain.

Answer 2

Yellow P4 is less stable and more reactive than other allotropes because the tetrahedral arrangement causes ring strain and instability.

White phosphorus is also known as yellow phosphorus because it is a translucent waxy solid which quickly becomes yellow when exposed to light. It is made up of discrete P4 tetrahedrons which are subjected to very high angular strain as the angles are 60 degrees. In yellow phosphorus P molecules are relatively more closer to one another than red phosphorus. Though red phosphorus is also tetrahedral it has a chain structure of P atoms so there is less strain. This makes it less reactive.