What element exists as an unstable white allotrope and a more stable red allotrope?

The element that exists as an unstable white allotrope and a more stable red allotrope is phosphorus. Phosphorus has several allotropes, with white phosphorus being highly reactive and unstable, typically stored underwater or in inert gases to prevent it from igniting in air. The white allotrope is known for its tetrahedral arrangement of phosphorus atoms, making it prone to spontaneous combustion.

In contrast, red phosphorus, which is a more stable allotrope, has a polymeric structure that greatly reduces its reactivity. This stability allows red phosphorus to be used safely in applications such as safety matches and fireworks. The transformation from white to red phosphorus also involves a significant change in the molecular arrangement, illustrating the different characteristics and properties of these allotropes.