The existence of an element in two or more different forms is called allotropy.

Here's a breakdown:

* Allotropy: The property of an element to exist in two or more different forms, known as allotropes. These allotropes have the same chemical composition but differ in their physical properties due to differences in their atomic arrangement.

You mentioned examples like carbon, oxygen, phosphorus, and sulfur. Here are some examples of their allotropes:

* Carbon: Diamond, graphite, fullerene, graphene

* Oxygen: Diatomic oxygen (O2), Ozone (O3)

* Phosphorus: White phosphorus, red phosphorus, black phosphorus

* Sulfur: Rhombic sulfur, monoclinic sulfur

Let me know if you have any other questions.