- Allotropy is the existence of an element in more than one physical form. It means that under different conditions of temperature and pressure an element can exist in more than one physical forms.
- Coal, graphite and diamond etc., are different allotropic forms of carbon.
- Polymorphism is the existence of a substance in more than one crystalline form.
- It means that under different conditions of temperature and pressure, a substance can form more than one type of crystal.
For example,
Mercuric iodide exists in the orthorhombic and trigonal form.
