Allotropy
The existence of an element in two or more distinct structural forms within the same physical state.
Unit
Dimensionless
In depth
Allotropes differ in how their atoms bond and pack, so they can differ enormously in hardness, conductivity and reactivity despite being chemically identical. Conversion between allotropes is a physical change, though it may require extreme conditions and is often kinetically frozen at room temperature.
Examples in the real world
Carbon as diamond, graphite, graphene and fullerenes; oxygen as O₂ and ozone O₃; phosphorus as white, red and black.