Allotropy

                 Some chemical elements exist in two or more different forms in the same physical state known as Allotropes of elements. In greek "allos" means another and trope means change. 

For example: Carbon exists as diamond and graphite. This phenomenon is called allotropism and the different forms of elements are called allotropes.

Allotropes of some elements:

Non-metals:

Carbon           -- Diamond, Graphite, Amorphous carbon, Fullerenes, Graphene, Carbon nanotubes.

Oxygen          -- Dioxygen, Ozone, Octaoxygen.

Selenium        -- Red selenium, Gray selenium, Black selenium, Monoclinic selenium.

Phosphorus    -- White phosphorus, Red phosphorus, Violet phosphorus, Black phosphorus, Scarlet                                          phosphorus.

Sulphur          -- Rhombus sulphur, Monoclinic sulphur.

Metalloids :

Tellurium     -- Amorphous tellurium, Crystalline tellurium.

Boron.          -- Amorphous boron,  ⍺ - rhombohedral boron, β - rhombohedral boron, 𝛾 - orthorhombic                                boron, ⍺ - tetragonal boron, β -tetragonal boron.

Germanium  --  ⍺ - germanium, β - germanium.

Arsenic         -- Yellow arsenic, Gray arsenic, Black arsenic.

Antimony     --  Yellow antimony, Black antimony, Blue-White antomony.

Silicon.         --  Amorphous silicon, Crystalline silicon.

Metals:

Tin        --  Gray tin, White tin, Rhombic tin, Sigma tin.

Iron      --  Ferrite, Austenite.

cobalt   --  ⍺ - cobalt, β - cobalt.