What is allotropy ? Mention about the allotropic forms of Pb, Ge , Si.

Explanation

Allotropy : The existence of the same element in two or more forms in the same physical state but having different properties is called allotropy. The different forms are called allotropes.
Pb: `to` No allotrope exists.
Ge : `to` Two ( Both are crystalline forms )
Si : `to` Two ( Both are crystalline forms )