[yalda]

So what is the exact diffrence between allotrpe and isomer??
I read that allotrophy is beyween same elements but SiO2 have 2 allotrpes 

[mjc123]

Allotropy (no h) refers to different forms of the same element, e.g. graphite and diamond.
Polymorphism refers to different crystal structures of the same compound, e.g. SiO₂.
Isomerism refers to different molecular structures of compounds with the same molecular formula, e.g. n-butane and 2-methylpropane (both C₄H₁₀).