isomer
/'aɪsəmər/
noun
a compound that exists in forms having different arrangements of atoms but the same molecular weight
That raises a question, what's the name for different crystalline structures of the same compound? Is that still an isomer or something different? I'm out of my element here.
So just for an example (I have no idea how to put a tetrahedron in a comment :) ) a chain of C-C-C-C and C-C=C-C (which you likely can not synthesize) would be an isomer but C-C-C-C in one crystal lattice versus C-C-C-C in another crystal lattice would be allotropes.
The carbon in graphite sits in sheets (hence the possibility to form graphene), whereas the carbon in a diamond always forms three dimensional lattices.
edit nah: bad example, sorry, I can't find a good way to visualize this in text the 2nd C-C-C-C chain should have the last C dangling from the 2nd down.
edit2: examples are wrong, I should have used a more complex molecule for the isomer and the carbon can have either three or four other carbons hanging of it for the allotrope. tricky...
allotrope
/'ælə,troʊp/
noun
a structurally different form of an element
https://www.differencebetween.com/difference-between-allotro...
You're looking at the word "polymorph": "(chemistry, geology) Any substance or mineral that forms different types of crystal."
It is the formation that is different not the molecules. C = C. They are technically allotropes:
https://en.wikipedia.org/wiki/Allotropy
And just like here that's a function of how the bulk carbon got to be formed, under extreme pressure and temperatures or less pressure and temperature. It's the recipe that makes the difference, not the ingredients.