Originally posted by Ponderable
Dimethylcyclopentane has a different number of atoms to Methylcyclohexane, thus the two CAN'T be isomeric to each other.

Isomers share the same number (and type) of atoms, but have a different arrangement of these.

Methylcyclohexane is C7H13. Dimethylcyclopentane is C7 H14.

But Methylcyclohexane is isomeric to Ethylcyclopentane or to Heptene, or ...[text shortened]... even calculate the various proportions of conformations using the enbergies, I gave further up.

They are isomeric as has been shown. Don't forget - in any hydrocarbon, there are 2n+2 as many hydrogens as there are carbons, minus two for each pi bond (that is, double bond, or four less for triple bond) or ring. Both have one ring and therefore 2n hydrogens.

Wikipedia agrees with you that conformations are a form of isomerism. I guess I was wrong. Well, in that case, shouldn't there be an infinite number of isomers, each differing by an infinitesimal amount?

For example look at the methyl group. What if it's rotated 1 degree? A new isomer! Then 2 degrees, 3 degrees, plus all the fractional degrees in between...