What is having the same chemical formula called?

Answer 1

Not quite sure what you mean.

You could mean isomerism, which is changing the structure of molecules (usually hydrocarbons).

You could also be referring to allotropes, which are different forms of the same element. For example, diamond and buckminsterfullerene are both allotropes of carbon.

Two different molecules with the same formula are known as isomers of each other.

Answer 2

Having the same chemical formula is called being isomeric. Isomers are compounds that have the same molecular formula but different structural arrangements of atoms.