What's the difference between an isotope and an isomer?

Isotopes are two atoms with the same number of protons, but a different number of neutrons. Because they have the same number of proton they are atoms of the same element, but with different masses. For example, most carbon is C-12 (with 6 protons and 6 neutrons); the radioactive isotope used for carbon dating is C-14 (with 6 protons and 8 neutrons).

Isomers are two molecules with the same atoms joined together in a different shape. For example, butane is C4H10, with the four carbon atoms joined in a straight chain; methylpropane is also C4H10, but with the carbon atoms joined in a T shape.

https://en.wikipedia.org/wiki/Isotope

https://en.wikipedia.org/wiki/Isomer