What is the difference between an isomer and allotrope?

Answer 1

An allotrope is a form in which an element exists. Generally these are relatively large molecules composed of a single element. For example diamond is a carbon allotrope in which the structure is set up on a lattice (similar to crystalline structure) while graphite is composed of layers of carbon atoms arranged in a hexagonal pattern, also buckminster fullerenes are an allotrope of carbon made of 60 carbon that form a structure with geometry equal to that of a soccer ball. By contrast, an isotope is a certain form of an element with a specific number of neutrons that varies from another form with a different number of neutrons. To illustrate this principle one must understand that there are three basic subatomic particles that compose all atoms: protons, electrons, and neutrons. The number of protons (which is usually balanced by an equal number of electrons) determine the atomic number and thus the identity of the atom. For example, carbon has the natural occurring carbon 12, 13, and 14 isotopes. This means that each isotope has 12, 13 , and 14 neutrons respectively. Generally, unstable isotopes are characterized by radioactivity.

In short, allotropes of of an element differ on the structure of the same atoms, (i.e. how the atoms are arranged), while isotopes of the element differ on the number of neutrons of the atoms (i.e. the composition of subatomic particles in an atom).

OR, may be simply:

Allotropes are different forms of a 'compound' whereas Isotopes are different forms of an 'atom'

Answer 2

Allotrope: the term is use in inorganic chemistry for the different forms of an element.

Examples: carbon allotropes are graphite, diamond, fullerene, graphene.

Isomer (1): the term is used in organic chemistry - compounds with the same chemical formula but with different structure.

Isomer (2): the term is used also in nuclear physics. The nuclear isomer has the same mass as another isotope but the nuclear properties are different.