Isotopes of an element, like An (which is likely a placeholder for a specific element), differ from one another in the number of neutrons they contain.

Here's a breakdown:

* Atoms: All atoms of a specific element have the same number of protons, which defines the element. This number is called the atomic number.

* Isotopes: Isotopes of the same element have the same number of protons but different numbers of neutrons.

* Mass Number: The sum of protons and neutrons in an atom is called the mass number. Since isotopes have different numbers of neutrons, they have different mass numbers.

For example:

Let's consider the element Carbon (C):

* Carbon-12 (¹²C): Has 6 protons and 6 neutrons (mass number = 12).

* Carbon-14 (¹⁴C): Has 6 protons and 8 neutrons (mass number = 14).

Both are carbon atoms because they have 6 protons, but they have different numbers of neutrons, making them isotopes.

Key Differences due to Different Neutrons:

* Mass: Isotopes have different masses due to the differing numbers of neutrons.

* Radioactivity: Some isotopes are radioactive, meaning they decay over time, emitting particles and energy. This is often due to an unstable ratio of protons to neutrons.

* Chemical Properties: While the number of neutrons affects mass, it does not significantly change the chemical properties of an element. The number of protons dictates how an element will bond with other atoms.

Important Note: The specific element you are referring to as "An" is not a recognized element symbol. Please provide the actual element you are interested in, and I can provide more specific information about its isotopes.