What are isotopes?

Isotopes are defined as atoms that have the same atomic number, which means they belong to the same element, but they differ in mass numbers due to varying numbers of neutrons. The atomic number denotes the number of protons in the nucleus, which determines the element's identity, while the mass number is the sum of protons and neutrons in the nucleus.

In the context of isotopes, variations in neutron count lead to different mass numbers, but the chemical properties of isotopes of the same element are generally similar since they have the same electron configuration and therefore behave similarly in chemical reactions. This underlying principle highlights the importance of mass number in differentiating isotopes even though they are chemically alike.

The other options do not accurately encapsulate this definition, as they either refer to protons and neutrons being equal, chemical properties diverging entirely, or identical atoms without numerical distinction in neutrons, which does not reflect the concept of isotopes properly.