The section discusses the concept of isotopes (đồng vị) in the context of atomic structure. It presents multiple-choice options regarding the definition of isotopes, focusing on the differences in subatomic particles among atoms. The key topics include:

- Definition of isotopes
- Subatomic particles: neutrons, protons, and electrons
- The correct answer is related to the number of neutrons in atoms.

Entities involved:
- Isotopes (đồng vị)
- Neutrons
- Protons
- Electrons

The correct answer to the question posed is option A: "Nguyên tử có số neutron khác nhau" (Atoms with different numbers of neutrons).