Two atoms of the same element hold different numbers of neutrons. What are these two atoms?

1. The identity of an element is fixed by its number of protons alone. 2. Adding or removing a neutron therefore leaves the element unchanged. 3. Atoms with the same number of protons but a different number of neutrons are isotopes. 4. An ion is different again, since an ion has gained or lost electrons and carries a charge. _Source: Siyavula Physical Sciences Grade 10 (Everything Science, CC BY 4.0), Chapter 4: The atom_