1.2.1—Terms: isotope

Isotopes are atoms of the same element. They have the same number of protons, so they have the same atomic number, but they contain different numbers of neutrons.

Because isotope proton numbers are the same but neutron numbers differ, their mass (nucleon) numbers are different. The nuclear identity can therefore be compared using proton number and neutron number separately.

In isotope notation, the lower-left atomic number stays constant for isotopes of one element, while the upper-left mass number changes. This lets the isotope be distinguished without changing the element symbol.

Changing neutrons makes an isotope; changing electrons makes an ion. Do not use a different mass number as evidence that the proton number, and therefore the element, has changed.