Question

which of the following will change the mass of an atom but not its identity?
adding protons
removing electrons
changing energy levels
adding neutrons

Explanation:

The identity of an atom is determined by the number of protons (atomic number). Adding protons changes the element. Removing electrons forms ions but has negligible mass - change as electrons are very light. Changing energy levels involves electrons moving between orbitals and doesn't affect mass or identity. Adding neutrons changes the mass number (sum of protons and neutrons) but not the atomic number, thus not changing the element's identity.

Answer:

Adding neutrons