Here's why:

* Atomic Number: The atomic number represents the number of protons in an atom's nucleus. Protons are fundamental particles with a positive charge, and they define the element. An atom with zero protons wouldn't be an atom at all.

* Mass Number: The mass number is the total number of protons and neutrons in an atom's nucleus. Even if an atom had zero protons, it would still need at least one neutron to exist. Neutrons, while neutral in charge, contribute to the mass of the atom.

The closest thing to a particle with zero mass is the photon, which is a fundamental particle of light. However, photons are not atoms, and they do not have an atomic number.