What is the atomic number for lithium-6?

A lithium atom contains 3 protons in its nucleus irrespective of the number of neutrons or electrons. Notice that because the lithium atom always has 3 protons, the atomic number for lithium is always 3. The mass number, however, is 6 in the isotope with 3 neutrons, and 7 in the isotope with 4 neutrons.

The atomic number of all isotopes of lithium is 3. The isotope doesn't matter because an isotope contains the same protons. All isotopes of a given element share the same atomic number