In this case, since isotopes are defined as atoms of the same element with the same atomic number (number of protons and electrons) but different mass number (atomic mass) we can represent them via the symbol of the element followed by a dash indicating the atomic mass of the occurring isotope, for instance, for carbon, we may find C-12, C-13 and even C-14 which is useful in dating techniques.
Therefore, Lithium-4 or Li-4 and Lithium-5 or Li-5 can be defined as isotopes.
