Final answer:
Lithium atoms always possess 3 protons, which corresponds to its atomic number and clearly identifies it as lithium on the periodic table. This property remains constant across its isotopes, making the answer D) 3.
Step-by-step explanation:
The atomic number of an element corresponds to the number of protons in the nucleus of an atom of that element. In the case of a lithium atom, which is represented by the chemical symbol 'Li', this atomic number is always 3. Therefore, it's the number of protons that defines lithium's place on the periodic table and distinguishes it from other elements.
While the atomic number is a constant for an element, the mass number can vary, as seen with lithium isotopes. Nevertheless, the isotope, whether it has 3 neutrons (Lithium-6) or 4 neutrons (Lithium-7), does not affect the number of protons. To directly answer the question: there are exactly 3 protons in a lithium atom, making the answer D) 3.