Final answer:
A lithium atom always contains three protons located in its nucleus, defining its atomic number as 3, regardless of the isotope.
Step-by-step explanation:
The number and location of protons in a lithium atom is universally three within its nucleus. This constant feature defines the element lithium (Li) and gives it an atomic number of 3. Lithium atoms can, however, have different numbers of neutrons, resulting in isotopes with different mass numbers such as Lithium-6 (three neutrons) and Lithium-7 (four neutrons). Despite these differences, the number of protons remains constant at three, which is a characteristic trait of the lithium element.