* Isotopes: Lithium has two naturally occurring isotopes: lithium-6 and lithium-7.
* Lithium-6 has 3 protons and 3 neutrons.
* Lithium-7 has 3 protons and 4 neutrons.
* Neutrons: The number of neutrons in an atom can vary. Since neutrons have mass, isotopes of the same element will have different atomic masses.
Therefore, lithium atoms can have two different masses depending on which isotope they are.
While the majority of lithium atoms are lithium-7, the existence of lithium-6 means that lithium atoms do not all have the same mass.
