Lithium-6 and lithium-7 are isotopes of lithium. This means they are both atoms of the same element (lithium) with the same number of protons (3) but differ in the number of neutrons.

Here's the breakdown:

* Lithium-6: Has 3 protons and 3 neutrons. Its atomic mass is approximately 6.

* Lithium-7: Has 3 protons and 4 neutrons. Its atomic mass is approximately 7.

Key Differences:

* Mass: Lithium-7 is heavier than lithium-6 due to the extra neutron.

* Natural abundance: Lithium-7 is much more abundant in nature (92.5%) than lithium-6 (7.5%).

* Nuclear properties: Lithium-6 is used in nuclear fusion reactions, while lithium-7 is used in medical imaging and therapy.

Periodic Table:

The periodic table only lists the average atomic mass of lithium, which is approximately 6.941. This average reflects the relative abundance of the two isotopes.

In summary:

The difference between lithium-6 and lithium-7 lies in their neutron count, which affects their mass and some of their nuclear properties. While the periodic table doesn't differentiate between isotopes, it's important to understand that elements can exist in different isotopic forms.