What member of the noble gases only has 5 rings?

beryllium-10

Explanation:

An isotope is an atom of the same element that has a different number of neutrons.

This means that an isotope of beryllium (Be) would have to be beryllium as well.

The number 10 references the fact that it has four protons and six neutrons:

4 protons + 6 neutrons = 10

While usually the element has 5 neutrons, so it is beryllium-9.