The Russian team wanted to use berkelium because the isotope of calcium used in the experiment, calcium-48, has 20 protons and 28 neutrons. This is the lightest stable or almost stable nucleus (the middle part of an atom) with much more neutron than proton. Zinc-68 is the second-lightest nucleus of this kind, but it is heavier than calcium-48. Since tennessine has 117 protons, they need another atom with 97 protons to be combined with the calcium atom, and berkelium has 97 protons.