Why does the band of stability curve upward at high atomic numbers? excess neutrons are required due to the repulsion between the protons. atoms with high atomic numbers have a large number of electrons orbiting the nucleus. this increased number of electrons requires a lot of extra mass in the nucleus to keep the electrons in their orbit?

The band of stability curves upward at high atomic numbers due to the fact that excess of neutrons are required due to the repulsion between protons.

Atomic number is the number of protons. As the number of protons (atomic number) increase, the electrical repulsion force, due to the same sign of the protons inside the nucleus, becomes more dominant compared to the nuclear force attractions, then the nucleus needs more neutrons to gain stability.The presence of more neutrons decrease the density of protons which decreases the repulsion among them.