Atoms with a neutron count greater than 90 (Option A) typically have unstable nuclei. This is primarily because the stability of an atomic nucleus is influenced by the ratio of neutrons to protons. In general, as the number of neutrons increases significantly beyond the number of protons, the forces that hold the nucleus together can weaken, leading to instability.
Heavy elements with a high neutron count often undergo radioactive decay to achieve a more stable configuration. This decay can occur through various processes such as alpha decay, beta decay, or gamma emission. Conversely, atoms with a neutron count less than 90 can also be unstable, but it’s more common for heavier elements with higher neutron counts to exhibit instability.