Consider the group 1A elements sodium (period 3) potassium (period 4) and rubidium (period 5). What would you predict about the ionization energies of these elements?

The ionization energies of the group 1A elements sodium, potassium, and rubidium decrease as you move down the group from sodium to rubidium. This trend occurs because the outermost electron is further from the nucleus in each successive element, leading to increased shielding and a weaker attraction between the nucleus and the outer electron. Consequently, less energy is required to remove this outer electron from rubidium compared to sodium.