Question

Explain why less energy is needed to ionise gaseous atoms of Rubidium that gaseous atoms of Sodium?

Answer 1

This is because of the screening effect of Rubidium(more shell) which is much more higher than Sodium. There electron is much more easier to be excited and hence less energy is required for the ionisation.

Answer 2

Sodium Na, and Rubidium Rb are both alkali metals meaning they both have one electron in their outermost shell so they display similar chemistry,
what makes Na and Rb different is that they have a different number of protons Sodium has 11, and Rb 37.
Picturing the atoms; Na is smaller and has only a few electron clouds , Rb is much larger and has many more electron clouds. The outer most electron is further away thus less tightly held in Rubidium and thus is removed (ionisation) with less energy