The first ionization energy generally decreases down a group due to the increasing distance of the valence electrons from the nucleus of an atom. The increasing gap lowers the attractive force of the protons that pull the electrons closer to the nucleus.
Ionization energy refers to the amount of energy needed to remove the outermost electrons, known as valence electrons, which are found on the highest energy level of a neutral atom. The magnitude of the ionization energy depends on the charge of the nucleus, distance of the nucleus from an electron and the number of electrons in the lower energy levels. The electrons that come in between the nucleus and the outermost electrons trigger a "shielding" or "screening" effect, which also reduces the pull of the protons on the valence electrons. Shielding generally increases down a group, which in turn causes a decrease in ionization energies.