Solution:

• The ionization energy is the energy required to remove an electron from its orbital around an atom to a point where it is no longer associated with that atom.
• Group 1 of the Periodic Table features metals whose valence electron singly occupies the outermost shell, the valence shell of the given atoms.
• Since the nuclear charge is necessarily diminished with respect to the valence shell, the alkali metals display the lowest ionization energies, and these energies (reasonably) decrease down the Group.