Why does ionization energy decrease moving down a group in the periodic table?

Electron shielding

Explanation:

Ionization energy decreases moving down a group in the periodic table because of a phenomen known as Electron shielding, in which valence electrons do not interact with the positively charged nucleus as strongly as inner electrons do, because these inner electrons shield the valence electrons. This means it's easier for these valence electrons to leave the atom the more inner electrons are between them and the nucleus, this translates into a decreased ionization energy value.