Answer:
decreases going down within a group
Explanation:
Ionization energy of an atom is defined as the energy required to remove electron from the gaseous form the atom. The energy required to remove the highest placed electron in the gaseous form of an atom is referred to as the first ionization energy.
In the periodic table, the first ionization energy decreases down the group because as the principal quantum number increases, the size of the orbital increases and the electron is easier to remove.
In addition, the first ionization energy increases across the period because electrons in the same principal quantum shell do not completely shield the increasing nuclear charge of the protons.
