Answer:
Within Groups: Ei decreases from top to bottom
Across Periods: Ei Increases from left to right
Step-by-step explanation:
Ionization Energy is defined as, "the minimum amount of energy required to remove the valence electron from the valence shell of an atom in its gaseous state".
Trends in Periodic table:
Across Periods:
Ionization Energy increases from left to right along the periods because moving from left to right in the same period the number of protons (atomic number) increases but the number of shells remain constant hence, resulting in strong nuclear interactions and electrons are more attracted to nucleus hence, requires more energy to knock them out.
Within Groups:
Ionization energy decreases from top to bottom along the groups because the number of shells increases and the distance between nucleus and valence electrons also increases along with increase in shielding effect provided by core electrons. Therefore, the valence electrons experience less nuclear attraction and are easily removed.