Describe the trend in ionization energy as you move across the periodic table and down the periodic table. Explain why thesetrends areobserved.

Ionization energy refers to the amount of energy needed to remove an electron from an atom in isolated and gaseous state.

 Ionization energy decreases as we go down a group. Ionization energy increases from left to right across the periodic table.

As  one progresses down a group on the periodic table, the ionization energy will likely decrease since the valence electrons are farther away from the nucleus and experience greater shielding. They experience a weaker attraction to the positive charge of the nucleus or in another way we can say that number of shells increase down the group so,size increases and size and ionization energy is inversely proportional to each other.