The graph shows three same structured gradual increases in three different areas- from elements 2 to 10, 11 to 18, 31 to 36. As the atomic number increases, the ionization energy slowly decreases. After Elements 2, 10, 18 and 36, the ionization energy level drops by a significant amount.
Between elements 20 and 29, the ionization energy level doesn’t change much, however it does gradually increase as the atomic number gets larger. Element 4, 13 and 31 all show a slight drop in energy from their previous element. This concept also applies to elements 8, 16, and 34.
From element 3 to 10, 11 to 18, 31 to 36, the ionization energy level increases as the numbers go up because these elements all belong in the same period. As you move towards the right of each period, the atomic radius decreases, therefore it requires more energy to remove electrons from the outer shell.
Elements 5, 13 and 31 all show a drop in ionization energy from the previous element, because they all belong to the same group with 3 valence electrons on the outer shell. As you move down each period, from period 1 to 7, the amount of energy required to remove electrons from the outer shell decreases, because the valence electron is further away from the nucleus, therefore there is less attraction. This is demonstrated by the graph, as the atomic number increases, the ionization energy decreases gradually.