Answer
As one moves across the fourth period, the first ionization energy would be larger and larger, due to the decrease in atomic radii.
Work Step by Step
The first ionization energy depends on two variables: the size of the atom and the effective nuclear charge of the atom on the outermost electrons.
As we move across a period, the effective nuclear charge would increase and the atomic radii would decrease. These two supporting effects would make the electrons experience more and more attraction from the nucleus and harder to remove. The first ionization energy, as a result, would be larger as we move across a period.
So, as one moves across the fourth period, the first ionization energy would be larger and larger.
Also, as one moves across the fourth period, the atomic radii would become smaller and smaller. This makes the outermost electrons experience greater attraction from the nucleus and harder and harder to remove. The first ionization energy, as a result, would be larger and larger, supporting the claim above.
