Why does ionization energy tend to increase as you move across a period?
As you move across a period, you add a proton in the nucleus and an electron in the outer shell. Ther greater number of protons causes a stronger pull on the electrons. The electrons are held tighter, basically, so the energy you have to put in to break one off increases.
What does the size do?
As you move across a period, the size of atoms tends to decrease. This is because the number of protons in the nucleus increases, leading to a stronger positive charge. This increased positive charge attracts the electrons more strongly, causing the electron cloud to be pulled closer to the nucleus. As a result, the atomic radius decreases.
The decrease in atomic size has an effect on ionization energy. Since the electron is closer to the nucleus in a smaller atom, it is held more tightly and requires more energy to remove. Therefore, as the atomic size decreases across a period, the ionization energy generally increases.
It's important to note that there may be exceptions or variations to these trends due to factors such as electron configuration, shielding effects, and electron-electron repulsion. However, the overall trend of increasing ionization energy across a period holds true in most cases.