How does ionization energy change when moving across a period in the periodic table?

When moving across a period in the periodic table, ionization energy generally increases. This trend can be attributed to several factors related to atomic structure and the effective nuclear charge experienced by the electrons.

As you move from left to right across a period, the number of protons in the nucleus increases, which results in a higher positive charge. This increased nuclear charge attracts the electrons more strongly. Although electrons are being added to the same principal energy level, the enhanced attraction from the nucleus makes it increasingly difficult to remove an electron.

Additionally, the increase in effective nuclear charge occurs because the shielding effect from inner-shell electrons does not significantly increase as new electrons are added to the same outer shell. Therefore, as the positive charge of the nucleus increases without a corresponding and significant increase in shielding, the overall attraction to the outer electrons becomes stronger, leading to higher ionization energy.

This explanation illustrates why the ionization energy trend displays an increasing pattern across a period rather than decreasing or remaining constant. In most cases, fluctuating patterns are more characteristic of specific groups or scenarios and do not reflect the consistent change seen across periods.