What trend is observed in first ionization energy as you move across a period from left to right?

As you move across a period from left to right on the periodic table, the first ionization energy generally increases. This trend occurs due to several key factors related to atomic structure.

First, as you progress across a period, the number of protons in the nucleus increases. This results in a greater positive charge that attracts the electrons more strongly. For each element, even though more electrons are being added, these additional electrons occupy the same principal energy level and experience a relatively constant shielding effect from the inner electron shells. Therefore, the effective nuclear charge—that is, the net positive charge felt by the valence electrons—increases.

As a result, when you attempt to remove one electron (which is what ionization energy measures), it requires more energy because the electrons are held more tightly by the increased nuclear charge. Generally, this leads to a higher ionization energy for elements located further to the right in a period compared to those on the left.

This increasing trend is consistent and is a fundamental concept in understanding the behavior of elements in the periodic table.