Final answer:
Within period 3 of the periodic table, the ionization energy tends to increase with increasing atomic number due to the stronger effective nuclear charge across the period, although there may be small deviations in this trend.
Step-by-step explanation:
Within period 3 of the periodic table, in order of decreasing atomic number, the ionization energy of each successive element generally increases.
This trend is due to the increasing effective nuclear charge across the period. This means that as we move from left to right in a period, the nuclei of atoms gain more protons, thus exerting a stronger hold on the valence electrons. The increasing pull from the nucleus requires more energy to remove an electron, hence the increase in ionization energy. Nonetheless, there are small deviations from this trend when a new subshell begins, such as the case with boron having a lower ionization energy than beryllium despite boron having a greater nuclear charge.
To summarize the periodic trend within a period, the first ionization energy (IE1) increases with increasing atomic number (Z), while down a group IE1 decreases with increasing Z, reflecting the greater distance of valence electrons from the nucleus.