Final answer:
The ionization energy generally increases from left to right within a period on the periodic table due to increasing effective nuclear charge and decreasing atomic radius.
Step-by-step explanation:
The ionization energy generally increases from left to right within a period on the periodic table. This means that elements on the right side of the periodic table have higher ionization energies compared to elements on the left side. The increase in ionization energy across a period can be explained by the increase in effective nuclear charge and the decrease in atomic radius.
For example, as you move from left to right in Period 2 (Li to Neon), the number of protons in the nucleus increases, resulting in a higher effective nuclear charge. At the same time, the atomic radius decreases, meaning that the valence electrons are held more tightly by the nucleus and require more energy to be removed.
As a result, elements on the left side of the periodic table, such as the alkali metals in Group 1, have low ionization energies because they have one valence electron that is relatively far from the nucleus. On the other hand, elements on the right side, such as the noble gases in Group 18, have high ionization energies because their valence electrons are closer to the nucleus and are held more tightly.