(a) What is the trend in first ionization energies as one proceeds down the group 7 A elements? Explain how this trend relates to the variation in atomic radii. (b) What is the trend in first ionization energies as one moves across the fourthperiod from \(\mathrm{K}\) to \(\mathrm{Kr}\) ? How does this trend compare with the trend in atomic radii?

As one proceeds down the group 7A elements, the ionization energy generally decreases. We will explain this trend by considering the changes in atomic radius.

When moving down the group 7A elements, the atomic radius increases. This is because additional electron shells are added as we move down the group. This leads to an increase in the shielding effect, causing a decreased hold of the nucleus on the valence electrons. As the atomic radius increases, the distance between the valence electron and the nucleus also increases, causing the electrostatic attraction between the electron and nucleus to decrease. As a result, the ionization energy decreases because less energy is required to remove the valence electron.

As one moves across the fourth period from \(\mathrm{K}\) (potassium) to \(\mathrm{Kr}\) (krypton), the ionization energy generally increases.

When moving across the fourth period, the atomic radius generally decreases. This is due to an increase in the effective nuclear charge experienced by the valence electrons. The shielding effects remain relatively constant, which means that the increasing nuclear charge results in stronger binding of valence electrons to the nucleus. The decreasing atomic radius means that the valence electrons are closer to the nucleus, resulting in stronger electrostatic attraction between the electrons and the nucleus. Consequently, the ionization energy increases across the period because more energy is required to remove the valence electron. In conclusion, when moving down the group 7A elements, the ionization energy decreases due to an increase in atomic radius and shielding effect. When moving across the fourth period from \(\mathrm{K}\) to \(\mathrm{Kr}\), the ionization energy increases because of a decrease in atomic radius and an increase in effective nuclear charge.