Which statement describes a periodic trend of first ionization energy? O A. It tends to decrease from left to right across a period. B. It remains the same within a period. O C. It tends to increase from top to bottom of a group. D. It tends to decrease from top to bottom of a group.

Answer: The correct statement that describes a periodic trend of first ionization energy is D. It tends to decrease from top to bottom of a group.

Extra: First ionization energy is the energy required to remove the outermost electron from a neutral atom in the gaseous state. Ionization energy tends to increase across a period from left to right. This is because, as you move across a period, protons are added to the nucleus, resulting in a stronger attraction between the positively charged nucleus and the negatively charged electrons. This makes it more difficult to remove an electron, and hence, the ionization energy increases.

In contrast, as you move down a group in the periodic table, the ionization energy tends to decrease. Even though there are more protons in the nucleus of these heavier atoms (which should increase the attraction), the effect is outweighed by the increase in the size of the atom. As atoms get larger, the outermost electron gets farther away from the nucleus and is also shielded by more inner-shell electrons. This reduces the electrostatic attraction between the nucleus and the outermost electron, making it easier to remove this electron and resulting in lower ionization energy.

So, as you go from top to bottom in a group, the increased distance and shielding outweigh the increased nuclear charge, leading to a decrease in the ionization energy.