Why does reactivity increase as you move down Group 1 elements?

Reactivity in Group 1 elements increases as you move down the group primarily due to the increasing atomic size. As the atomic number increases, additional electron shells are added, which makes the atoms larger. This increase in size results in the outermost electron being further from the nucleus.

Because the outer electron is located farther away, the electrostatic attraction between the positively charged nucleus and the negatively charged outer electron weakens. Consequently, it becomes easier for the outer electron to be lost during chemical reactions, leading to increased reactivity of the alkali metals.

In contrast, as one considers the other options, atoms do not become smaller as you move down the group; they actually become larger. The statement about outer electrons being closer to the nucleus is also inaccurate since they are farther away from the nucleus in heavier atoms, which contributes to their increased reactivity. The number of neutrons does not directly influence reactivity for these elements, as reactivity is primarily determined by the behavior of the outermost electrons.