As you move down a group in the periodic table, what trend in reactivity is observed?

As you move down a group in the periodic table, particularly for the alkali metals, reactivity increases. Alkali metals, such as lithium, sodium, and potassium, become more reactive as you go down the group. This trend is due to the increasing atomic radius and shielding effect, which results in the outermost electron being further away from the nucleus and thus more easily lost in reactions.

Choosing the option about reactivity decreasing for alkali metals is not accurate in this context. For halogens, reactivity actually decreases as you move down the group, contrary to the statement provided in the answer. The larger atomic size and increased shielding effect render the ability to gain an electron less effective, thus decreasing reactivity down the group.

In summary, while reactivity trends can vary based on the group, focusing on alkali metals or halogens respectively gives a clearer understanding of their behavior as you move down the group.