What trend occurs as you go down Group 1 in the periodic table?

As you progress down Group 1 in the periodic table, the reactivity of the alkali metals increases significantly. This trend can be attributed to the atomic structure of the elements in this group. Each successive element down the group has an additional electron shell, which means that the outermost electron is further away from the nucleus and is shielded by more inner electrons.

The outer electron's increased distance from the positively charged nucleus results in a weaker electrostatic attraction between the nucleus and the outer electron. Consequently, it becomes easier for the alkali metal atoms to lose this outer electron during chemical reactions. As a result, the elements become more reactive as you move from lithium at the top to cesium and francium further down the group.

In contrast, other options do not reflect the correct trends seen within the group. Hardness is not a characteristic that increases down Group 1; in fact, the metals become softer. The relative atomic mass increases down the group, and the melting and boiling points of the metals typically decrease rather than increase. Therefore, the increase in reactivity is the defining trend for Group 1 alkali metals.