In the periodic table, the alkali metals belong to Group 1. How does reactivity trend within this group?

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In Group 1, reactivity increases as you go down the group because these metals have one outer electron to lose. As you move down, atoms get larger and more electron shielding increases, so the outer electron feels less pull from the nucleus. This lowers the energy required to remove it (ionization energy), making it easier for the metal to react. The result is more vigorous reactions with water and other reactants for heavier alkali metals. So the trend is that reactivity grows down the group. The other patterns would contradict what’s observed: the reactivity does not stay the same, and it does not increase as you move up the group.