What happens to reactivity as you go down group 1 in the periodic table?

As you go down the group, what changes? The number of shells.

As you go down the group, what doesn't change? The number of electrons in the outer shell - it is always one.

For group 1, they become reactive when they can lose that electron. It is easier to lose it as the electron moves further away from the nucleus as the attraction is weaker. Therefore, reactivity increases down the group as the electron is further away.

Always remember NASA: nucleur charge, atomic radius, shielding and attraction.