Why do group 1 elements get more reactive as you go down the group?

The atoms are composed of a positive nucleus surrounded by negative electrons. In order to ionically bond, each group 1 element wants to lose its outer electron to whatever it is going to bond to, eg Cl. As you go down the group, each element has more and more layers of electrons between the inner positive nucleus and the outer negative electron. This increases the shielding, so there is less attraction between the outer electron and the nucleus, meaning as you go down the group it is easier for the elements to lose their outer electron and therefore react.