When a group 1 metal reacts, its outer shell electron is lost.

Here's why:

* Group 1 metals (alkali metals) have one valence electron in their outermost shell.

* They are highly reactive because this single electron is easily lost.

* To achieve a stable electron configuration (like the noble gases), they tend to lose this electron and form a +1 cation.

For example, when sodium (Na) reacts, it loses its outer electron to become a sodium ion (Na+), which has a stable electron configuration.

In summary, the reaction of group 1 metals involves the loss of their outer shell electron, resulting in the formation of a positively charged ion.