Answer:
Because they both have one electron in their outermost shell
Explanation:
Both lithium and cesium are classified as alkali metals because they both have just one electron in their outermost shell.
The electronic configuration of both atoms are:
Lithium = 2, 1
Cesium = 2, 8, 18, 18, 8, 1
Now we can see that the outer shell of both elements have just one electrons.
Division into groups is based on the number of outer most shell of each element
