Why are both lithium and cesium alkali metals, when lithium has three electrons and cesium has 55

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

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6