The elements are arranged in the periodic table in order of their atomic number

The number of electrons in the outer most shell is the group number

This means all the elements in the same group react in the same way.

Metal atoms lose their outer most electron, forming positive ion surrounded by a sea of delocalised mobile electrons.

Group 1 metals are very different:

Sodium 2.8.1 EC

-Grey/silver
-Cuts like butter
In water:
-forms a ball
-does everything that Li does
-UI goes blue

When Universal Indicator is added it turns blue. Alkali metals produce alkalis in water- in this case Lithium Hydroxide