(1) There are 18 vertical columns in the modern periodic table and are called groups. These groups are 1 and 2, 13 to 18 and 3 to 12.
(2) The number of valence electrons in all these elements from the group 1, i.e., the family of alkali metals, is the same. Similarly, the elements from any other group, the number of their valence electrons to be the same. For example, the elements beryllium (Be), magnesium (Mg) and calcium (Ca) belong to the group 2, i.e. the family of alkaline earth metals. There are two electrons in their outermost shell the number of valence electrons are 2.
Similarly, there are seven electrons in the outermost shell of the elements such as fluorine (F) and chlorine (Cl) from the group 17, i.e. the family of halogens the number of valence electron is 1. As a result, all elements belonging to the same group have the same valence electrons and show similar chemical properties.
(3) while going from top to bottom within any group, one electronic shell is added at a time. Atomic radius and atomic size increases and hence shows gradation of properties of the elements down the group. From this, the electronic configuration of the outermost shell is characteristic of a particular group.
