hello mam!on whwt basis the groups and periods of elements are classified?
The elements which have the same outer electron arrangement are placed within a column called a group. The outer electrons are called valence electrons. Because they have the same number of valence electrons, elements in a group share similar chemical properties.
Whereas a new row (period) is started when a new shell has its first electron. The period number of an element signifies the highest unexcited energy level for an electron in that element.
Or we can say that the total number of electron shells an atom of an element has determines the period to which it belongs and the number of valence shell electrons determines the group to which the element belongs.
2 2s 2p
3 3s 3p