The elements are arranged in the long periodic table in the increasing order of atomic numbers. Neils Bohr constructed the long form of the periodic table based on the electronic configuration of elements.
The important features of the long periodic table are :
It consists of seven horizontal rows called periods and 18 vertical columns which are classified into 16 groups only.
Periods : Every period starts with an alkali metal and ends with an inert gas. The first period consists of two elements only (H, He) and is called very short period. Second period consists 8 elements (Li to Ne) and is called first short period. The third period consists (Na to Ar) 8 elements and is called second short period.
Fourth period contains 18 elements (K to Kr) and s called first long period. Fifth period is the second long period with 18 elements (Rb to Xe).
Sixth period is the longest period with 32 elements which starts with Cs and end with Rn. This period includes 14 lanthanides.
Seventh period is an incomplete period with 20 radioactive elements.
Groups : There are 16 groups in the long form of the periodic table (in transition elements three vertical columns are fused and designated as VIII group). These groups are IA, IIA, IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIB, IIIA, IVA, VA, VIA, VIIA and zero group.
The elements of IA, IIA, IIIA, IVA, VA, VIA, VIIA groups are called representative elements or normal elements. Elements of IB, IIB, IIIB, IVB, VB, VIB, VIIB and VII groups have their ultimate and penultimate shell incomplete. These are called transition elements. IIB elements have outermost electronic configuration.
Zero group elements have a stable electronic configuration. These elements are called inert gases, noble gases. These elements have been grouped at the extreme right of the periodic table.
In this long periods have been expanded and short periods are broken to accommodate the transitional elements in the middle of the long period.
Lanthanides and actinides have been grouped separately and placed at the bottom of the periodic table.