What is the main feature of the modern periodic table and how are the elements arranged in it?

The main feature of the modern periodic table is that the elements are arranged in the increasing order of atomic numbers. The elements are arranged in 7 periods and 18 groups. The horizontal rows are called periods and the vertical columns are called groups. The elements are placed in periods based on the number of shells in their atoms, with the shortest period (the first period) containing only two elements - Hydrogen and Helium, and the longest period (the sixth period) containing elements from Caesium to Radon. The bottom of the periodic table contains the Lanthanides and Actinides, which are placed in group IIIB of the sixth and seventh periods, respectively.