Numerous forms of the periodic table have been revised from time to time. A modern verion, which is most convenient and widely used is the long or extended from the periodic table. The aufbau principle (electrons are filled in the progressive order of their increasing energy i.e., by n + l rule) and the electronic configuration of atom provides a theoretical foundation for the periodic classification. The horizontal rows are called periods. There are altogether seven periods. The first period consists of 2 elements. The subsequent periods consist of 8, 8, 18 and 32 elements respectively. The seventh maximum of 32 elements. Elements having similar outer electronic configurations in their atoms are grouped in vertical columns. These are referred to as groups or famillies. According to the recommendation of IUPAC, the groups are numbered 1 to 18 replacing the older notation of groups 0, IA, IIA ..... VIIA, VIII, IB, ...... VII B. Each successive period in the periodic table is associated with the filling up of next higher principal energy level following aufbua sequence. The number of elements in each period is twice the number of atomic orbitals avaliable in the energy level that is being filled. All the elements are classified into four blocks, i.e., s-block, p-block, d-block and f-block depending on the type of atomic orbitals that are being filled with the last electron of the element.
If aufbau rule is not followed, Ca-20 will be placed in........block.