Numerous forms of the periodic table have been devised from time to time. A modern version, which is most convenient and widely used is the long or extended form of periodic table aufbau principle and the electronic configuration of atoms provide 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 \( 8,8,18,18 \) and 32 elements respectively. The seventh period is incomplete and like the sixth period would have maximum of 32 elements.
Elements having similar outer electronic configurations in their atoms are grouped in vertical columns. These are refered to as groups or families. 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 next higher principal energy level following aufbau sequence.
The number of elements in each period is twice the number of atomic orbitals available 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 electrons.
Elements in the same vertical group of the periodic table have generally the same:
(A) atomic number
(B) electronic configuration
(C) atomic mass
(D) number of electrons in the outermost shell of their atoms
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