The modern periodic table is divided into four main blocks named as s, p, d and f-blocks. The classification of elements into blocks is primarily based upon their electronic configuration. Below is the explanation of these blocks and how the elements follow the general electronic configuration.
1. s-Block Elements
The last electron enters the s-sub-shell of their outermost energy level. This block is situated at extreme left of the periodic table. It contains elements of groups 1 and 2. Their general electronic configuration is ns1-2, where n represents the outermost shell.
The elements of group 1 are called alkali metals whereas the elements of group 2 are called alkaline earth metals. The two groups I A or 1 and II A or 2
2. p-Block Elements
The elements in which the last electrons enters the p-sub-shell of their outermost energy level are called p-Block elements. The only exception is helium (1s2). General electronic configuration is ns2 np1-6.
This block is situated at the extreme right of the periodic table and contains elements of groups 13, 14, 15, 16, 17 and 18 of the periodic table. Most of these, elements are non-metals, some are metalloids and a few others are heavy elements which exhibit metallic character.
3. d-Block Elements
The elements in which the last electron enters the d-sub-shell of the penultimate energy level are called d-block elements. General electronic configuration (n-1) d 1-10 ns 1-2.
The exception is 46Pd whose configuration is 4d105s0. d-Block contains three complete, rows of ten elements in each. The three rows are called first, second and third transition series. They involve the filling of 3d, 4d and 5d orbitals respectively.
The d-block contains elements of groups 3 to 12 of the periodic table. Their two outermost shell are incomplete. 10 groups are there IIIB, IVB, VB, VIB, VIIB, VIIIB (Triad) , IB, IIB or 3, 4, 5, 6, 7, (8, 9, 10), 11, 12.
4. f-Block Elements (Inner Transition)
The elements in which the last electron enters the sub-level of the anti-penultimate (third to the outer most shell) shell are called f-block elements.
General electronic configuration f-block elements (n-2)f1-14(n-1)d0-1ns2. They consist of two series of elements placed at the bottom of the periodic table.
The elements of first series follow lanthanum (57La) and are called lanthanoids. The elements of second series follow actinium (89Ac) and are called actinoids.
Maximum Number of Elements in a Period= Maximum number of electrons in that period. If n is the period number then,
Maximum number of elements in a odd period number 
Maximum number of elements in a even period number 
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