The important characteristic property of an element is found to be its .atomic number. and not atomic weight. Accordingly, the . periodic law was modified - "The physical and chemical properties of the elements are periodic functions of their atomic numbers". Later on it was found that while deciding the properties of an element, its electronic configuration plays a very important role. So Bohr constructed the long form of the periodic table, based on the electronic configurations of the elements. Now, the .periodic law can also be stated as "The physical and chemical properties of the elements are periodic functions of their electronic configurations.“
Salient features :
1) This table is prepared based on a fundamental property "atomic number".
2) This table can be easily studied, remembered and reproduced.
3) Similarities, differences and trends in properties are more clearly reflected in this table.
4) Vertical columns are known as groups and horizontal rows are called periods.
5) There are seven periods in this table. The first period consists of two elements only. Second and third periods contain 8 elements each. Fourth and fifth periods contain 18 elements each. Sixth period consists of 32 elements. Seventh period is an incomplete period and consists of 19 elements.
6) There are eighteen groups in this table. They are designated 1A, IIA, IIB, IVB, VB, VIB, VIIB, VIII, IB, IIB, IIIA, IVA, VA, VIA, VIIA, O. (American Convention of naming).
7) The elements in IA, IIA, IIIA, IVA, VA, VIA and VIIA are known as representative elements or normal elements.
8) The elements in IB, IIB, IIB, IVB, VB, VIB, VIIB and VIII are called Transition elements.
9) Zero group elements are placed at the extreme right of the table. These are called inert gases or noble gases. They possess stable ns.np configuration.
10) Short periods are broken and long periods are extended to accommodate transition elements.
11) Lanthanides and actinides are placed separately at the bottom of the periodic table.
12) Based on the entrance of differentiating electron, the table is divided into four blocks. They are s - block, p - block, d - block and f. - block. In the elements of s - block, differentiating electron enters into s-orbital. Similarly in the elements of p-block, d-block and f-block, the differentiating electron enters into p, d and f-orbitals respectively.
13) Based on complete and incomplete electron shells and chemical properties, the elements are classified into four types. They are 1) Type 1 (Inert gas "elements) 2) Type II . (Representative elements) 3) Type Ill (Transition elements) 4) Type IV (Inner transition elements).
14) All the elements in a group possess similar properties, because they possess the same valence electron configuration.
