Definition of periodic table:
The representation of chemical elements by atomic number,electronic configuration and chemical properties is called periodic table
A German chemist dobereiner observed relationship between atomic masses of several group of three elements called triads.
In these group, the central or middle element had atomic mass average of the other two elements. One triad group example is that of calcium(40),strontium (88) and barium(138).
After successful determination of correct atomic masses of elements by Cannizzaro in 1860, attempts were again initiated to organize elements. In 1864 British chemist New-lands put forward his observations in the form of law of octaves.
He noted that there was a repetition in chemical properties of every eighth element if they were arranged by their increasing atomic masses. He compared it with musical notes . His work could not get much recognition as no space was left for undiscovered element. The noble gases were not known at that time.
Mendeleev,s periodic table
Russian chemist Mendeleev arranged the known elements ( only 63) in order of increasing atomic masses in horizontal rows called periods. So, the elements with similar properties were in the same vertical columns. This arrangement was called period table.
He put forward the result of his work in the form of periodic law, which is stated as ” properties of the elements are periodic function of their atomic masses. Although Mendeleev periodic table was the first ever attempt to arrange the elements ,yet it has a few demerits in jt. His failure to explain the position of isotopes and wrong order of the atomic masses of some elements suggested that atomic mass of an elements cannot serve as the basis for the arrangements of elements .
The properties of the elements are the periodic function of their atomic massed.
Modern Periodic Law
The physical and chemical properties of element are the periodic function of their atomic numbers.
Features of the Long Form of periodic Table
(1) This table is based on atomic number which is a more fundamental properties of the atom.
(2) The position of an element in the periodic Table is related to its electronic configuration.
(3) Long form of periodic table reflects more clearly the changes in the properties of various elements while moving with in a period from left to right or going down a group.
(4) The representative and transition elements have been assigned separate position.
(5) This table can also separate physical and chemical properties of metals and nonmetals
(6) Hydrogen (H) resembles both the alkali metals as well as the halogens .lt also resemble elements of carbon family. Howe-ever,it is placed in a separate box just above the group 1,which is not totally correct.
(7) Helium (He) is placed at the top of group eighteen (18) in periodic table. Noble gases. These are p_ block elements. The outermost orbital of He is s, so its position is not justified.