Genesis of periodic classification:-

(1) Doberneir triad / law of triad:-  He arranged similar elements in groups of three and showed that Their atomic Weights ware either nearly the same or the atomic weight of the middle element was approximately. The arithmetic mean of the other two. These groups of three elements were called Doberneir’s triad and this generalization was law of triad.

e.g.	(1)	Triad	Lithium	Sodium	Potassium	Atomic Weight are nearly same
			Li	Na	K	Mean of I & III
		Atomic Weight	7	23	39	Atomic Weight of Na
	(2)	Triad	Chlorine(CL)	Bromine(Br)	Iodine I	(Middle element
		Atomic weight	35.5	80	127

 

(2). Telluric screw / Helix:- Next reported attempt to classify elements was made by( Chancourtis gave the real classification of elements). He arranged the then known elements in order of increasing atomic weights and made a cylindrical table of elements to display the periodic recurrence of properties.

(3). Newland’s law of Octave:- John Alexander in 1865 profound the law of Octave He arranged the elements in increasing order of atomic weights and noticed that every eight element had properties similar to the first element.

Drawback:- It is true only for elements upto calcium

e.g.	1	2	3	4	5	6	7
	Li	Be	B	C	N	O	F
	8	9	10	11	12	13	14
	Na	Mg	Al	Si	P	S	CL
	15	16
	K	Ca

 

The sodium is similar to that of Li and next higher eight member K and soon.

(4) Lothar Meyer Arrangement:- He plotted a graph between atomic volume and atomic weight of element and observed that the elements V similar properties occupied similar position on curve.

example (a) Most Electronegative elements (F, CL, Be, I) occupy assending position on curve.

(b) Most Electropositive electropositive alkali metals occupy peaks on curve.

(C)

(5) Mendeleev’s Periodic law:- The physical and chemical properties of element are a periodic function of their atomic weights Ionisation Energy when the elements are arranged in order of their increasing atomic weights elements v similar properties are repeater after certain regular inter volume.

This repetition of properties of elements after certain regular intervals is called periodicity of elements.

(6) Modern periodic law:- It is based upon Moseley’s saying that physical and chemical properties of elements are function of their atomic number.

We will discuss Modern Periodic Law and Table in next Topic