Chapter 1st
B.sc 1st year Book
(Page 13)

Periodic Table

Introduction :

Initially elements were classified into metals and non metals, After this, classification of elements were done by Dobereiner as ‘triads’; Newland law of octaves and Lothar Meyer’s atomic volume curves. In the development of chemistry the classification of periodic system plays an outstanding role in the determination of their properties. Most of the facts of inorganic chemistry has come to the knowledge of chemists by the help of their systematic relationship between the elements. The main points of periodic systems are as follows :
1- To make an attempt to natural classification of elements to facilitate the study of a large number of elements.
2- To establish a basic relationship between some fundamental characteristics of the elements and their physical and chemical properties.
3. To recognize the periodicity in these elements.
For this all the attempts has been made in the arrangements of all the known elements in a systematic manner. Out of all these attempts Mendeleef’s periodic table plays a most important role because modern periodic table is also based on it. According to Mendeleef “The physical and chemical properties of elements are periodic functions of their atomic weights.”
Thus, according to Mendeleef the word function means a change in the properties of the elements with the change in their atomic weights and the word periodic point out the direction of this change that is the change is continuous either in decreasing direction or in increasing direction. In this table elements were arranged from left to right horizontally in the order of increasing atomic weight and from top to bottom (vertically) according to their similarities in their properties. This arrangement was most useful even today along with a slight modification In it. D.I. Mendeleef in 1869 created a certain gaps in the periodic table corresponding to those elements which latter on discovered or yet to be discovered.

Main features of Mendeleef’s Periodic table :

This table includes all the elements which was discovered up-to that time. Mendeloef’s Periodic table described as follows :
(a) In the table the vertical columns are called groups which includes I to VIII and Zero groups. Each group is further divided into two sub-groups A and B except VIII and zero. The left portion of the column is called ‘sub-group A’ and those of right one is called ‘sub-group B’.
(b) The horizontal rows of the table are called periods which are seven in number. The first six periods have complete number of elements while seventh period is incomplete. There are only two elements in the first period, the first period is called ‘shortest period’ whereas 8 elements are in each 2nd and 3rd periods and these periods are called short periods: 4th  and 5th  are ‘long periods’ and have 18 elements. The 6th one have 32 elements which includes 14 rare earth elements, is called ‘longest periods’. The lost period l.e. 7th  one is incomplete and has only 6 naturally occurring elements whereas rests are developed by nuclear reactions.
Typical elements :
All the elements which belongs to 2nd and 3rd periods are called ‘typical elements’ e.g.
Second period: Li Be B C N O F 
Third period: Na Mg Al Si P S Cl

Normal and transitional elements :

All the elements which resemble with typical elements of second short period are called ‘normal elements’ and which show difference with them are called “transitional elements’ e.g. K,Ab and Cs are normal elements and Cu, Ag and Au are transitional elements.

Defects of Mendeleef’s periodic table :

The Mendeleet’s periodic table has a great advancement over all other previous attempts to arrange the elements in the form of a table yet it has several defects. All the anomalies which has been reported in it are as follows :
  1. Position of Hydrogen – The position of hydrogen is uncertain because it shows the similarities with IA group (alkali metals) as well as VII A group (halogens), Mendeleef could not decide about this uncertainty.
  2. Position of rare earth elements: There are 14 elements of atomic number 58 to 71 are called’ ‘Lanthanides’. They are trivalent due to which they have a claim to be placed in III group. But they have higher value of atomic weight due to which their inclusion in III group disturb the symmetry of table as well as group. Similar is the case with actinides (Atomic number 90 to 103 ) which are also trivalent and have the same claim.
  3. Position of Isotopes : Isotopes are those atomic particles which have same atomic numbers but different atomic weights. For example h1, h2, h3are three isotopes of hydrogen. The arrangement of the isotopes according to their atomic weights vanish the periodic character of the table. However, isotopes show same chemical properties hence they should be placed at the same place in the periodic table.
  4. Position of metals and non metals : Mendeleef has made no attempt to place them separately in the periodic table.
  5. Position or noble gases : Mendeleef has not predicted the possibility of the existence of noble gases and he made no provision for their accommodation in this classification.
  6. Diagonal relationship : Mendeleef was unable to assign any explanation regarding the diagonal relationship. Elements of second short period show similarities in properties with the elements diagonally placed in the next group of the third period e.g.

diagonal relationship

7- Separate position of similar elements : There are certain elements of the periodic table which show similar properties but are placed separately in the Mendeleef periodic table a.g.

 Elements  Cu  Hg  Ag  TI  

 Groups      I   II   I   III 

8- Dissimilar elements are placed together : There are few elements which have dissimilar properties are placed together in the same group e.g. Mn has been placed along with halogens.

9. Transition elements : Mendeleef treated the three triads of group VIII as only the transition elements. But according to the modem atomic theory there are 3 series of transition elements (3 d,4 d and 5 d), each series includes 10 elements and the transition triads of Mendeleaf form are only a part of these series.

10. Causes of periodicity : Mendeleaf was unable to explain about the reasons of periodicity of elements in their properties when they are arranged according to their increasing atomic weights.

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