Chapter 1st:- Atomic Structure and Periodic Table

B.sc 1st year Book
(Page 13)

Periodic Table

Introduction :

Initially, Periodic table elements were classified into metals and non-metals, After this, the classification of elements was 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 systems plays an outstanding role in the determination of their properties. Most of the facts of inorganic chemistry have come to the knowledge of chemists with the help of their systematic relationship between the elements.

The main points of periodic systems are as follows :

1- To make an attempt at the 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 have been made in arranging all the known elements in a systematic manner. Out of all these attempts, Mendeleev’s periodic table plays the most important role because the modern periodic table is also based on it. According to Mendeleev “The physical and chemical properties of elements are periodic functions of their atomic weights.”
Thus, according to Mendeleev, 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 an 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. Mendeleev in 1869 created a certain gap in the periodic table corresponding to those elements which later on discovered or are yet to be discovered.

Main features of Mendeleev’s Periodic table :

This table includes all the elements which were discovered up to that time. Mendeleev’s Periodic table is described as follows :
(a) In the table the vertical columns are called groups which include 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 the 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 the complete number of elements while the seventh period is incomplete. There are only two elements in the first period, the first period is called the ‘shortest period’ whereas 8 elements are in each 2nd and 3rd period, and these periods are called short periods: 4th  and 5th  are ‘long periods’ and have 18 elements. The 6th one has 32 elements which include 14 rare earth elements, and is called the ‘longest periods’. The lost period l.e. the 7th  one is incomplete and has only 6 naturally occurring elements whereas the rest are developed by nuclear reactions.

Typical elements :

All the elements which belong to the 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 typical elements of the second short period are called ‘normal elements’ and which show differences 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 Mendeleev’s periodic table :

The Mendeleev 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 have been reported in it are as follows :
1. Position of Hydrogen – The position of hydrogen is uncertain because it shows similarities with the IA group (alkali metals) as well as VII A group (halogens), Mendeleev could not decide about this uncertainty.
2. Position of rare earth elements: There are 14 elements of atomic numbers 58 to 71 called’ ‘Lanthanides‘. They are trivalent due to which they have a claim to be placed in the III group. But they have a higher value of atomic weight due to which their inclusion in the III group disturbs the symmetry of the table as well as a 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 that have the same atomic numbers but different atomic weights. For example are three isotopes of hydrogen. The arrangement of the isotopes according to their atomic weights vanishes the periodic character of the table. However, isotopes show the 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: Mendeleev was unable to assign any explanation regarding the diagonal relationship. Elements of the second short period show similarities in properties with the elements diagonally placed in the next group of the third period e.g.

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

Elements  Cu  Hg  Ag  TI

Groups      I   II   I   III

8- Dissimilar elements are placed together: There are a few elements that have dissimilar properties that 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 Mendeleev form are only a part of these series.

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