Chapter 1st:- Atomic Structure and Periodic Table
B.sc 1st year Book (Page 13)
The main points of periodic systems are as follows :
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.
Main features of Mendeleev’s Periodic table :
Typical elements :
Normal and transitional elements :
Defects of Mendeleev’s periodic table :
- 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.
- 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.
- 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.
- Position of metals and non-metals: Mendeleef has made no attempt to place them separately in the periodic table.
- 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.
- 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.