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 Mendeleef’s Periodic table :
Normal and transitional elements :
Defects of Mendeleef’s periodic table :
- 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.
- 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.
- Position of Isotopes : Isotopes are those atomic particles which have same atomic numbers but different atomic weights. For example are 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.
- 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 : 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.
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.