Merits and dimerits of Mendeleev's periodic table.
1. He could classify all the 63 elements known at that time on the basis of similarities in properties.
2. He left gaps for the elements yet to be discovered.
3. He predicted the properties of those yet to be discovered elements and thus, helped in the discovery of other elements later on.
4. He named these elements by prefixing a sanskrit numeral 'Eka' (one), 'Divi' (two), 'Tri' (three), etc.to the names of the preceding preceding similar element in the same group. For example : eka - boron, eka - aluminium, eka - silicon, divi - manganese, eka - tantulum, etc.
All these elements were discovered later and did have the properties similar to those predicted by mendeleev. For example -
PROPERTIES EKA - ALUMINIUM GALLIUM
Atomic mass 68 69.7
Formula of oxide E2O3 Ga2O3
Formula of chloride ECL3 GaCl3
1. Although most of the elements were placed in the order of increasing atomic weights but this increasing order could not be maintained in all cases. For example - Cobalt atomic mass is 58.93u preceded Nickel 58.71u preceded Tellurium 127.6 preceded Iodine 126.90, thus he couldnt maintain similarities in properties.
2. Mendeleev periodic table did not provide space for noble gases which were discovered later.
3. Moreover there was no space left for isotopes in medeleev's periodic table.
- systematic arrangement of 63 elements in groups and periods.
- corrected doudtful masses of some elements like Be , Pd , Pt .
- predicted properties of elememts which were not discovered like scandium , gallium , germanium which were correct.
- zero group of noble elements was added without disturbance.
- contravarsile position of hydrogen.
- position of isotopes was not correct.
- two similar elements like Cu & Hg were grouped differently.
- numbers of elements to be dicovered would not be predicted.
- cobalt with higher atomic no. was placed before lower atomic no. nickel which was contradicting arrangement of element in increasing atomic no.
Merits of Mendeleev's periodic table
Mendeleev's periodic table was one of the greatest achievements in Chemistry with some of its important contributions as follows:
- As the arrangements are classified into groups, it was very useful in studying and remembering the properties of a large number of elements in a systematic way.
- This classification helped in correcting the atomic masses of some of the elements like gold, beryllium and platinum based on their positions in the periodic table.
- Mendeleev could predict the properties of some undiscovered elements like scandium, gallium and germanium. By this intuition, he had left gaps for the undiscovered elements while arranging elements in his periodic table.
Demerits of Mendeleev's periodic table
In spite of the above advantages, Mendeleev's periodic table suffered defects as follows:
- The position of hydrogen was not correctly defined. It was placed in group I although it resembles both the group I elements - the alkali metals and the group VII elements-the halogens, in their properties.
- In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6).
- Isotopes were not given separate places in the periodic table although Mendeleev's classification is based on the atomic masses.
- Some similar elements were grouped separately while some dissimilar elements were grouped together. For example copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.
- Mendeleev could not explain the cause of periodicity in the elements.
- The position for lanthanides and actinides were not included in this table.