DRAWBACKS OF DOBEREINER LAW OF TRIADS
DRAWBACKS OF NEWLANDS LAW OF OCTAVE
DRAWBACKS OF MENDELLEVS PERIODIC TABLE
Dobereiner’s law of triads failed for the following reasons :
all the then known elements could not be arranged in the form of triads.
for very low mass or for very high mass elements, the law was not holding good. Take the example of F, Cl, Br. Atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
The only advantage of Dobereiner’s research was that it made chemists look at elements in terms of groups of elements with similar chemical and physical properties. This eventually led to rigorous classification of elements and the modern periodic table of elements, as we now know it, was discovered.
Newlands ' Law of octaves failed for the following reasons :
1. It was not valid for elements that had atomic masses higher than Ca.
2. When more elements were discovered, such as elements from the noble gases such as He, Ne, Ar, they could not be accommodated in his table.
But the most important contribution in the process of classification of elements was the periodicity he saw in every eighth element. The modern periodic table, that we shall study in later sections, drew heavily from the concept of periods of eight. Also it must be noted that Dobereiner’s triads occurred in the octaves of Newlands.
- DRAWBACKS OF MENDELEEV’S PERIODIC TABLE
1. For placing the elements in proper groups, the order of the elements according to atomic mass was reversed in certain cases. He placed Iodine (127) after Tellurium (128) Potassium (39) and Ni (58) after Co (59). Which is against his periodic law but correct according to properties.
2. Mendeleev’s periodic table does not provide a clear idea about the structure of atom.
3. Lanthanide and Actinide have been assigned places in the periodic table which is against the periodic law.
4. Alkali metal and coinage metals (Cu, Ag and Au) which differ widely in properties are placed into the same group.
5. There is no separate position for isotopes in his periodic table.
6. The change in atomic mass of two successive elements is not constant. Hence it is not possible to predict the number of missing elements by knowing the atomic masses of two known elements.