what were the drawbacks of Mendeleev's periodic table..?? What were the drawbacks of Doberiner periodic table..??
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Answer:
Drawback of Dobereiner's triads:
- Dobereiner could identify only three triads. He was not able to prepare triads of all the known elements.
- All the 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.
→Drawbacks of Mendeleev's periodic table.
❥ He was not able to locate hydrogen because it resembles alkali metals by forming positive ions and resembles halogens by forming diatomic molecule.
❥ Increase in atomic mass was not regular while moving from one element to another.
❥ Mendeleev's periodic table was based on arranging elements in increasing order of atomic masses. But isotopes were not included in his periodic table.
→Drawbacks of Dobereiner periodic table.
❥ Dobereiner suggested that nature encompassed triads of elements whereby the middle element had properties that were an average of the other two elements. However, the poor accuracy of measurements, such as atomic weights, hindered the classification of more elements.
❥Dobereiner was not able to prepare triads of all the known elements. He could identify only 3 triads.
❥Metals are grouped with non-metals, e.g. oyxgen is in the same group as iron.
❥Not all of the elements in a group have the same properties, e.g. copper is unreactive and it is in the same group as potassium which explodes on contact with water.
Increase in atomic mass was not regular while moving from one element to another. Hence, the number of elements yet to be discovered was not predictable.
Later on, isotopes of elements were found which violated Mendeleev’s periodic law.