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प्रश्न
What challenges did Mendeleev face in following periodic law?
उत्तर
The challenges faced by Mendeleev while following periodic law were:
- Mendeleev had to arrange the elements with the thought that the information available till then was not final but it could change.
- He had to revise the atomic masses of some elements so as to give them a proper place in the periodic table in accordance with their properties. For example, the previously determined atomic mass of beryllium, 14.09, was changed to the correct value 9.4, and beryllium was placed before boron.
- He had to keep some vacant places in the periodic table for elements not discovered till then. However, he predicted the atomic masses and properties of some these unknown elements.
- There was no place reserved for noble gases in Mendeleev’s original periodic table. So, when noble gases were discovered, he had to include them. He did this without disturbing the original periodic table by creating ‘zero’ group.
- He had to indicate a question mark before the symbol of the element due to uncertainty in the names of some of the elements.
- There was ambiguity regarding the sequence of elements having a whole number atomic mass such as Co and Ni.
- It was difficult to place isotopes of elements as they have the same chemical properties but different atomic masses.
- It was not possible to predict how many elements could be discovered between two heavy elements as the rise in atomic mass is not uniform. In addition to this, it was difficult to decide the position of hydrogen in the periodic table.
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Mendeleev’s periodic law.
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