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Question
Which of these pairs were put by Mendeleev in the wrong order on the basis of increasing atomic mass in his table?
Which of these pairs were put by Mendeleev in the wrong order on the basis of increasing atomic mass in his table?
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Solution
The correct option is D Cobalt and nickel
Mendeleev arranged the elements on the basis of increasing atomic mass. In certain cases, in order to preserve the similar properties in groups exceptions were made. Cobalt (Co) has the atomic mass of 58.93 u and nickel (Ni) has an atomic mass of 58.71 u still cobalt was kept before nickel in his periodic table. Mendeleev could not give a proper justification to the position of these two elements at that time.
Later with the establishment of modern periodic law, which is based on atomic number, it became clear that the order kept was correct as the atomic number of cobalt is 27 and of nickel is 28.
Mendeleev arranged the elements on the basis of increasing atomic mass. In certain cases, in order to preserve the similar properties in groups exceptions were made. Cobalt (Co) has the atomic mass of 58.93 u and nickel (Ni) has an atomic mass of 58.71 u still cobalt was kept before nickel in his periodic table. Mendeleev could not give a proper justification to the position of these two elements at that time.
Later with the establishment of modern periodic law, which is based on atomic number, it became clear that the order kept was correct as the atomic number of cobalt is 27 and of nickel is 28.
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