For which of the following elements the sequence of increasing atomic masses was inverted in Mendeleev’s periodic table?

Cobalt (Co) and nickel (Ni)

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Lithium (Li) and sodium (Na)

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Beryllium (Be) and magnesium (Mg)

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Potassium (K) and calcium (Ca)

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Solution

The correct option is A Cobalt (Co) and nickel (Ni)
While developing the periodic table, there were a few instances where Mendeleev had to place an element with a slightly higher atomic mass before an element with a slightly lower atomic mass. The sequence was inverted so that elements with similar properties could be grouped together. For example, cobalt (58.9) appeared before nickel (58.7).

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Similar questions

Q. For which of the following elements the sequence of increasing atomic masses was inverted in Mendeleev’s periodic table?

Q. The elements in the Mendeleev’s periodic table were arranged in the increasing order of their atomic masses. But, the sequence was inverted so that two elements with similar properties could be grouped together. Identify those two elements.

Q. Which of the following statements are correct about Mendeleev's periodic table?
i) It is based on increasing order of atomic numbers.
ii) Mendeleev corrected the atomic weight of some elements like Be, In etc.
iii)

(A r, H_{2}), (C o, C l_{2}), (T e, F_{2})

are three inverted pairs.
iv) It is based on increasing order of atomic weights.

Q. The atomic masses of Nickel (Ni) and Cobalt (Co) are 58.7 and 58.9 respectively. Identify the element that was placed first according to the Mendeleev’s periodic table.

In Mendeleev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with an atomic mass of $58.93$ amu was placed before nickel having an atomic mass of $58.71$ amu. Give a reason for the same.

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