Question

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.

• A. Ni was placed before Co.
• B. Co was placed before Ni.
• C. Both were placed in the same group.
• D. Ni and Co were not discovered.

Answer 1

The correct option is B Co was placed before 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) even though its atomic mass is more than that of Nickel's.