Question

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

• A. Potassium (K) and calcium (Ca)
• B. Cobalt (Co) and nickel (Ni)
• C. Beryllium (Be) and magnesium (Mg)
• D. Lithium (Li) and sodium (Na)

Answer 1

The correct option is B 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).