Question

(a) How were the positions of isotopes of an element decided in the modern periodic table?
(b) How were the positions of cobalt and nickel resolved in the modern periodic table?
(c) where should hydrogen be placed in the modern periodic table? Give reason for your answer.

Answer 1

(a) The isotopes of elements are not given a separate place in the periodic table. This is because isotopes have the same atomic number and a different atomic mass, and the modern periodic table is based on the atomic number of elements. Hence, to give it a separate place, the elements should be arranged according to their atomic mass.

(b) As per Mendeleev's classification, elements are arranged in an increasing order of atomic masses, but cobalt, with a higher atomic mass was placed before nickel. Cobalt has an atomic mass of 58.9 and nickel has an atomic mass of 58.7, which is slightly lower than that of cobalt. Mendeleev could not explain this point. This problem was resolved in the modern periodic table. According to the modern periodic law, elements are arranged in the increasing order of their atomic numbers. The atomic numbers of cobalt and nickel are 27 and 28 respectively. So, cobalt with a lower atomic number should come before nickel.

(c) Generally, hydrogen is treated as a special element and placed alone at the head of the periodic table. The position of hydrogen in the periodic table is unclear because it resembles both the alkali metals and halogens in some of its properties. However, it is placed in the first group, above all the alkali metals in the modern periodic table because its electronic configuration is similar to that of alkali metals. But due to the small size of hydrogen, it exhibits properties different from that of the alkali metals.