• (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? Where should hydrogen be placed in the modern periodic table? Give reason for your answer.
ChemistryClass 10ICSEAnswered 27 Mar 2026
Answer
- Isotopes have same atomic number and different atomic mass, So they are not given separate place in periodic table and is based on the atomic number of elements.
- Elements are arranged in the increasing order of their atomic masses as per the Mendeleev’s classification. Cobalt has higher atomic mass was placed before nickel. It has atomic mass of 58.9 and Nickel has 7. In modern periodic table, elements are arranged in the increasing order of therir atomic numbers and hence the cobalt and nickel has 27,28 atomic numbers and cobalt with lower atomic number comes before nickel.
- Hydrogen is placed at the topmost of periodic table because it resembles both alkali metals and halogens some of the properties so the position in the periodic table is unclear. But it is placed in first group above the alkali metals in the modern periodic table because its electronic configuration is similar to that of alkali metals. As the size is small, it exhibits different properties than of alkali metals.