(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.

(a) Isotopes of an element have the same atomic number but a different mass number. So they are not given a separate place in the periodic table as the modern periodic table is based on the atomic number of elements.

(b) The atomic number of cobalt is 27 and that of nickel is 28. According to modern periodic law, the elements are arranged in order of increasing atomic numbers. Therefore, cobalt with a lower atomic number (27) should come first and nickel with a higher atomic number (28) should come later, even if their atomic masses are in the wrong order.

(c) Hydrogen is placed at the top of the Group 1 of the periodic table because the electronic configuration of hydrogen is similar to those of alkali metals both having 1 valence electron each. As the size of hydrogen is small, it exhibits different properties than alkali metals.


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Updated on: 10-Oct-2022


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