(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) Position of isotopes: All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since, all the isotopes of an element have the same atomic number; they can be put at one place in the same group of the periodic table.
(b) Position of Cobalt and Nickel: 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. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.
(c) Position of hydrogen: Hydrogen has been placed at the top of group 1, above the alkali metals in the modern periodic table because the electronic configuration of hydrogen is similar to those of alkali metals. Both, have 1 valence electron each.