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Position of the elements in the periodic table:

1. How is the problem regarding the position of cobalt (⁵⁹Co) and nickel (⁵⁹Ni) in Mendeleev’s periodic table resolved in modern periodic table?
→ In spite of being heavier, Cobalt (⁵⁹Co) had to be placed before Nickel (⁵⁹Ni) in the Mendeleev’s Periodic Table. This had to be done so as to match the properties of these elements in their respective groups. The modern periodic table is based on atomic numbers. Thus, this problem does not arise, because the atomic number of Cobalt (27) is less than that of Ni (28). Hence, Cobalt has to be placed before Nickel. 2 Marks
2. How did the position of ${}_{17}^{35}{Cl}$ and ${}_{17}^{37}{Cl}$ get fixed in the modern periodic table?
→ Modern periodic table is based on atomic numbers. As all the isotopes of an element have the same atomic number, the question of giving them separate positions does not arise. 2 Marks
3. Can there be an element with atomic mass 53 or 54 in between the two elements, chromium ${}_{24}^{52}{Cr}$ and manganese ${}_{25}^{55}{Mn}$ ?
→ Since atomic numbers are natural numbers, it is not possible to have any element between atomic number 24 and 25. Thus, there can’t be any element with atomic mass 53 or 54 between chromium ${}_{24}^{52}{Cr}$ and manganese ${}_{25}^{55}{Mn}$ . 2 Marks
4. What do you think? Should hydrogen be placed in the group 17 of halogens, or group 1 of alkali metals in the modern periodic table?
→ Hydrogen shows the properties of alkali metals as well as halogens both. However, most of the times it behaves like an alkali metal. Hence, it should be placed in group 1 of the modern periodic table. 2 Marks

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Position of the elements in the periodic table: