Mendeleev's periodic table
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Mendeleev’s Periodic Table
Mendeleev arranged the elements based on their atomic masses. He observed that when the elements were arranged in increasing order of their atomic masses, there was a periodic recurrence in their physical and chemical properties. Thus, Mendeléev formulated a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’. Mendeléev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.
When noble gases like helium (He), neon (Ne) and argon (Ar) were discovered, they could be placed in a new group without disturbing the existing order.
However, it also had some drawbacks:
Some elements in Mendeleev’s Table have not been arranged in the increasing order of their atomic masses. For example, Co and Ni – even though Co has a higher atomic mass than Ni, it is placed before Ni in his table.
Cobalt Nickel
58.9 amu 58.7 amu
Hydrogen forms similar compounds as Group 1 elements. However, it also forms similar diatomic molecules as Group 7 elements (H2, F2, Cl2, Br2, I2) . Hence, it could not be assigned a fixed position in the table.
Compounds of H Compounds of Na
HCl NaCl
H2O Na2O
H2S Na2S
Isotopes posed a challenge to Mendeleev’s table. For example, Cl has two major isotopes – Cl-35 and Cl-37. Hence, it can be placed in accordance with its mass being 35 amu or 37 amu or both of them could be placed together owing to their similar chemical properties!
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