a) Discuss any two achievements of the Modern Periodic Table.
b) Discuss any two limitations of Mendeleev's classification.
c) How were these removed in the Modern Periodic Table?

Explanation

a) Two achievements of Modern Periodic Table are:
  i] It is based on the atomic number which increases  uniformly by one from element to element. Hence, it is more accurate.
ii] All elements having similar properties are placed in the same group. It has accommodated all the known elements till date.
b] Two limitations of Mendeleev.s classification:
i] Isotopes discovered several years later posed a challenge to Mendeleev.s table. These had different atomic masses but same chemical properties and hence, did not obey periodic law. They could neither be placed in the same group (since they had different atomic masses) nor in different groups (since their properties were same). Thus, their position in the Periodic Table could not be decided properly.
ii] Like alkali metals, hydrogen combines with halogens, oxygen, and sulphur to form compounds having similar formulae (for example: HCI, H2O, H2S which can be compared with NaCl, Na2O and Na2S respectively). On the other hand, like halogens, hydrogen exists as a diatomic molecule and combines with metals and non-metals to form covalent compounds. Hence, Mendeléey could not give a fixed position to hydrogen in his Periodic Table.
c] i]  In Mendeléey.s Periodic Table, hydrogen could notbe allotted a fixed position since it showed properties similar to both alkali metals and halogens. In the Modern Periodic Table, this anomaly is resolved by placing hydrogen in the top left corner separately since it has special properties that are not found in any other element.
ii] In Mendeléey.s Periodic Table, isotopes of an element (same chemical properties) could not be placed in the same position since they have different atomic masses. But in the Modern Periodic Table, isotopes (same atomic number) can be placed in the same position.