Question

Write two demerits of the modern periodic table

Answer 1

Hint In the past, scientists assumed that the properties of the elements mainly depended on their atomic masses and on this assumption Mendeleev proposed a periodic table in which 63 elements were placed in a vertical column called groups and in horizontal rows called periods.

Complete Step by step solution:
Mendeleev’s periodic table is rejected as it is not able to explain the position of certain elements like rare earth metals and isotopes. This defect was removed by the scientist named Henry Moseley and he discovered the modern periodic table.
Modern periodic table consists of 18 vertical columns known as groups arranged as left to right and seven horizontal rows known as periods arranged from top to bottom. In this periodic table, elements are arranged according to their atomic number and this table divides the elements into s, p, d and f-block elements on the basis of their valence shell electrons. The main demerits of the modern periodic table are as follows:
1. Position of the hydrogen atom: Hydrogen shows similarities with both electropositive alkali metals and electronegative non-metallic halogens but it placed with alkali metals which is not well explanatory.
2. Elements which have the same properties as barium-lead and copper-mercury are put separately as they placed earlier in Mendeleev’s periodic table.

Note: Group 1,2,13-17 are known as main group elements, group 3-12 are known by the name transition elements and group 18 is of inert gases or noble gases i.e. those elements which have fully filled outermost atomic shell and have high stability.