What were the defects and anomalies in Mendeleeff's Periodic Table and how are they resolved by Moseley.

Defects in Mendeleeff's Periodic Table :

1. Anomalous pairs of elements — Certain pairs of elements having higher atomic weights have been given positions before the elements having lower atomic weights.
   This defect disappears if elements were arranged according to their atomic numbers.
   e.g., Co [at. wt. 58.9, at. no. 27] was placed before Ni [at. wt. 58.6, at. no. 28].
2. Position of rare earths and actinides — Could be justified only if arranged to their atomic numbers
3. Position of isotopes — Isotopes had to be placed in same position according to atomic numbers

Most of the defects in Mendeleeff's Periodic Table disappear if the basis of classification of elements is changed from atomic weights to atomic numbers.
Moseley arranged the elements in increasing order of atomic number in Seven horizontal rows called 'periods' and Eighteen vertical columns called 'groups'.
This forms the basis of the Modern Periodic Table.