Question

list two things which possed a challenge to mendeleev's periodic table

Answer 1

Mendeleev's periodic table is based on atomic mass no and not the atomic no. Hydrogen has not stable position it act like alkali metal as well as halogen Due to atomic mass no isotopes should given separate place but Mendeleev put isotopes at Sam place

Answer 2

 *isotopes of all element 
*position of H2