Question

Write two achievements of mendeleev periodic Table? Why could not isotopes be placed in it.

Answer 1

achievements he arranged all the 63 known element in increasing order of their atomic masses . The elements with similar physical and chemical propertiescame under same group . Isotopes of an element have same atomic number where as mendeleev adjusted few elements with greater atomic mass before the elements with lower atomic mass
.so he could not placed isotopes on proper way.

Answer 2

While developing the periodic table there were few instances where Mendeleev had to place an element with a slightly greater atomic mass before an element with lower atomic mass.

Mendeleev boldly predicted the existence of some elements that had not been discovered at that time.