(a) Achievements of Modem periodic table are:
(i) Prediction of properties of elements could be made with more precision since elements were arranged on the basis of increasing atomic number.
(ii) Metals are separated from non-metals by a zig-zag line. On this zig-zag line borderline elements-like Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium and Polonium which are also known as metalloids are present.
(b) Limitations of Mendeleev’s classification are:
(i) Anomalous pairs : Mendeleev couldn’t
explain the presence of anomalous pairs in which an element with more atomic mass preceded the element with lower atomic mass, e.g., Co and Ni.
(ii) Isotopes could not be accommodated in the same slot since it was Based upon atomic masses.
© (i) The anomaly of higher atomic mass element
preceding the lower atomic mass element was easily removed when these elements were placed in the order of increasing atomic number.
(ii) Isotopes could be placed in the same slot since elements are arranged on the basis of their valence electrons (atomic number).