Statement (I) : Mendeleev’s periodic table is based upon increasing order of atomic masses. 

Statement (II) : Mendeleev’s periodic table is fail to explain cause of periodicity.

Both statements I and II are correct

Explanation:

• Statement I: Mendeleev’s periodic table is based upon increasing order of atomic masses.
  • This statement is correct. Dmitri Mendeleev, a Russian chemist, developed the periodic table in 1869 by arranging elements in order of increasing atomic masses. He observed that elements with similar chemical properties recurred at regular intervals, leading to the formulation of the periodic law. This arrangement allowed Mendeleev to predict the existence and properties of undiscovered elements by leaving gaps in his table for them. For example, he predicted the properties of elements like gallium and germanium before they were discovered, and their actual properties closely matched his predictions.
• Statement II: Mendeleev’s periodic table fails to explain the cause of periodicity.
  • This statement is also correct. While Mendeleev's periodic table effectively organized elements and highlighted periodic trends in properties, it did not provide an explanation for the underlying cause of this periodicity. The true cause of periodicity was later understood with the development of quantum mechanics and the discovery of atomic structure, particularly the arrangement of electrons in atomic orbitals. The modern periodic table arranges elements by increasing atomic number (the number of protons) rather than atomic mass, which more accurately reflects periodic trends.

Therefore, both statements are accurate: Mendeleev's periodic table is based on increasing atomic masses, but it did not explain the underlying cause of the periodicity observed in element properties.