Which among the given option is not a demerit of Mendeleev’s periodic table?

Hydrogen (Z = 1) is placed at the top of the alkali metal family because it resembles alkali metals in its properties. So, we can say that the Anomalous positions of some elements are a demerit of Mendeleev’s periodic table.

Isotopes of an element have different atomic masses but the same atomic number. Since the periodic table has been framed on the basis of increasing atomic masses of the elements, different positions must have been allotted to all the isotopes of a particular element. So 'positions of isotopes' is also considered a demerit of Mendeleev's periodic table.

Hydrogen possessed characteristics that resemble both the alkali metals and the halogens. So, the 'Position of hydrogen is not fixed' and is considered a demerit of  Mendeleev's periodic table.

Mendeleev left gaps in his table to place elements not known at the time. By looking at the chemical properties and physical properties of the elements next to a gap, he could also predict the properties of these undiscovered elements. Hence, It is not a demerit, it is considered as the merit of Mendeleev’s periodic table.

Hence, the 'Gap after some elements' is not a demerit of Mendeleev's periodic table.