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Elements are classified in mendleeve's periodic table in the order of their increasing atomic masses.
merits :-
1. his prediction about discovery of new elements were proved correct.
2. he left gaps in his table so that new elements could be easily fitted.
3. nobel gases could be placed in his table without disturbing other elements

1. he couldn't define the position of hydrogen and isotopes
2. at some places in his table he put the element with more atomic mass before the element with less atomic mass.
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Merits are:1. The classification of elements is based on the atomic number, which is a more fundamental property.2. The reason for placing isotopes at one place is justified as the classification is on the basis of atomic number

DEMERITS 1 .Hydrogen resembles both the alkali metals and halogens. But it has been placed with the alkalis 2. The lanthanides and actinides have not been placed in the main body of the table.