1.
Hydrogen
was placed in group 1 and in group 17 as it resembles alkali metals as well as
halogens. Therefore, no fixed position could be given to hydrogen in
Mendeleev’s periodic table.
2.
Though
isotopes of the same elements have different atomic masses, they cannot be
placed separately in the periodic table. Since isotopes are chemically similar,
they had to be given the same position in the periodic table.
3.
Some
elements with higher atomic masses are placed before those having lower atomic
masses, e.g. cobalt with atomic mass 58.93 is placed before nickel having
atomic mass 58.71.
4.
Dissimilar
elements were placed in the same group. E.g. Mn (Manganese) was placed in the
group of halogens which totally differ in the properties.
