Question

differentiate between the arrangements of elements in mendeleev's and modern perodic table

Answer 1

In mendleev's periodic table elements are arranged in the order if increasing atomic mass while in modern periodic table elements are arranged in the order of increasing atomic number.

Answer 2

the arrangement in Mendeleev's periodic table were 1 according to the periodic properties which at that time differed with the help of atomic weight.
the arrangement in moseley periodic table or the long form of periodic table are according to the periodic properties which now differ by atomic number that is the number of protons and electrons
and now stating the major difference the major difference comes in the placement of isotopes which are not placed at the correct position in Mendeleev periodic table because of it being arranged you to atomic mass. position of Nickel and Cobalt were mismatched. the properties of bridge elements was also not very well satisfied in the Mendeleev's periodic table
Mendeleev periodic table had 8 groups and now the modern periodic table as 18 groups at 7 period