Dmitri Mendeleev tried to order the first periodic table
according to ascending atomic weights; however,
he had to place tellurium ahead of iodine,
even though tellurium has the greater atomic weight,
to better align the chemical properties of the two
with the chemical properties of the elements above them.
The modern periodic table is arranged by atomic number.
Francis William Aston discovered isotopes in 1922, and
James Chadwick discovered the neutron in 1932.
See also in The book of science:
Readings in wikipedia: