What did Mendeleev claim about the first periodic table ?
" Here Mendeleev seems to be wrong as the " atomic mass " of tellurium ( 127.6 ) remains higher than that of iodine ( 126.9 ) as displayed on modern periodic tables , but this is due to the way atomic masses are calculated , based on a weighted average of all of an element 's common isotopes , not just the one - to - one proton / neutron - ratio version of the element to which Mendeleev was referring . "