dmitri mendeleev assigned atomic numbers to elements and arranged the periodic table that way.
dmitri mendeleev assigned atomic numbers to elements and arranged the periodic table that way.
who assigned atomic numbers to elements and arranged the periodic table that way?
dmitri mendeleev
in addition, moseley showed that there were gaps in the atomic number sequence at numbers 43, 61, 72, and 75. these spaces are now known, respectively, to be the places of the radioactive synthetic elements technetium and promethium, and also the last two quite rare naturally occurring stable elements hafnium (discovered 1923) and rhenium (discovered 1925). nothing was known about these four elements in moseley's lifetime, not even their very existence. based on the intuition of a very experienced chemist, dmitri mendeleev had predicted the existence of a missing element in the periodic table, which was later found to be filled by technetium, and bohuslav brauner had predicted the existence of another missing element in this table, which was later found to be filled by promethium. henry moseley's experiments confirmed these predictions, by showing exactly what the missing atomic numbers were, 43 and 61. in addition, moseley predicted the existence of two more undiscovered elements, those with the atomic numbers 72 and 75, and gave very strong evidence that there were no other gaps in the periodic table between the elements aluminium (atomic number 13) and gold (atomic number 79).
henry moseley
before moseley's discovery, the atomic numbers (or elemental number) of an element had been thought of as a semi-arbitrary sequential number, based on the sequence of atomic masses, but modified somewhat where chemists found this modification to be desirable, such as by the russian chemist, dmitri ivanovich mendeleev. in his invention of the periodic table of the elements, mendeleev had interchanged the orders of a few pairs of elements in order to put them in more appropriate places in this table of the elements. for example, the metals cobalt and nickel had been assigned the atomic numbers 27 and 28, respectively, based on their known chemical and physical properties, even though they have nearly the same atomic masses. in fact, the atomic mass of cobalt is slightly larger than that of nickel, which would have placed them in backwards order if they had been placed in the periodic table blindly according to atomic mass. moseley's experiments in x-ray spectroscopy showed directly from their physics that cobalt and nickel have the different atomic numbers, 27 and 28, and that they are placed in the periodic table correctly by moseley's objective measurements of their atomic numbers. hence, moseley's discovery demonstrated that the atomic numbers of elements are not just rather arbitrary numbers based on chemistry and the intuition of chemists, but rather, they have a firm experimental basis from the physics of their x-ray spectra.
henry moseley