Escutcheons of Science
 Coat-of-arms of Dmitri Mendeleev

Dmitri Mendeleev (1834-1907).   Russian chemist.
[ Dimitri Mendeleyev, Mendelejev, Mendeleyeff, etc. ]

This coat-of-arms of  Dmitri Mendeleev (1834-1907)
is a copyrighted image drawn by the renowned heraldic artist
Carl-Alexander von Volborth (1919-2009).

Quarterly;   [1 & 4]  Gules, an ironclad arm from a cloud Proper holding a sword Argent;
  [2 & 3]  Vert, a horseshoe below a stirrup Argent strapped Gules.
On an inescutcheon Azure, a cross patonce Or, 3 mullets in chief and a crescent in base Argent.

 Dimitri Mendeleev
Dmitri Mendeleev

In the 1860's, several chemists  (working independently)  observed that the basic properties of chemical elements with similar atomic weights repeat themselves in sequence, by order of atomic mass, for heavier elements.  Dmitri Mendeleev  carried this idea further than anyone else in his influential chemistry textbook (1869).  In particular,  Mendeleev  noticed several  gaps  in this scheme and attributed them to undiscovered elements whose chemical properties he correctly predicted.

Mendeleev's classification can be construed as an empirical discovery of what we now call the  atomic number  (Z)  of a chemical element  (now known to be the number of electrons orbiting its nucleus).  The chemical properties of an atom are entirely due to its electronic structure and the allocation of electrons under quantum rules  (explained by Schrödinger in 1926)  is the cause of the periodicity presented by Mendeleev in 1869.  Because the atomic weights are not always increasing with Z,  Dmitri Mendeleev  postulated that the observed discrepancies were due to measurement errors.  He was wrong on that particular detail:  For example, the atomic mass of  tellurium  (127.6 g/mol for Z=52)  is indeed higher than that of  iodine  (126.9 g/mol for Z=53).

Incidentally,  Dmitri Mendeleev  also sought to give chemical explanations for two scientific mysteries of his times by speculating the existence of elements with an atomic mass lower than that of hydrogen:

Well,  Dmitri Mendeleev  was not entirely wrong about such speculations, since there is indeed an overlooked unstable element which is barely heavier than hydrogen  (although it has nothing to do with the aether or the corona).  It has a half-life of about  15  minutes and it's sometimes called  neutronium.  The atomic number of  neutronium  is zero and its nucleus consists of a single free neutron.  Of course, due to its utter lack of electrons,  neutronium  cannot take part in  any  chemical reaction whatsoever; it's the ultimate inert gas!  Arguably,  neutronium  can form extremely heavy isotopes under the influence of gravity, since we may view every  neutron star  as a huge nucleus of  neutronium Just a joke!      Mendeleev Periodic Table of the Elements
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