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.
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:
- One very light element as the constituent of the electromagnetic aether. (At the time, it was firmly believed that lightwaves existed in a vacuum because of some kind of unknown vibrating medium permeating it.)
- A slightly more substantial element (which Mendeleev dubbed coronium) to explain a prominent spectral line in the solar corona (actually due to highly-ionized iron).
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.