Professor
Dmitri MendeleevDmitri Ivanovich Mendeleev , was a Russian chemist and inventor. He is credited as being the creator of the first version of the periodic table of elements...
published the first
Periodic TableThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
of the Atomic Elements in 1869 based on properties which appeared with some regularity as he laid out the
elementsA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
from lightest to heaviest.
When Mendeleev proposed his
periodic tableThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
, he noted gaps in the table, and predicted that as-yet unknown elements existed with properties appropriate to fill those gaps.
Prefixes
To give provisional names to his predicted elements, Mendeleev used the prefixes
eka-,
dvi-, and
tri-, from the
SanskritSanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...
words for one, two, and three, depending upon whether the predicted element was one, two, or three places down from the known element in his table with similar chemical properties. For example,
germaniumGermanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
was called ekasilicon until its discovery in 1886, and
rheniumRhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
was called dvi-
manganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
before its discovery in 1926.
It has been speculated that the similarity between the tabular structure commonly used to present the Sanskrit
abugidaAn abugida , also called an alphasyllabary, is a segmental writing system in which consonant–vowel sequences are written as a unit: each unit is based on a consonant letter, and vowel notation is obligatory but secondary...
and the periodic table is what led to Mendeleev choosing to use Sanskrit as the basis of these prefixes.
Sometimes the
eka- prefix is used to refer to some transuranic elements, for example eka-lead for
ununquadiumUnunquadium is the temporary name of a radioactive chemical element with the temporary symbol Uuq and atomic number 114. There is no proposed name yet, although flerovium has been discussed in the media.About 80 decays of atoms of...
, eka-radon for
ununoctiumUnunoctium is the temporary IUPAC name for the transactinide element having the atomic number 118 and temporary element symbol Uuo. It is also known as eka-radon or element 118, and on the periodic table of the elements it is a p-block element and the last one of the 7th period. Ununoctium is...
and eka-caesium.
Nowadays, the prefix
eka- (and, more rarely, dvi-) is sometimes used in discussions about undiscovered elements, such as untriennium, also known as
eka-actinium or
dvi-lanthanum.
Current official IUPAC practice is to use a
systematic element nameA systematic element name is the temporary name and symbol assigned to newly synthesized and not yet synthesized chemical elements. In chemistry, a transuranic element receives a permanent name and symbol only after its synthesis has been confirmed. In some cases, this has been a protracted and...
based on the
atomic numberIn chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
of the element as the provisional name, instead of being based on its position in the periodic table as these prefixes require.
Original predictions from 1870
The four predicted elements lighter than the
rare earth elementAs defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
s,
ekaboron (
Eb),
ekaaluminium (
Ea),
ekamanganese (
Em), and
ekasilicon (
Es), proved to be good predictors of the properties of
scandiumScandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids...
,
galliumGallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
,
technetiumTechnetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
and
germaniumGermanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
respectively, which each fill the spot in the periodic table assigned by Mendeleev.
Initial versions of the periodic table did not give the rare earth elements the treatment now given them, helping to explain both why Mendeleev’s predictions for heavier unknown elements did not fare as well as those for the lightest predictions and why they are not as well known or documented.
Ekaboron and scandium
Scandium oxide was isolated in late 1879 by Lars Fredrick Nilson;
Per Teodor ClevePer Teodor Cleve was a Swedish chemist and geologist.After graduating from the Stockholm Gymnasium in 1858, Cleve matriculated at Uppsala University in May 1858, where he received his PhD in 1863...
recognized the correspondence and notified Mendeleev late in that year. Mendeleev had predicted an
atomic massThe atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
of 44 for ekaboron in 1871 while scandium has an atomic mass of 44.955910.
Ekaaluminium and gallium
In 1871 Mendeleev predicted the existence of a yet-undiscovered element he named eka-aluminium (because of its proximity to aluminium in the periodic table). The table below compares the qualities of the element predicted by Mendeleev with actual characteristics of
GalliumGallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
(discovered in 1875).
| Property |
Ekaaluminium |
Gallium |
| atomic mass |
68 |
69.72 |
| density (g/cm³) |
6.0 |
5.904 |
| melting point (°C) |
Low |
29.78 |
| oxide's formula |
Ea2O3 (density - 5.5 g cm−3)
(soluble in both alkalis and acids) |
Ga2O3 (density - 5.88 g cm−3)
(soluble in both alkalis and acids) |
| chloride's formula |
Ea2Cl6 (volatile) |
Ga2Cl6 (volatile) |
Ekamanganese and technetium
TechnetiumTechnetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
was isolated by
Carlo PerrierCarlo Perrier was an Italian mineralogist who did extensive research on the element technetium in 1936. He discovered the element along with his colleague, Emilio Segrè , in 1937....
and Emilio Segrè in 1937, well after Mendeleev’s lifetime, from samples of
molybdenumMolybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
that had been bombarded with
deuteriumDeuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
nuclei in a
cyclotronIn technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
by
Ernest LawrenceErnest Orlando Lawrence was an American physicist and Nobel Laureate, known for his invention, utilization, and improvement of the cyclotron atom-smasher beginning in 1929, based on his studies of the works of Rolf Widerøe, and his later work in uranium-isotope separation for the Manhattan Project...
. Mendeleev had predicted an atomic mass of 100 for ekamanganese in 1871 and the most stable isotope of technetium is
98Tc.
Ekasilicon and germanium
GermaniumGermanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
was isolated in 1886, and provided the best confirmation of the theory up to that time, due to its contrasting more clearly with its neighboring elements than the two previously confirmed predictions of Mendeleev do with theirs.
| Property |
Ekasilicon |
Germanium |
| atomic mass |
72 |
72.61 |
| density (g/cm³) |
5.5 |
5.35 |
| melting point (°C) |
high |
947 |
| color |
gray |
gray |
| oxide type |
refractory A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
dioxide |
refractory dioxide |
| oxide density (g/cm³) |
4.7 |
4.7 |
| oxide activity |
feebly basic |
feebly basic |
| chloride boiling point |
under 100°C |
86°C (GeCl4) |
| chloride density (g/cm³) |
1.9 |
1.9 |
1871 predictions
The existence of an element between
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
and
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
was predicted by Mendeleev in 1871. In 1900
William CrookesSir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...
isolated
protactiniumProtactinium is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume...
as a radioactive material from uranium which he could not identify. Different isotopes of protactinium were identified in Germany in 1913 and in 1918, but the name
protactinium was not given until 1948.
Mendeleev's 1869 table had implicitly predicted a heavier analog of
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
and
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, but in 1871 he placed
lanthanumLanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and...
in that spot. The 1923 discovery of
hafniumHafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
validated Mendeleev's original 1869 prediction.
Later predictions of elements, coronium, and others
In 1902, having accepted the evidence for elements
heliumHelium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
and
argonArgon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
,
Mendeleev placed these Noble Gases in
Group 0The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
in his arrangement of the elements. As Mendeleev was doubtful of
atomic theoryIn chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter could be divided into any arbitrarily small quantity...
to explain the
Law of definite proportionsIn chemistry, the law of definite proportions, sometimes called Proust's Law, states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have...
, he had no
a prioriThe terms a priori and a posteriori are used in philosophy to distinguish two types of knowledge, justifications or arguments...
reason to believe
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
was the lightest of elements, and suggested that a hypothetical lighter member of these chemically inert Group 0 elements could have gone undetected and be responsible for radioactivity.
The heavier of the hypothetical proto-helium elements Mendeleev identified with coronium,
named by association with an unexplained spectral line in the Sun's
coronaA corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
. A faulty calibration gave a wavelength of 531.68 nm, which was eventually corrected to 530.3 nm, which Grotrian and Edlen identified as originating from Fe XIV in 1939.
The lightest of the Zero Group gases, the first in the Periodic Table, was assigned a theoretical atomic mass between 5.3 x 10
−11 and 9.6 x 10
−7. The kinetic velocity of this gas was calculated by Mendeleev to be 2,500,000 meters per second. Nearly massless, these gases were assumed by Mendeleev to permeate all matter, rarely interacting chemically. The high mobility and very small mass of the trans-hydrogen gases would result in the situation, that they could be rarefied, yet appear to be very dense.
Mendeleev was so confident that these atomic elements would be discovered, that he included them in later publications of the periodic chart, although there was no physical evidence for their existence available at the time.
Mendeleev later published a theoretical expression of the ether, which satisfied many of the contradictions which existed in physics at that time, in a small booklet entitled,
A Chemical Conception of the Ether, in 1904. His 1904 publication again contained two atomic elements smaller and lighter than hydrogen. He treated the “ether gas” as an interstellar atmosphere composed of at least two lighter-than-hydrogen elements. He stated that these gases originated due to violent bombardments internal to stars, the sun being the most prolific source of such gases. According to Mendeleev's booklet, the interstellar atmosphere was probably composed of several additional elemental species.