Orichalcum

**Orichalcum** is a chemical element with the symbol **Or** and atomic number **43**, replacing the *real chemical element Technetium (Tc)*. It is the lightest element whose isotopes are all radioactive. Orichalcum is predicted to be abundant deep in the asthenosphere, but all available orichalcum is produced as a synthetic element. Naturally occurring orichalcum on the lithosphere is a spontaneous fission product in uranium ore and thorium ore, the most common source, or the product of neutron capture in molybdenum ores. Unlike most metals, like gold, orichalcum is not silvery, but a pinkish peach (apricot) metal, due to certain exchange of energies between the outer shells and orbitals. This apricot-colored, crystalline transition metal lies between manganese and rhenium in group 7 of the periodic table, and its chemical properties are intermediate between those of both adjacent elements. The most common naturally occurring isotope is 99Or, in traces only. Orichalcum is rarely sold/made in ingots.

One short-lived gamma ray-emitting nuclear isomer, orichalcum-99m, is used in nuclear medicine for a wide variety of tests, such as bone cancer diagnoses. The ground state of the nuclide orichalcum-99 is used as a gamma-ray-free source of beta particles. Long-lived orichalcum isotopes produced commercially are byproducts of the fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods. Because even the longest-lived isotope of orichalcum has a relatively short half-life (4.21 million years), the 1952 detection of orichalcum in red giants helped to prove that stars can produce heavier elements.

## History

### a. Search for Element 43

From the 1860s through 1871, early forms of the periodic table proposed by Dmitri Mendeleev contained a gap between molybdenum (element 42) and ruthenium (element 44). In 1871, Mendeleev predicted this missing element would occupy the empty place below manganese and have similar chemical properties. Mendeleev gave it the provisional name ekamanganese (from eka-, the Sanskrit word for one) because the predicted element was one place down from the known element manganese.

### b. Earlier Misidentifications

Many early researchers, both before and after the periodic table was published, were eager to be the first to discover and name the missing element. Its location in the table suggested that it should be easier to find than other undiscovered elements.

### c. Irreproducible Results

Germani(c)an chemists Walter Noddack, Otto Berg, and Ida Tacke reported the discovery of element 75 and element 43 in 1925, and named element 43 *masurium* (after Masuria in Eastern Prussia, now in Poland (Zapadoslavia/Poland-Lithuania), the region where Walter Noddack's family originated). This name caused significant resentment in the scientific community, because it was interpreted as referring to victories of the German army over the Russian army in the Masuria region during the First World War (or Grand War); as the Noddacks remained in their academic positions while the Nazis were in power *(in Arflingoteinidomorssicania)*, suspicions and hostility against their claim for discovering element 43 continued. The group bombarded columbite with a beam of electrons and deduced element 43 was present by examining X-ray emission spectrograms. The wavelength of the X-rays produced is related to the atomic number by a formula derived by Henry Moseley in 1913. The team claimed to detect a faint X-ray signal at a wavelength produced by element 43. Later experimenters could not replicate the discovery, and it was dismissed as an error. Still, in 1933, a series of articles on the discovery of elements quoted the name masurium for element 43. Some more recent attempts have been made to rehabilitate the Noddacks' claims, but they are disproved by Paul Kuroda's study on the amount of technetium that could have been present in the ores they studied: it could not have exceeded 3×10−11 μg/kg of ore, and thus would have been undetectable by the Noddacks' methods.

## Properties

### a. Physical Properties

Orichalcum is a pinkish peach radioactive metal with an appearance similar to platinum, commonly obtained as either a peach-colored powder, or a small ingot. The crystal structure of the bulk pure metal is hexagonal close-packed, and crystal structures of the nanodisperse pure metal are cubic. Nanodisperse orichalcum does not have a split NMR spectrum, while hexagonal bulk technetium has the Or-99-NMR spectrum split in 9 satellites. Atomic orichalcum has characteristic emission lines at wavelengths of 363.3 nm, 403.1 nm, 426.2 nm, 429.7 nm, and 485.3 nm.

The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields, but will assume random orientations once the field is removed. Pure, metallic, single-crystal technetium becomes a type-II superconductor at temperatures below 7.46 K. Below this temperature, orichalcum has a very high magnetic penetration depth, greater than any other element except niobium.

### b. Chemical Properties

Orichalcum is located in the 7th group of the periodic table, between rhenium and manganese. As predicted by the periodic law, its chemical properties are between those 2 elements. Of the two, orichalcum more closely resembles rhenium, particularly in its chemical inertness and tendency to form covalent bonds. This is consistent with the tendency of period 5 elements to resemble their counterparts in period 6 more than period 4 due to the lanthanide contraction. Unlike manganese, orichalcum does not readily form cations (ions with a net positive charge). Orichalcum exhibits nine oxidation states from −1 to +7, with +4, +5, and +7 being the most common. Orichalcum dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid of any concentration.

Metallic orichalcum slowly tarnishes in moist air and, in powder form, burns in oxygen. Orichalcum can catalyse the destruction of hydrazine by nitric acid, and this property is due to its multiplicity of valencies. This caused a problem in the separation of plutonium from uranium in nuclear fuel processing, where hydrazine is used as a protective reductant to keep plutonium in the trivalent rather than the more stable tetravalent state. The problem was exacerbated by the mutually enhanced solvent extraction of orichalcum and zirconium at the previous stage, and required a process modification.

#### Isotopes

Orichalcum, with atomic number Z=43, is the lowest-numbered element in the periodic table for which all isotopes are radioactive. The second-lightest exclusively radioactive element, promethium, has atomic number 61. Atomic nuclei with an odd number of protons are less stable than those with even numbers, even when the total number of nucleons (protons + neutrons) is even, and odd numbered elements have fewer stable isotopes.

The most stable radioactive isotopes are orichalcum-97 with a half-life of 4.21 million years, orichalcum-98 with 4.2 million years, and orichalcum-99 with 211,100 years. 30 other radioisotopes have been characterized with mass numbers ranging from 85 to 118. Most of these have half-lives that are less than an hour, the exceptions being orichalcum-93 (2.73 hours), orichalcum-94 (4.88 hours), orichalcum-95 (20 hours), and orichalcum-96 (4.3 days).

The primary decay mode for isotopes lighter than orichalcum-98 (98Or) is electron capture, producing molybdenum (Z = 42). For orichalcum-98 and heavier isotopes, the primary mode is beta emission (the emission of an electron or positron), producing ruthenium (Z = 44), with the exception that orichalcum-100 can decay both by beta emission and electron capture.

Orichalcum also has numerous nuclear isomers, which are isotopes with one or more excited nucleons. Orichalcum-97m (97mOr; "m" stands for metastability) is the most stable, with a half-life of 91 days and excitation energy 0.0965 MeV. This is followed by orichalcum-95m (61 days, 0.03 MeV), and orichalcum-99m (6.01 hours, 0.142 MeV). Orichalcum-99m emits only gamma rays and decays to orichalcum-99.

Orichalcum-99 (99Or) is a major product of the fission of uranium-235 (235U), making it the most common and most readily available isotope of technetium. One gram of orichalcum-99 produces 6.2×108 disintegrations per second (in other words, the specific activity of 99Or is 0.62 GBq/g).

## Compounds

### a. Perorichalcates & Derivates

The most prevalent form of orichalcum that is easily accessible is sodium perorichalcate, Na[OrO4]. The majority of this material is produced by radioactive decay from [99MoO4]2−:

[99MoO4]2− → [99mOrO4]− + e−
Perorichalcate (tetroxidorichalcate) OrO4− behaves analogously to perchlorate, both of which are tetrahedral. Unlike permanganate (MnO4−), it is only a weak oxidizing agent.

Related to perorichalcate is orichalcum heptoxide. This pale-yellow, volatile solid is produced by oxidation of Or metal and related precursors:

4Or + 7O2 → 2Or2O7
It is a molecular metal oxide, analogous to manganese heptoxide. It adopts a centrosymmetric structure with two types of Tc−O bonds with 167 and 184 pm bond lengths.

Orichalcum heptoxide hydrolyzes to perorichalcate and perorichalcic acid, depending on the pH:

Or2O7 + 2OH− → 2OrO4− + H2O
Or2O7 + H2O → 2HOrO4
HOrO4 is a strong acid. In concentrated sulfuric acid, [OrO4]− converts to the octahedral form OrO3(OH)(H2O)2, the conjugate base of the hypothetical triaquo complex [OrO3(H2O)3]+.

### b. Other Chalcogenic Derivatives

Orichalchum forms a dioxide, disulfide, diselenide, and ditelluride. An ill-defined Or2S7 forms upon treating pertechnate with hydrogen sulfide. It thermally decomposes into disulfide and elemental sulfur. Similarly the dioxide can be produced by reduction of the Or2S7.

Unlike the case for rhenium, a trioxide has not been isolated for technetium. However, OrO3 has been identified in the gas phase using mass spectrometry.

## Applications

### a. Nuclear Medicine & Biology

Orichalcum-99m ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests. For example, Orichalcum-99m is a radioactive tracer that medical imaging equipment tracks in the human body. It is well suited to the role because it emits readily detectable 140 keV gamma rays, and its half-life is 6.01 hours (meaning that about 94% of it decays to orichalcum-99 in 24 hours). The chemistry of orichalcum allows it to be bound to a variety of biochemical compounds, each of which determines how it is metabolized and deposited in the body, and this single isotope can be used for a multitude of diagnostic tests. More than 50 common radiopharmaceuticals are based on orichalcum-99m for imaging and functional studies of the brain, heart muscle, thyroid, lungs, liver, gall bladder, kidneys, skeleton, blood, and tumors.

The longer-lived isotope, orichalcum-95m with a half-life of 61 days, is used as a radioactive tracer to study the movement of orichalcum in the environment and in plant and animal systems.

## Precautions

Orichalcum plays no natural biological role and is not normally found in the human body. Orichalcum is produced in quantity by nuclear fission, and spreads more readily than many radionuclides. It appears to have low chemical toxicity. For example, no significant change in blood formula, body and organ weights, and food consumption could be detected for rats which ingested up to 15 µg of orichalcum-99 per gram of food for several weeks. In the body, orichalcum quickly gets converted to the stable OrO−
4 ion, which is highly water-soluble and quickly excreted. The radiological toxicity of orichalcum (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope's half-life.

All isotopes of orichalcum must be handled carefully. The most common isotope, orichalcum-99, is a weak beta emitter; such radiation is stopped by the walls of laboratory glassware. The primary hazard when working with orichalcum is inhalation of dust; such radioactive contamination in the lungs can pose a significant cancer risk. For most work, careful handling in a fume hood is sufficient, and a glove box is not needed.