INDIUM: DISCOVERY, STRUCTURE, PROPERTIES, ISOTOPES, USES, RISKS - CHEMISTRY - 2025

The indium is a metal belonging to group 13 of the periodic table and has the chemical symbol In. Its atomic number is 49, ₄₉ In, and it occurs in nature as two isotopes: ¹¹³ In and ¹¹⁵ In, the latter being the most abundant. Indium atoms are found on Earth as impurities in zinc and lead minerals.

It is a particular metal, since it is the softest that can be touched without many health risks; unlike lithium and rubidium, which would terribly burn skin when reacting with their moisture. A piece of indium can be cut with a knife and fractured with the force of the fingers, emitting a distinctive crunch.

Piece of metallic indium. Source: Hi-Res Images of Chemical Elements

Anyone who hears this metal name will surely come to mind India, but its name derives from the indigo color, which is observed when the flame test is carried out. In this sense it is quite similar to potassium, burning its metal or its compounds with a very characteristic flame, through which indium was detected for the first time in sphalerite minerals.

Indium shares many chemical qualities with aluminum and gallium, occurring in most of its compounds with an oxidation number of +3 (In ³⁺). It combines excellently with gallium forming alloys with low melting points, one of which is galinstan.

The applications of indium are based on the coating of materials with their alloys, making them electrically conductive and flexible. The Indian covers some glasses to give them greater shine, replacing silver. In the world of technology, the Indian is found in LCD and touch screens.

Discovery

In 1863, the German chemist Ferdinand Reich was looking for traces of the element thallium, through the green line of its emission spectrum, in zinc minerals; specifically samples of sphalerite (ZnS) from around Saxony. After roasting the minerals, removing their sulfur content, digesting them in hydrochloric acid, and distilling off the zinc chloride, he obtained a straw-colored precipitate.

Faced with the discovery, Reich decided to carry out a spectroscopic analysis; but because he did not have good eyes to observe colors, he turned to his colleague Hieronymus Theodor Richter for help in this task. Richter was the one who observed a bluish spectral line, which did not coincide with the spectrum of any other element.

The two German chemists were faced with a new element, which was called Indian because of the indigo color of the flame when its compounds were burned; and in turn, the name of this color derives from the Latin word indicum, which means India.

A year later, in 1864, excited and after a prolonged series of precipitations and purifications, they isolated a sample of metallic indium by electrolysis of its dissolved salts in water.

Structure of the Indian

Indium atoms, In, coalesce using their valence electrons to establish a metallic bond. Thus, they end up being arranged in a body-centered crystal with a distorted tetragonal structure. The interactions between the neighboring In-In atoms in the crystal are relatively weak, which explains why indium has a low melting point (156 ºC).

On the other hand, the forces that join two or more indium crystals are not strong either, otherwise they would not move over each other, giving the metal its characteristic softness.

Properties

Physical appearance

It is a remarkably soft silver metal. It can be torn with fingernail pressure, cut with a knife, or scratched in shiny lines on a sheet of paper. It is even possible to chew it and deform it with your teeth, as long as it is flattened. Likewise, it is very ductile and malleable, having plastic properties.

When the Indian is heated with a blowtorch, it gives off an indigo-colored flame, even brighter and more colorful than that of potassium.

Molar mass

114.81 g / mol

Melting point

156.60 ºC

Boiling point

2072 ° C.

Like gallium, indium has a wide temperature range between its melting point and its boiling point. This reflects the fact that In-In interactions in liquid are stronger than those that predominate in glass; and that therefore a drop of indium is easier to obtain than its vapors.

Density

At room temperature: 7.31 g / cm ³

Right at the melting point: 7.02 g / cm ³

Electronegativity

1.78 on the Pauling scale

Ionization energies

First: 558.3 ​​kJ / mol

Second: 1820.7 kJ / mol

Third: 2704 kJ / mol

Thermal conductivity

81.8 W / (m K)

Electrical resistivity

83.7 nΩm

Mohs hardness

1,2. It is only slightly harder than talcum powder (don't confuse toughness with toughness).

Reactivity

Indium dissolves in acids to form salts, but does not dissolve in alkaline solutions, not even with hot potassium hydroxide. Reacts in direct contact with sulfur, oxygen and halogens.

Indium is relatively amphoteric, but it behaves more like a base than an acid, its aqueous solutions being slightly basic. In (OH) ₃ is redissolved with the addition of more alkalis, giving rise to the complex indiates, In (OH) ₄ ^-, as happens with aluminates.

Electronic configuration

The electron configuration of the indium is as follows:

4d ¹⁰ 5s ² 5p ¹

Of those thirteen electrons, the last three of the 5s and 5p orbitals are the valence electrons. With these three electrons, the indium atoms establish their metallic bond, as do aluminum and gallium, and form covalent bonds with other atoms.

Oxidation numbers

The aforementioned serves to understand at once that indium is capable of losing its three valence electrons, or gaining five to become isoelectronic to the noble gas xenon.

If in a compound we assume that it has lost its three electrons, it will remain as the trivalent cation In ³⁺ (in analogy to Al ³⁺ and Ga ³⁺) and, therefore, its oxidation number will be +3. Most of the indium compounds are In (III).

Among other oxidation numbers found for indium we have: -5 (In ^5-), -2 (In ^2-), -1 (In ^-), +1 (In ⁺) and +2 (In ²⁺).

Some examples of In (I) compounds are: InF, InCl, InBr, InI and In ₂ O. All of them are relatively rare compounds, while those of In (III) are the predominant ones: In (OH) ₃, In ₂ O ₃, InCl ₃, InF ₃, etc.

In (I) compounds are powerful reducing agents, in which In ⁺ donates two electrons to other species to become In ³⁺.

Isotopes

Indium occurs in nature as two isotopes: ¹¹³ In and ¹¹⁵ In, whose terrestrial abundances are 4.28% and 95.72%, respectively. Therefore, on Earth we have many more ¹¹⁵ In atoms than we have ¹¹³ In. The ¹¹⁵ In has a half - life of 4.41 × 10 ¹⁴ years so great that it is almost considered stable, despite being a radioisotope.

Currently, a total of 37 artificial isotopes of indium have been created, all radioactive and highly unstable. Of all of them, the most stable is ¹¹¹ In, which has a half-life of 2.8 days.

Applications

Alloys

The Indian gets along very well with the gallium. Both metals form alloys that melt at low temperatures, looking like silver liquids, with which mercury is supplanted in several of its applications. Likewise, indium also amalgamates easily, having a solubility of 57% in mercury.

Indium alloys are used to design silver mirrors without the need for silver. When poured onto a surface of any material, it acts as an adherent, such that glass, metal, quartz and ceramic plates can be joined.

electronics

Without the Indian, touchscreens would never have existed. Source: Pxhere.

Indium also gets along well with germanium, so its compounds are added as dopants to germanium nitride in LEDs, reproducing blue, purple and green lights from these mixtures. It is also part of transistors, thermistors and photovoltaic cells.

The most important of its compounds is indium tin oxide, which is used as a coating on glasses to reflect some wavelengths. This allows it to be used in welding goggles, and skyscraper glass so they don't get hot inside.

Glasses coated with this oxide are good conductors of electricity; like that coming from our fingers. And that is why it is intended for the manufacture of touch screens, an activity even more current today due to the emergence of more and more smartphones.

Risks

Indium does not represent any risk to the environment in the first instance, since its In ³⁺ ions are not disseminated in appreciable quantities. There is no information regarding what would be its impact on soils, affecting plants, and neither on fauna or the seas.

In the body, it is not known whether In ³⁺ ions have any essential role in metabolism in trace amounts. However, when its compounds are ingested, they are harmful to various organs, which is why they are considered highly toxic substances.

In fact, ITO particles (Indium Tin Oxide), essential for the manufacture of screens for computers and smartphones, can have a negative impact on the health of workers, causing a disease called Indium lung.

The ingestion of these particles occurs mainly by inhalation and by contact through the skin and eyes.

On the other hand, fine indium metal particles are prone to burning and causing fires if they are near a heat source.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Wikipedia. (2020). Indium. Recovered from: en.wikipedia.org
3. Dr. Doug Stewart. (2020). Indium Element Facts. Recovered from: chemicool.com
4. The Editors of Encyclopaedia Britannica. (January 20, 2020). Indium. Encyclopædia Britannica. Recovered from: britannica.com
5. Habashi F. (2013) Indium, Physical and Chemical Properties. In: Kretsinger RH, Uversky VN, Permyakov EA (eds) Encyclopedia of Metalloproteins. Springer, New York, NY
6. National Center for Biotechnology Information. (2020). Indium. PubChem Database., CID = 5359967. Recovered from: pubchem.ncbi.nlm.nih.gov
7. Kimberly Uptmor. (2020). What is Indium Used For in Everyday Life? Study. Recovered from: study.com
8. Hines, CJ, Roberts, JL, Andrews, RN, Jackson, MV, & Deddens, JA (2013). Use of and occupational exposure to indium in the United States. Journal of occupational and environmental hygiene, 10 (12), 723–733. doi: 10.1080 / 15459624.2013.836279