AMPHIGENS: ELEMENTS, PROPERTIES AND COMPOUNDS - CHEMISTRY - 2024

The chalcogens or chalcogen are chemical elements belonging to the group or family of oxygen of the periodic table. They are in group VIA or 16, located on the right side or block p.

The head of the group, as its name implies, is occupied by the element oxygen, which differs physically and chemically from its elements of the same group. The word 'chalcogen' derives from the Greek word chalcos, which means copper.

Source: Pxhere

Many chemists have named these elements as formers of ash, chalks, bronzes and chains. However, the most accurate interpretation corresponds to that of 'mineral formers'.

Thus, chalcogens are characterized by being present in innumerable minerals; such as silicates, phosphates, oxides, sulfides, selenides, etc.

On the other hand, the word 'antigen' means capable of forming acidic or basic compounds. A simple example of this is in the fact that there are acidic and basic oxides.

Oxygen can not only be found in the air you breathe, but it is also part of 49% of the earth's crust. For this reason, it is not enough to look at the clouds to have it head on; and to contemplate the maximum physical manifestation of chalcogens, it is necessary to visit a mountain or an ore.

Chalcogenic elements

Source: Gabriel Bolívar

What are the elements of group 16? The upper image shows the column or group with all its elements, headed by oxygen. Naming them in descending order we have: oxygen, sulfur, selenium, tellurium and polonium.

Although it is not shown, below the polonium is located the synthetic, radioactive element, and the second heaviest after oganeson: livermorio (Lv).

Oxygen

Oxygen is found in nature primarily as two allotropes: O ₂, molecular or diatomic oxygen, and O ₃, ozone. It is gas under terrestrial conditions and is obtained from the liquefaction of air. In liquid state, it has pale bluish tones, and in the form of ozone it can form reddish-brown salts called ozonides.

Sulfur

It naturally has twenty different allotropes, the most common of all being S ₈ "the sulfur crown." Sulfur is capable of forming with itself cyclic molecules or helical chains with covalent bond SSS…; This is known as catenation.

Under normal conditions it is a yellow solid, whose reddish and greenish colorations depend on the number of sulfur atoms that make up the molecule. In the gas phase, only, it is found as a diatomic molecule S = S, S ₂; similar to molecular oxygen.

Selenium and tellurium

Selenium forms shorter chains than sulfur; but with sufficient structural diversity to find allotropes of red, greyish crystalline, and amorphous black.

Some consider it a metalloid, and others a non-metallic element. Surprisingly, it is essential for living organisms, but at very low concentrations.

Tellurium, on the other hand, crystallizes as a grayish solid and possesses characteristics and properties of a metalloid. It is a very scarce element in the earth's crust, being found in extremely low concentrations in rare minerals.

Polonium

Of all the chalcogens, it is the only metallic element; but like its 29 isotopes (and others) it is unstable, highly toxic and radioactive. It is found as a trace element in some uranium minerals and in tobacco smoke.

Properties

Electronic configuration and valence states

All chalcogens have the same electronic configuration: ns ² np ⁴. They therefore have six valence electrons. Being in block p, on the right side of the periodic table, they tend to gain electrons more than lose them; therefore, they gain two electrons to complete their valence octet, and consequently acquire a valence of -2.

Likewise, they can lose all six of their valence electrons, leaving them with a state of +6.

The possible valence states for chalcogens vary from -2 to +6, these two being the most common. As one moves down the group (from oxygen to polonium), the tendency of the elements to adopt positive valence states increases; which is equal to an increase in the metallic character.

Oxygen, for example, acquires a valence state of -2 in almost all its compounds, except when it forms bonds with fluorine, forcing it to lose electrons due to its higher electronegativity, adopting a valence state of +2 (OF ₂). Peroxides are also an example of compounds where oxygen has a valence of -1 and not -2.

Metallic and non-metallic character

When you go down the group, the atomic radii increase, and with them the chemical properties of the elements are modified. For example, oxygen is a gas, and thermodynamically it is more stable as a diatomic molecule O = O, than as an "oxygen chain" OOOO…

It is the element with the largest non-metallic character of the group and, therefore, forms covalent compounds with all the elements of the p block and with some transition metals.

The non-metallic character decreases as the metallic character increases. This is reflected in physical properties such as boiling and melting points, which increase from sulfur to polonium.

Another characteristic of the increase in metallic character is the increase in the crystalline configurations of the compounds formed by tellurium and polonium.

Compounds

Some compounds formed by chalcogens are mentioned in general below.

Hydrides

-H ₂ O

-H ₂ S

According to the IUPAC nomenclature, it is named as hydrogen sulfide, and not sulfur hydride; since H lacks a valence of -1.

-H ₂ Se

Likewise, it is named as hydrogen selenide, like the rest of the hydrides.

-H ₂ Te

-H ₂ Po

Oxygen hydride is water. The others are smelly and poisonous, with H ₂ S being the best known of all, even in popular culture.

Sulfides

All have in common the anion S ^2- (the simplest). Among them are:

-MgS

-FeS

-CuFeS ₂

-Na ₂ S

-BaS

Similarly, there are selenides, Se ^2-; telenuros, Te ^2-, and polonuros, Po ^2-.

Halides

Chalcogens can form compounds with halogens (F, Cl, Br, I). Some of them are:

-TeI ₂

-S ₂ F ₂

-OF ₂

-SCl ₂

-SF ₆

-SeBr ₄

Oxides

Finally there are the oxides. In them, oxygen has a valence of -2, and they can be ionic or covalent (or have characteristics of both). You have for example the following oxides:

-SO ₂

-TeO ₂

-Ag ₂ O

-Fe ₂ O ₃

-H ₂ O (hydrogen oxide)

-SeO ₃

There are hundreds of thousands of other compounds, which involve interesting solid structures. In addition, they can present polyanions or polycations, especially in the cases of sulfur and selenium, whose chains can acquire positive or negative charges and interact with other chemical species.

References

1. López A. (2019). Oxygen and its group (the oxygen family). Academy. Recovered from: academia.edu
2. Shiver & Atkins. (2008). Inorganic chemistry. In The elements of group 16. (Fourth edition). Mc Graw Hill.
3. Wikipedia. (2018). Chalcogen. Recovered from:
4. Catherine H. Banks. (2019). Chalcogens. Advameg. Recovered from: chemistryexplained.com
5. William B. Jensen. (1997). A Note on the Term «Chalcogen». Journal of Chemical Education, 74 (9), 1063. DOI: 10.1021 / ed074p1063.
6. Chemistry Libretexts. (May 16, 2017). The Elements of Group 16 (The Chalcogens). Recovered from: chem.libretexts.org.