10 julio, 2024

Chalcogens or anfigens: what they are, properties and compounds

What are chalcogens or anphigens?

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

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

Many chemists have named these elements as ash, chalk, bronze and chain formers. However, the most accurate interpretation corresponds to that of ‘formers of minerals’.

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 also makes up 49% of the earth’s crust. For this reason, it is not enough to look at the clouds to have it face to face, and to contemplate the maximum physical manifestation of chalcogens, it is necessary to visit a mountain or an ore.

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 not shown, below polonium is the second-heaviest radioactive synthetic element after oganeson: livermorium (Lv).

Oxygen

Oxygen occurs in nature mainly as two allotropes: O2, molecular or diatomic oxygen, and O3, ozone. It is a gas under terrestrial conditions and is obtained from the liquefaction of air. In the liquid state, it has pale bluish hues, and in the form of ozone it can form reddish-brown salts called ozonides.

Sulfur

It naturally presents twenty different allotropes, the most common of all being S8, «the sulfur crown». Sulfur is capable of forming with itself cyclic molecules or helical chains with SSS covalent bond… This is known as catenation.

Under normal conditions it is a yellow solid, whose reddish and greenish colors 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, S2, similar to molecular oxygen.

selenium and tellurium

Selenium forms shorter chains than sulfur, but with sufficient structural diversity to find allotropes of red, crystalline gray, 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 very low concentrations in rare minerals.

Polonium

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

Properties

Electronic configuration and valence states

All chalcogens have the same electronic configuration: ns2np4. Therefore, they have six valence electrons. Being in the p block, on the right side of the periodic table, they tend to gain electrons more than to lose them, therefore, they gain two electrons to complete their valence octet, and consequently acquire a valence of -2.

They can also lose their six valence electrons, leaving them with a +6 state.

The possible valence states for chalcogens range from -2 to +6, these two being the most common. As you go down the group (from oxygen to polonium), the tendency of the elements to assume positive valence states increases, which is equal to an increase in 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 greater electronegativity, adopting a valence state of +2 (OF2). Peroxides are also an example of compounds where oxygen has a valence of -1 and not -2.

Metallic and non-metallic character

As you move down the group, the atomic radii increase, and with them the chemical properties of the elements change. For example, oxygen is a gas, and thermodynamically it is more stable as a diatomic molecule O=O, than as an «oxygenated chain» OOOO…

It is the most nonmetallic element of the group and therefore forms covalent compounds with all p-block elements 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 the 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

-H₂O

-H2S

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

-H2Se

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

-H2Te

-H2Po

Oxygen hydride is water. The others are stinky and poisonous, with H2S being the best known of all, even in popular culture.

sulphides

They all have the S2- anion in common (the simplest). Among them are:

-MgS

-FeS

-CuFeS2

-Na2S

-BaS

Similarly, there are selenides, Se2-, telelides, Te2-, and polonides, Po2-.

halides

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

-TeI2

-S2F2

-OF2

-SCl2

-SF6

-SeBr4

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). For example, there are the following oxides:

-SO2

-TeO2

-Ag2O

-Fe2O3

-H₂O (hydrogen oxide)

-SeO3

There are hundreds of thousands of other compounds, all of 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

Chalcogens. Advameg. Retrieved from chemistryexplained.com.

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