Group 16 Elements: The Chalcogens

The elements of Group 16, which include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po), have the general electronic configuration ns²np⁴ and are referred to as the oxygen family. These elements are also collectively known as chalcogens. Notably, polonium is a radioactive element.

1.0What Are Group 16 Elements?

Group 16 consists of the following elements:

• Oxygen (O)
• Sulfur (S)
• Selenium (Se)
• Tellurium (Te)
• Polonium (Po)

The first four elements of the group—oxygen, sulfur, selenium, and tellurium—are known as chalcogens, which means "ore-forming elements." This name arises because many metals in the Earth's crust are oxides and sulfides, such as Cu₂O, CuO, Ag₂S, ZnS, and FeS. Metals can also occur as selenides and tellurides.

2.0Occurrence of Group 16 Elements

• Oxygen: Oxygen is the most abundant element on Earth, constituting about 46.6% of the Earth's crust by mass. In dry air, oxygen makes up 21.0% by volume.
• Sulphur (S)Sulphur, symbolized as S, is the ninth most abundant element in the cosmos. It is found both in combined and free states. In the combined state, sulphur is present as sulfates in seawater, making up about 0.09% of its composition. In its free state, sulphur can be found in underground deposits, often in dome-like structures formed by the action of anaerobic bacteria on sulfate minerals such as gypsum. Meteorites also contain about 12% sulphur, highlighting its cosmic significance. Sulphur allotropes exhibit distinct physical properties and stability ranges, making sulphur a versatile element with various applications.
• Selenium (Se)Selenium, represented by the symbol Se, is rarer than oxygen and sulphur. It is found in a few minerals and often in combination with heavy metals such as lead, silver, or mercury. The most stable form of selenium under normal conditions is its grey metallic form, which is the predominant form in typical settings.
• Tellurium (Te)Tellurium, with the atomic number 52, exhibits intermediate properties between metals and nonmetals. It is one of the rarest stable elements in the earth’s crust. Tellurium can be found in its pure form (free state) and compounds with other elements like copper, lead, silver, and gold. Its rarity and unique properties make it significant in various scientific and industrial applications.
• Polonium (Po)Polonium, symbolised as Po, is the rarest element in group sixteen. It is a radioactive element known for its applications in alpha radiation. Due to its radioactive nature, polonium is occasionally used in scientific research and technological applications that require a source of alpha particles. Its extreme rarity and radioactivity make it a notable element within its group.

3.0General Properties of Oxygen Family 

4.0Chemical Properties of Oxygen Family 

Hydrides (H₂E, E = O, S, Se, Te, Po):

• Acidic character increases from H₂O to H₂Te due to decreasing H-E bond enthalpy.
• Thermal stability decreases from H₂O to H₂Po.
• All hydrides, except H₂O, have reducing properties, increasing from H₂S to H₂Te.

Reactivity with Oxygen:

• Form oxides of EO₂ and EO₃ (E = S, Se, Te, Po).
• SO₂ and O₃ are gases; SeO₂ is a solid.
• The reducing property decreases from SO₂ (reducing) to TeO₂ (oxidizing).
• Both EO₂ and  EO₃ oxides are acidic.

Reactivity with Halogens:

• Form halides EX₆, EX₄, and EX₂ (X = halogen).
• Stability order: F ^⁻ > Cl^⁻ > Br^⁻ > I^⁻.
• Hexafluorides (EX₆) are stable and gaseous with octahedral structures (e.g., SF₆).
• Tetrafluorides (EX₄) may vary:  SF₄ (gas), SeF₄ (liquid), and TeF₄ (solid), with see-saw geometry.
• Dichlorides and dibromides (EX₂) have tetrahedral structures.
• Known monohalides (e.g., S₂F₂, S₂Cl₂) are dimeric and undergo disproportionation.

5.0Anomalous Behaviour of Oxygen 

Oxygen's small size and high electronegativity significantly influence its chemical behaviour. This leads to strong hydrogen bonding and limits its covalency due to the lack of d orbitals. In contrast, heavier Group 16 elements can bond with their d orbitals, allowing for greater covalency and a broader range of chemical compounds.

Small Size and High Electronegativity:

• Oxygen's small atomic radius leads to a high charge density. This high charge density results in stronger attractions between the nucleus and bonding electrons, contributing to oxygen's high electronegativity.
• In water (H₂O), oxygen's high electronegativity leads to strong hydrogen bonding between molecules, which accounts for its high boiling and melting points, high specific heat capacity, and excellent solvent properties.

Hydrogen Bonding:

• Hydrogen bonding in H₂O creates a network structure in liquid water, leading to unique properties such as a higher boiling point and strong intermolecular forces. In contrast, the lower electronegativity of sulfur in H₂S results in weaker attractions and the absence of such a network, giving H₂S a lower boiling point and weaker intermolecular forces.

Absence of d Orbitals:

• As a second-period element, oxygen lacks d orbitals, limiting it to typically forming two covalent bonds, as seen in H₂O and O₂. Although it can reach a maximum covalency of four in rare cases, such as in [O₂F₂]⁺, it usually adheres to two covalency in most stable compounds.

Covalency in Heavier Group 16 Elements:

• Sulfur, selenium, and tellurium have accessible d orbitals, allowing them to expand their valence shells and form higher covalencies. For example, sulfur can form up to six bonds in SF₆. This capability enables these elements to exhibit a wider range of oxidation states and forms more complex compounds, such as SO₂, SF₄, and SF₆.

Dioxygen

Dioxygen (O₂), known as molecular oxygen or oxygen gas, is the most common allotrope of oxygen.

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Simple Oxides

A binary compound of oxygen with another element is called an oxide. Oxygen reacts with most elements in the periodic table to form simple oxides, and one element can form multiple oxides. These oxides vary widely in nature and properties.

Ozone: An Allotropic Form of Oxygen

Ozone is an allotropic form of oxygen. Its molecular formula is O₃, and its molar mass is 48 g mol^-1.

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6.0Sulfur 

Symbol: S

Atomic Number: 16

Sulfur is primarily extracted using the Frasch process, which utilises superheated water and compressed air to bring liquid sulfur to the surface. Additionally, sulfur can be obtained by reducing hydrogen sulfide (H₂S) from oil and natural gas. Most sulfur produces sulfur dioxide (SO₂) and sulfuric acid (H₂SO₄). Pure sulfur is tasteless, odourless, and light yellow, though common laboratory samples often have a noticeable odour. It is the tenth most abundant element in the known universe.

7.0Selenium 

Symbol: Se

Atomic Number: 34

Selenium is most stable in its grey metallic form, but it also occurs in other allotropes, including red amorphous powder and red crystalline forms. Chemically, selenium is less reactive than sulfur but shares similar compound formation tendencies. It commonly exhibits oxidation states of -2, +4, and +6. Selenium forms selenides, similar to sulfides, as well as selenates and selenites, comparable to sulfates and sulfites. Notably, hydrogen selenide (H₂Se) is a toxic gas with a foul odour, while selenium dioxide (SeO₂) is used as a catalyst and in organic synthesis. It is used in metallurgy as an alloy with copper, lead, and iron and in technology for manufacturing solar panels and memory chips.

8.0Tellurium 

Symbol: Te

Atomic Number: 52

Tellurium is a metalloid in the oxygen family, characterised by its silvery-white colour and metallic lustre, similar to tin at room temperature. Like selenium, tellurium exhibits photoconductivity. This element is scarce and is most commonly found as a telluride of gold. It is frequently used in metallurgy, often in combination with copper, lead, and iron. Additionally, tellurium finds applications in solar panels and memory chips for computers. While it is not toxic or carcinogenic, excessive exposure to tellurium can cause a garlic-like odour on a person's breath.

9.0Polonium 

Symbol: Po

Atomic Number: 84

Polonium is highly radioactive and found in trace amounts in uranium and thorium ores. Common compounds of polonium include polonium chloride (PoCl₂) and polonium hydride (PoH₂). Due to its significant radioactivity, polonium is used primarily as an alpha radiation source in scientific applications and antistatic devices. However, it is highly toxic and poses serious health risks if ingested or inhaled. Discovered by Marie Curie in 1898, polonium was named in honour of Poland.