What are Chalcogens?
Chalcogens are the elements belonging to the group 16 according to the modern periodic table. There are five different chalcogen elements, which are, oxygen, sulfur, selenium, tellurium, and polonium. Livermorium, denoted by Lv, is a synthetic element which is believed to belong to the chalcogen group. However, it is not an uncommon situation wherein oxygen is excluded when it comes to the chalcogen family. The oxygen family characteristics are quite different from the other elements of the groups.
All the elements of the chalcogen family have 6 valence shell electrons. They are also referred to as ore-forming elements because a huge of metals exist in the oxides or sulphides form in the Earth’s crust. Many chalcogens from the chalcogen group also tend to occur as different allotropes, for example, oxygen consists of 9 allotropes and on the other hand, sulphur has more than 20. However, tellurium has just one allotrope known so far. In this article, we will learn about what are chalcogens, and their physical and chemical properties.
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Electron Affinity Order of Chalcogens
The general electronic configuration of the chalcogens elements is written as ns²np⁴, wherein, ‘n’ refers to the value of the principal quantum number which corresponds to the valence shell of an element. The electronic configurations of chalcogens are shown below.
The electron configuration of livermorium, which is believed to be a chalcogen element, is predicted as [Rn]5f¹⁴6d¹⁰7s².
Physical Properties of Chalcogens
Atomic/Ionic Radii of the Chalcogens
The atomic radii also called as the ionic radii of elements tend to increase when it progresses down a group. The chalcogen having the lowest atomic radius is oxygen, whereas the chalcogen having the largest atomic radius is polonium (excluding livermorium).
The atomic radii of elements tend to decrease across periods because of the addition of protons and the increasing effective nuclear charge. Hence, the atomic radius of oxygen is much smaller when compared to lithium.
Ionization Enthalpies of Chalcogens
The ionisation enthalpy decreases as the radius or the size of the atom increases. Hence, the ionisation enthalpies of the chalcogens tend to decrease when progressing down in the group. Amongst all the chalcogen elements, oxygen is known to have the highest ionization enthalpy.
The ionisation enthalpy tends to increase when progressing across the group since there is an increase in the effective nuclear charge as we progress across the period. Hence, oxygen has a higher ionisation enthalpy than lithium.
Electron Gain Enthalpies of Group 16 Elements
As the size of the atom tends to increase, the electron gain enthalpy tends to decrease. Hence, the electron gain enthalpies of chalcogens tend to decrease while progressing down the group. However, oxygen has a lesser negative electron gain enthalpy than sulfur, which is explained by the compressed atomic structure that oxygen has, which tends to contribute to the interelectronic repulsion that occurs between the valence electrons and other approaching electrons.
Electronegativities of Chalcogens
The electronegativity tends to decrease when progressing down the group because of several factors like the increase in the electron-electron repulsion and the increase in the atomic radius. The most electronegative known chalcogen element is oxygen and the least electronegative known chalcogen element is polonium when we do not consider livermorium.
Metallic Nature of the Group 16 Elements
Oxygen and sulfur are non-metals.
Selenium and tellurium are metalloids.
Under the standard conditions, polonium tends to exhibit metallic characteristics. However, polonium is a radioactive element.
Chemical Properties of the Chalcogens
Allotropy Exhibited by Group 16 Elements
Almost all of the chalcogens have more than one allotrope. The most common allotropes of oxygen are known as dioxygen and ozone. Oxygen has 9 different allotropes that are known. Furthermore, sulfur has more than 20 known allotropes.
Selenium has at least 5 different allotropes and polonium has 2 allotropes. The two most stable allotropes of sulfur are known as monoclinic sulfur and rhombic sulfur. Selenium and tellurium tend to exist in both the crystalline and amorphous forms.
Reactions Between Group 16 Elements and Hydrogen
Chalcogens tend to form hydrides when they react with dihydrogen and has a generic formula H₂M, wherein, M refers to any given chalcogen element. The generic format of the chemical reaction is as follows:
M (chalcogen) + H₂ (dihydrogen) → H₂M (hydride of chalcogen)
FAQs on Chalcogens
1. Which elements belong to the chalcogen family (Group 16)?
The chalcogen family, or Group 16 of the periodic table, consists of the following elements: Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te), and the radioactive element Polonium (Po). The synthetically created radioactive element Livermorium (Lv) is also a member of this group.
2. Why is Group 16 called the chalcogens?
The name "chalcogen" is derived from the Greek words "khalkos" (meaning copper or ore) and "gen" (meaning born or former). This translates to "ore-forming" because the most common ores of many metals, particularly copper, are oxides or sulphides. For example, many important metal ores like cuprite (Cu₂O) and chalcopyrite (CuFeS₂) are compounds of chalcogens.
3. What are the common oxidation states exhibited by chalcogens and why?
Chalcogens have a general valence shell electronic configuration of ns²np⁴. To achieve a stable octet, they most commonly exhibit a -2 oxidation state by gaining two electrons. However, as we move down the group from sulphur, they also show positive oxidation states like +2, +4, and +6. This is because the larger elements have accessible d-orbitals which allow them to expand their octet. Oxygen, being highly electronegative and lacking d-orbitals, primarily shows a -2 state.
4. How do the melting and boiling points of chalcogens change down the group?
The melting and boiling points of chalcogens generally increase down the group from oxygen to tellurium. This is due to the increase in atomic size and mass, which leads to stronger van der Waals forces between the atoms. There is a significant jump between oxygen and sulphur because oxygen exists as a discrete diatomic molecule (O₂), while sulphur exists as a larger, polyatomic molecule (S₈), requiring more energy to overcome intermolecular attractions.
5. Are chalcogens considered transition elements or representative elements?
Chalcogens are representative elements, also known as main-group elements. Representative elements are those in the s-block and p-block of the periodic table. Chalcogens belong to Group 16, which is part of the p-block. Transition elements are located in the d-block (Groups 3-12) of the periodic table.
6. How do chalcogens differ from halogens?
Chalcogens (Group 16) and halogens (Group 17) are neighbouring non-metal groups with several key differences:
- Valence Electrons: Chalcogens have 6 valence electrons (ns²np⁴), while halogens have 7 (ns²np⁵).
- Common Oxidation State: The most common oxidation state for chalcogens is -2, whereas for halogens it is -1.
- Valency: Chalcogens are typically divalent (form two covalent bonds), while halogens are typically monovalent (form one covalent bond).
- Reactivity: Halogens are generally more reactive and more electronegative than the chalcogens found in the same period.
7. Why does oxygen exist as a diatomic gas (O₂) while sulphur exists as a polyatomic solid (S₈)?
This significant difference arises from the ability of oxygen to form strong pπ-pπ multiple bonds. Oxygen's small atomic size allows it to form a stable double bond with another oxygen atom, creating the O=O molecule. In contrast, the larger sulphur atom cannot form effective pπ-pπ bonds. Instead, it achieves stability by forming two strong S-S single bonds and catenating (linking to itself), resulting in a stable, puckered eight-membered ring (S₈) structure, which is a solid at room temperature.
8. How does the acidic character of hydrides change down the chalcogen group?
The acidic character of the hydrides of chalcogens (H₂O, H₂S, H₂Se, H₂Te) increases down the group. While water (H₂O) is neutral, H₂S is a weak acid, and H₂Se and H₂Te are progressively stronger acids. This trend occurs because as the size of the central chalcogen atom increases, the strength of the H-E bond (where E is the chalcogen) decreases. A weaker bond is easier to break, allowing the hydride to donate a proton (H⁺) more readily in solution.
9. What are some important real-world applications of chalcogens?
Chalcogens and their compounds are vital in many areas. For example:
- Oxygen is essential for biological respiration and industrial combustion processes.
- Sulphur is used to manufacture sulphuric acid, a crucial chemical for producing fertilisers, detergents, and batteries.
- Selenium is a semiconductor used in photocopiers and solar cells, and is also an essential micronutrient.
- Tellurium is added to alloys to improve the machinability of steel and copper.
