What are the Chalcogens?
The chalcogens are the elements that belong to group 16 of the modern periodic table (or the oxygen family). The Chalcogens consist of five elements – oxygen, sulfur, selenium, tellurium, and polonium. It can be noted that the synthetic element livermorium (denoted by the symbol Lv) is also believed to belong to the chalcogen family. It can also be noted that it is not uncommon for oxygen to be excluded from the chalcogen family while generalizing their chemical properties since its chemical behaviour is quite different from the rest of the member elements.
All chalcogens have a total of 6 electrons in their respective valence shells. These elements are also known as ore-forming elements since a large number of metals are known to exist in the form of sulfides or oxides in the Earth’s crust. Many chalcogens are known to occur in different allotropes. For example, oxygen is known to have a total of 9 allotropes and sulfur is known to have over 20. However, it can be noted that only one allotrope of tellurium has been discovered so far.
How can the Chalcogens be Isolated?
- Oxygen can be isolated via the separation of air into oxygen and nitrogen.
- Sulfur can be extracted from natural gas and oil.
- The refining of copper is known to yield tellurium and selenium as by-products.
- Livermorium and polonium can be created with the help of particle accelerators.
General Electronic Configuration of the Chalcogens
The general electronic configuration of the chalcogens can be written as ‘ns2np4’, where ‘n’ denotes the value of the principal quantum number corresponding to the valence shell of the element. The electronic configurations of the chalcogens are tabulated below.
The electron configuration of the synthetic element livermorium (believed to be a chalcogen) is predicted to be [Rn]5f146d107s27p4.
Physical Properties of the Chalcogens
Atomic/Ionic Radii of the Chalcogens
The atomic radii or the ionic radii of elements increases while progressing down a group. The chalcogen with the lowest atomic radius and ionic radius is oxygen, whereas the chalcogen with the largest atomic/ionic radius (excluding livermorium) is polonium.
It can also be noted that the atomic radii of elements decrease across periods due the addition of protons and the increase in the effective nuclear charge. Therefore, the atomic radius of oxygen will be much smaller than that of lithium.
Ionization Enthalpies of the Chalcogens
As the size or the radius of the atom increases, the ionization enthalpy decreases (it is easier to remove an electron from an atom with a large atomic radius since the distance between the nucleus and the valence shell will be relatively large). Therefore, the ionization enthalpies of the chalcogens decrease while progressing down the group. Oxygen has the highest ionization enthalpy among the chalcogens.
It can also be noted that ionization enthalpy increases while progressing across a period (due to the increase in the effective nuclear charge across the period). Therefore, the ionization enthalpy of oxygen will be much higher than that of lithium.
Electron Gain Enthalpies of the Group 16 Elements
As the size of the atom increases, the electron gain enthalpy decreases. Therefore, the electron gain enthalpies of the chalcogens decrease down the group. It is important to note that oxygen has a less negative electron gain enthalpy when compared to sulfur, which can be explained by the compressed atomic structure of oxygen, which contributes to interelectronic repulsion between the valence electrons and any other approaching electron.
Electronegativities of the Chalcogens
The electronegativity decreases while progressing down a group due to many factors such as the increase in the atomic radius and the increase in the electron-electron repulsion. The most electronegative chalcogen is oxygen and the least electronegative chalcogen is polonium (livermorium not considered).
Metallic Nature of the Group 16 Elements
- Oxygen and sulfur are classified as non-metals.
- Selenium and tellurium are classified as metalloids.
- Under standard conditions, polonium exhibits metallic characteristics. However, it is important to note that polonium is a radioactive element.
Trends in the Melting and Boiling Points of the Chalcogens
Due to the increase in atomic sizes and atomic masses down a group, the melting and boiling points of the elements also increase while progressing down a group (as a result of increased intermolecular forces of attraction). Among the chalcogens, oxygen is known to have the lowest melting and boiling point.
The considerable difference in the melting and boiling points of sulfur and oxygen can be explained by the fact that oxygen exists in the atmosphere as a diatomic molecule whereas sulfur usually exists in the form a polyatomic molecule.
Chemical Properties of the Chalcogens
Allotropy Exhibited by Group 16 Elements
Almost all chalcogens have more than one allotrope. The most common allotropes of oxygen are dioxygen and ozone. In fact, oxygen has 9 known allotropes. Furthermore, sulfur is known to have over 20 known allotropes.
Selenium is known to have at least 5 different allotropes and polonium is known to have 2 allotropes. The two most stable allotropic forms of sulfur are monoclinic sulfur and rhombic sulfur. It can be noted that selenium and tellurium can exist in both crystalline and amorphous forms.
Oxidation States Exhibited by the Chalcogens
Since the general electronic configuration of the chalcogens is ‘ns2np4’, they can obtain a stable electronic configuration by gaining two electrons or participating in covalent bonding. When they gain 2 electrons, the general formula of the ion formed is M2- (where M denotes a chalcogen). The regular oxidation states shown by the chalcogens include -2, +2, +4, and +6.
Reactions Between Group 16 Elements and Hydrogen
When reacted with dihydrogen, the chalcogens usually form hydrides with the general formula H2M (where M denotes any chalcogen – oxygen, sulfur, selenium, tellurium, or polonium). The general format of this chemical reaction is:
M (chalcogen) + H2 (dihydrogen) → H2M (hydride of the chalcogen)
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