[Chemistry Class Notes] on Chalcogens Pdf for Exam

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

1. Oxygen and sulfur are non-metals.

2. Selenium and tellurium are metalloids.

3. 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)