The elements oxygen, sulphur, selenium, tellurium and polonium constitute group 16 elements of the periodic table. These are named as oxygen group elements after the name of the first member of the group. The oxygen group number is 16. The first four elements (oxygen, sulphur, selenium, and tellurium) of the 16th group are known as chalcogens. This is because many naturally occurring metal ores occur as oxides and sulphides. Group 16 elements are called chalcogens.
Group 16 Elements
Oxygen is the most abundant of all elements. It occurs in the free form as an oxygen molecule and makes up 20.946% of the volume of the atmosphere. Most of it has been produced by photosynthesis. Oxygen makes up the major component of the earth crust. It occurs in the silicates mineral. It also occurs as metal oxide ores and deposits of oxo salts such as carbonates, sulphates, nitrates, and borates. As water, it comprises 89% by weight of the oceans. Oxygen occurs as ozone, an allotrope of oxygen in the upper atmosphere and is of great importance.
Sulphur- Sulphur is the non-metallic element and it is the sixteenth most abundant element found on the earth crust. It constitutes 0.03-0.1% by the mass of the earth’s crust. It occurs in the combined form of sulphides ores and sulphate ores. The other elements are comparatively rare.
Selenium and tellurium are more electronegative than metals. Therefore, they occur as metal selenides and tellurides in the naturally occurring sulphide ores. Selenium and tellurium are found in anode mud or the anode slime deposited during the electrolytic refining of copper.
General Characteristics of the Oxygen Group Elements
1. Electronic Configuration
The elements of the oxygen family have six electrons in the outermost shell and have the general electronic configuration s ns2 np4. The electronic configurations of the oxygen family are given below:
2. Atomic and Ionic Radii
The atomic and ionic radii of the elements of the oxygen periodic table group is smaller than that of the corresponding elements of group 15. The atomic radii and ionic radii of the group 16th elements is expected to increase on moving down the group. The comparatively smaller atomic and ionic radii of the oxygen group elements compared to group 15 elements are due to the increased effective nuclear charge of group 16 elements. Due to this, there is a greater attraction of the electrons by the nucleus and hence radii are less. The increase in the atomic radii of the 16th group elements, moving down the group is due to the increment in the number of electronic shells.
3. Ionisation Enthalpies
The ionisation enthalpies of the elements of the oxygen family are less than those of the nitrogen family. On moving down the group from oxygen element to polonium element, the ionisation enthalpy or potential decreases.
The general trend of the ionization enthalpy is, it increases when we move from left to right. But when we move from the nitrogen group to the oxygen group the ionization enthalpy decreases. This happens because the nitrogen completely half-filled p-orbitals. This half-filled configuration is stable because a half-filled and filled configuration has extra stability due to more exchange energy. But the configuration of oxygen is less stable and therefore, has less ionisation enthalpy. However, it may be noted that the second ionization enthalpies (IE2) of the members of group 16 are higher than those of group 15. This is because, after the removal of the first electron, the second electron has to be removed from a more symmetrical half-filled configuration, which is more stable.
The elements of the 16th group (chalcogen group) have higher values of electronegativity than the corresponding group 15 elements. The electronegativity decreases on going down the group. The decrease in electronegativity down the group is due to an increase in the size of the atoms.
5. Electron Gain Enthalpy
The oxygen family elements have high negative electron gain enthalpies. The value decreases down the group from sulphur to polonium. Oxygen exceptionally has low negative electron gain enthalpy. This characteristic is attributed to the small size of the oxygen atom so that its electron cloud is distributed over a small region of space and therefore, it repels the incoming electrons.
Did You Know?
Oxygen is the second most electronegative element, the first being fluorine.
Only sulphur is an element in the oxygen group elements that shows catenation property.
All the group 16 elements exhibit allotropy.