The elements oxygen, sulphur, selenium, tellurium and polonium constitute the 16th
vertical column or VIA group elements in the long form of periodic table.
The first four elements of the group are collectively called chalcogens or ore-forming elements
, because a large number of metal ores are found in the earth’s crust as oxides or sulphides.
Oxygen is the most abundant element that is available in nature. It forms 20.946% of air by volume and 46.6% of the earth’s mass mostly in the form of silicates and other compounds like carbonates, sulphates and oxides.
Most of the oxygen in the atmosphere is produced by photosynthesis in plants. It also occurs in the form of ozone. Sulphur is the 16th
most abundant element. Sulphur in its combined form is found in ores.
Include gypsum, CaSO4.
O, Epsom salt MgSO4
O and barytes, BaSO4
Include zinc blende (ZnS), galena (PbS) and copper pyrites (CuFeS2
Sulphur is also present in organic substances like mustard seeds, eggs, onion, garlic, wool and hair.
Selenium and tellurium are present in sulphides ores as metal selenides and tellurides.
Polonium is a radioactive element.
All the elements of this group have 6 electrons in their outer shell and have the general electronic configuration of ns2np4.
Atomic and physical properties and their trends:
Atomic and ionic radii:
||[He] 2s2 2p4
||[Ne] 3s2 3p4
||[Ar] 3d10 4s2 4p4
||[Kr] 4d10 5s2 5p4
||[Xe] 4f14 5d10 6s2 64
The atomic and ionic radius increases from oxygen to polonium.
Down a group ionization enthalpy decreases with increase in size of the atom.
Electron gain enthalpy:
Down the group electron gain enthalpy decreases with increase in size of the atom. Oxygen atom has a less negative electron gain enthalpy than sulphur. This is because, oxygen, due to its compact nature experience more repulsion between the electrons already present and the incoming electron. Due to these electron-electron repulsions, oxygen atom has lesser tendency than sulphur atom to accept the extra electron.
Trend in electronegativity:
Down the group from oxygen to polonium, with increase in atomic size there is a gradual decrease in electronegativity. Oxygen is the second most electronegative atom after fluorine.
Physical properties of the group 16 elements:
‘Oxygen’ and ‘sulphur’ are non-metals, ‘selenium’ and ‘tellurium’ are metalloids and ‘polonium’ is a metal under normal conditions. As polonium is a radio-active element.
All the elements of group 16 exhibit allotropy.
Oxygen - 2 allotropes.
Sulphur has many allotropic forms only two of them are stable.
1) Rhombic sulphur
2) Monoclinic sulphur
Selenium and tellurium exist in both amorphous and crystalline forms.
The melting and boiling points
The melting and boiling points increases from oxygen to tellurium as the atomic size increases.
The large difference between the melting and boiling points of oxygen and sulphur may be explained on the basis that oxygen exists as a diatomic molecule while sulphur exists as polyatomic molecule.