Group 16 Elements - Oxygen Family


 Oxygen, sulphur, selenium, tellurium, polonium and livermorium constitute Group 16 of the periodic table. This is sometimes known as group of chalcogens. The name is derived from the Greek word for brass and points to the association of sulphur and its congeners with copper. Most copper minerals contain either oxygen or sulphur and frequently the other members of the group.


Occurrence of group 16 elements     

Oxygen is the most abundant of all the elements on earth. Oxygen forms about 46.6% by mass of earth’s crust.

Dry air contains 20.946% oxygen by volume.

However, the abundance of sulphur in the earth’s crust is only 0.03-0.1%. Combined sulphur exists primarily as sulphates such as gypsum CaSO4.2H2O, epsom salt MgSO4.7H2O, baryte BaSO4 and sulphides such as galena PbS, zinc blende ZnS, copper pyrites CuFeS2. Traces of sulphur occur as hydrogen sulphide in volcanoes. Organic materials such as eggs, proteins, garlic, onion, mustard, hair and wool contain sulphur.


Selenium and tellurium are also found as metal selenides and tellurides in sulphide ores.


Polonium occurs in nature as a decay product of thorium and uranium minerals.


Livermorium is a synthetic radioactive element. Its symbol is Lv, atomic number 116, atomic mass 292 and electronic configuration [Rn] 5f 146d107s27p4. It has been produced only in a very small amount and has very short half-life (only a small fraction of one second). This limits the study of properlies of Lv.


Electronic Configuration of group 16 elements    

The elements of Group16 have six electrons in the outermost shell and have ns2np4general electronic configuration.


Atomic and Ionic Radii of group 16 elements        

Due to increase in the number of shells, atomic and ionic radii increase from top to bottom in the group. The size of oxygen atom is, however, exceptionally small.


Ionisation enthalpy of group 16 elements  

1.     Ionisation enthalpy of elements of group 16 is lower than group 15 due to half-filled p-orbitals in group 15 which is more stable.

2.     However, ionization enthalpy decreases down the group.


Electronegativity of group 16 elements      

Next to fluorine, oxygen has the highest electronegativity value amongst the elements. Within the group, electronegativity decreases with an increase in atomic number. This implies that the metallic character increases from oxygen to polonium.


Physical Properties of group 16 elements  

 Some of the physical properties of Group 16 elements are given in Table 7.6. Oxygen and sulphur are non-metals, selenium and tellurium metalloids, whereas polonium is a metal. Polonium is radioactive and is short lived (Half-life 13.8 days). All these elements exhibit allotropy. The melting and boiling points increase with an increase in atomic number down the group. The large difference between the melting and boiling points of oxygen and sulphur may be explained on the basis of their atomicity; oxygen exists as diatomic molecule (O2) whereas sulphur exists as polyatomic molecule (S8).

Chemical Properties of group 16 elements

Oxidation states and trends in chemical reactivity of group 16 elements  

 The elements of Group 16 exhibit a number of oxidation states (Table 7.6). The stability of -2 oxidation state decreases down the group. Polonium hardly shows –2 oxidation state. Since electronegativity of oxygen is very high, it shows only negative oxidation state as –2 except in the case of OF2 where its oxidation state is + 2. Other elements of the group exhibit + 2, + 4, + 6 oxidation states but + 4 and + 6 are more common. Sulphur, selenium and tellurium usually show + 4 oxidation state in their compounds with oxygen and + 6 with fluorine. The stability of + 6 oxidation state decreases down the group and stability of + 4 oxidation state increases (inert pair effect). Bonding in +4 and +6 oxidation states is primarily covalent.


 Anomalous properties of oxygen      

 The anomalous behaviour of oxygen, like other members of p-block present in second period is due to its small size and high electronegativity. One typical example of effects of small size and high electronegativity is the presence of strong hydrogen bonding in H2O which is not found in H2S.

The absence of d orbitals in oxygen limits its covalency to four and in practice, rarely exceeds two. On the other hand, in case of other elements of the group, the valence shells can be expanded and covalence exceeds four.


Reactivity of group 16 elements towards hydrogen       

1.     All group 16 elements form hydrides.

2.     They possess bent shape.

3.     Bond angle:

·         Acidic nature:
This is because the H-E bond length increases down the group. Therefore, the bond dissociation enthalpy decreases down the group.

·         Thermal stability:
This is because the H-E bond length increases down the group. Therefore, the bond dissociation enthalpy decreases down the group.

·         Reducing character:

This is because the H-E bond length increases down the group. Therefore, the bond dissociation enthalpy decreases down the group.


Reactivity of group 16 elements towards oxygen

1.     Reducing character of dioxides decreases down the group because oxygen has a strong positive field which attracts the hydroxyl group and removal of becomes easy.

2.     Acidity also decreases down the group.

3.     is a gas whereas SeO2 is solid. This is because  has a chain polymeric structure whereas SO2 forms discrete units.


Reactivity of group 16 elements towards halogens        

1.     The stability of halides decreases in the order

2.     This is because E-X bond length increases with increase in size.

3.     Among hexa halides, fluorides are the most stable because of steric reasons.

4.     Dihalides are  hybridised and so, are tetrahedral in shape.

5.     Hexafluorides are only stable halides which are gaseous and have  hybridisation and octahedral structure.

6.      is a liquid while H2S is a gas. This is because strong hydrogen bonding is present in water. This is due to small size and high electronegativity of O.