Physical Characteristics Group 16 Elements

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The important physical properties of the elements of group 16 are recorded as shown below:

Some physical properties of the elements of group 16

Property	Oxygen	Sulphur	Selenium	Tellurium	Polonium
Atomic radius (Ao)	0.73	1.09	1.16	1.35	-
Ionic (M2+) radius (Ao)	1.40	1.85	1.98	2.21	-
Ionization energy (kJ mol-1)	1314	1000	941	869	-
Electronegativity	3.5	2.5	2.4	2.1	2.0
Electron affinity (k.J mol-1)	141.4	208.8	195.5	190.0	-
Melting point (K)	54	392	490	723	527
Boiling Point (K)	90	718	958	1263	1235
Oxidation state	- 2	- 2, + 2 + 4, + 6	- 2, + 2 + 4,+6	- 2, + 2 + 4,+6	- 2, + 4
Density (g cm-3) (in solid state)	1.14	2.07	4.79	6.25	9.4

The general trends in characteristic properties are discussed below:

1. Atomic and ionic radii

The atomic and ionic radii of the elements of this group increase on going down the group. This is due to the increase in the number of electron shells.

2. Ionization energies

The ionization energies of the elements of oxygen family are less than those of nitrogen family. As we move down the group from oxygen to polonium, the ionization energy decreases.

Explanation:

We expect that the ionization energy of oxygen should be more than that of N because of decrease in size. However, oxygen has unexpectedly low ionization energy than N. This is due to the reason that nitrogen has completely half filled orbitals and the configuration is stable because half filled and completely filled configurations have extra stability. But the configuration of O is less stable and therefore, has less ionization energy.

As one moves down a group there is increase in nuclear charge. But at the same time the atomic size as well as the number of inner electrons, which shield the valence electrons from the nucleus increase. The overall effect of increase in atomic size and the shielding effect is much more than effect of increase in nuclear charge. Consequently, the outermost electron is less and less tightly held by the nucleus as we move down the group and hence ionization energy decreases.

3. Melting and boiling points

The melting and boiling points increase with the increase in atomic number as we go down the group.

Explanation:

When we move down the group, the molecular size increases. As a result, the magnitude of the van der Waals forces also increases with increase in atomic number and therefore melting point also increases. The melting point of polonium is, however, small.

4. Electronegativity

Oxygen is the second most electronegative element, the first being fluorine. The electronegativity decreases on going down the group. This is due to increase in size of the atoms.

5. Metallic and non-metallic character

The first four elements namely oxygen, sulphur, selenium and tellurium are non-metals. The non-metallic character is stronger in O and S and weaker in Se and Te. On the other hand, last element is markedly metallic. However, it is radioactive and is only short-lived. .

6. Electron affinity

The elements of this family have high electron affinities. The values decrease down the group from sulphur to polonium. Oxygen unexpectedly has low electron affinity. This is attributed to the small size of oxygen atom so that its electron cloud is distributed over a small region of space and therefore, it repels the incoming electron. Thus, the electron affinity of oxygen is unexpectedly less in the family.

7. Catenation

Catenation is the tendency of an atom to form bonds with identical atoms. In this group only sulphur has a strong tendency for catenation. Oxygen also shows this tendency to a limited extent. Thus the polyoxides and polysulphides of the following types are known:

H₂O₂, H - O - O - H

Polyoxides

H₂S₂, H-S-S-H

H₂S₃, H-S-S-S-H

H₂S, H-S-S-S-S-H

Polysulphides

8. Elemental state

Oxygen exists as diatomic molecule. Under normal conditions oxygen exists as a gas. In oxygen molecule there is pp-pp overlap between two oxygen atoms forming double bond, O = O. The intermolecular forces in oxygen are weak van der Waals forces and therefore, oxygen exists as a gas. On the other hand the other elements of family do not form stable pp-pp bonds and do not exist as M₂ molecules. On the other hand the other atoms are linked by single bonds and form polyatomic complex molecules. For e.g., sulphur and selenium molecules have eight atoms per molecule (S₈ and Se₈) and have puckered ring structure. The puckered ring structure of S is as shown below.

The Puckered ring structure of S₈ molecule

9. Allotropy

All the elements of the group exhibit allotropy. For e.g., oxygen exists as O₂ and O₃ (ozone). Sulphur exists in a number of allotropic forms such as rhombic, monoclinic, plastic sulphur. All these allotropic forms of sulphur are non-metallic. Selenium has two common forms-red and grey. Similarly tellurium and polonium occur in allotropic forms.