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General physical properties of oxygen family elements

Property
Element
Oxygen
Sulphur
Selenium
Tellurium
Polonium
Atomic number
8
16
34
52
84
Atomic mass
16.00
32.06
78.96
127.61
210
Atomic radius (pm)
74
104
117
137
164
Oxidation states
2
2, 4, 6
2, 4, 6
2, 4, 6
2, 4
Ionic radius (pm) M2–
140
184
198
221
M2+
89
102
Ionization energy (kJ/mol)
1358
999
940
869
[MM+ + e]
3.5
2.5
2.4
2.1
Electronegativity
1.27
2.06
4.80
6.25
9.51
Density (kg/L)
55
392
490
723
527
Melting point (K)
90
718
958
1663
1235
Boiling point (K)
Electron affinity (kJ/mol)
–142
–200
–195
–190
….
[M + e M ]
700
332
230
….
….
[M + 2e M2–]
2s22p4
3s23p4
4s24p4
5s25p4
6s26p4
  1. Atomic and ionic radii: Atomic (covalent) radius increases as we go down the group.
     
    O < S < Se < Te < Po
     
    Ionic radius for dinegative ions (M2–) also increases from oxygen to polonium. The order is: O2– < S2–< Se2–< Te2–.
  2. Catenation: The self-linking property of atoms with identical atoms is called catenation. Oxygen shows a little tendency towards catenation, e.g., in peroxides, [–O–O–]2–.
     
    Sulphur shows a strong tendency towards catenation, e.g., in polysulphides Description: 51428.png, sulphanes (H – Sn – H), polysulphuric acid (HO3S SnSO3H), and in various allotropes.
  3. Density: Density of group VIB elements increases in going from oxygen to polonium, i.e., the density of group 16 elements follows the order: O < S < Se < Te < Po.
  4. Melting points and boiling points: The melting and boiling points show regular increase with the increase in atomic number. As we go down the group, the molecular size increases. As a result van der Wall forces increase, and hence the melting and boiling points also increase.
  5. Ionization energy or ionization enthalpy (ΔiH): The ionization energies of group VIB elements are quite high. Due to their high ionization energies, it is extremely difficult to remove electrons from the atoms of these elements.
     
    The ionization energy decreases as we go from oxygen to polonium.
     
    The first ionization energies of lighter elements of group VIB (oxygen family) are lower than those of group VB (nitrogen family).
  6. Oxidation states: The outer electronic configuration of group VIB elements can be described as ns2np1. Being strongly electronegative, these elements complete their outer shells by gaining two electrons. Thus, all the elements of group 16 shows an oxidation state of –2. However, these elements also show other oxidation states as follows. Oxygen shows an oxidation state of +2 in F2O, and –1 in peroxides (O22–). Other elements of group VIB exhibit oxidation states of +2, +4, and +6 also. The oxidation states of +4 and +6 being more stable.
     
    For sulphur, selenium, and tellurium, the oxidation states of +4 and +6 are important. The +4 state is more stable for Se, Te, and Po, than +6 state. This is due to the availability of d-orbitals in the valence shells of the atoms of these elements.

Allotropic forms of sulphur

Sulphur exists in several allotropic forms. The important ones are discussed here.
  1. Rhombic or octahedral or α-sulphur: This is the common form of sulphur. It is pale yellow in color. It melts at 114.5°C. Its specific gravity is 2.06. It is insoluble in water but readily soluble in CS2. It is a crystalline variety and consists of S8 structural units packed together into octahedral shape. This is the stable variety at ordinary temperature and all other forms gradually change into this form.
  2. Monoclinic or prismatic or β-sulphur: This form is formed by melting sulphur in a dish and cooling until a crust is formed. Two holes are made in the crust and liquid is poured out. On removing the crust, needle shaped crystals of monoclinic sulphur are obtained.
     
    This form of sulphur is stable above 95.6°C. Crystals melt at 119°C. It is also soluble in CS2. Below 95.6°C, it changes into rhombic form. Thus, 95.6°C is the transition temperature.
     
    Description: 51436.png

Compounds of oxygen family

  1. Hydrides: All these hydrides (H2M type) have angular structure. The central atom in these hydrides shows sp3 hybridization. Thus, there are two bond pairs and two lone pairs of electrons in the molecules of these hydrides. Due to stronger lone pair–bond pair repulsions, the H–M–H angles in hydrides are less than the tetrahedral angle of 109°28′. The bond angle in H2M hydrides of group VIB elements decreases as we go from oxygen to tellurium in the group.
     
  2. Physical state: Water has unusually high boiling point as compared to other hydrides of this group. The volatility of these hydrides follows the order:
     
    H2O < H2S > H2Se > H2Te.
     
    The existence of hydrogen bonds between water molecules is mainly responsible for its abnormally high melting and boiling points (or for its low volatility).
  3. Thermal stability: The thermal stability of hydrides of group 16 elements decreases with the increase in the size of the central atom.
     
    H2O > H2S > H2Se > H2Te
  4. Acidic nature: The hydrides of group VIB elements are weakly acidic. The acidic character of these hydrides increases with increasing atomic number. Thus, the acid strength of these hydrides increases as we move from O to Te because of the increase in the distance between central atom and hydrogen, which favor the release of hydrogen as proton.
  5. Reducing character: All hydrides of group VIB elements, except H2O, are reducing agents. The reducing power of these hydrides increases in going from H2S to H2Te, which may be due to increase in the size of the central atom and hence decrease in the M–H bond energy.

Oxoacids of sulphur

The oxoacids of sulphur are more numerous and more important than those of Se and Te. Many of the oxoacids of sulphur do not exist as free acids, but are known as anions and salts. Acids ending in -ous have S in the oxidation state (+IV) and form salts ending in -ite. Acids ending in -ic have S in the oxidation state (+VI) and form salts ending in -ate.

Sulphurous acid series

H2SO3 sulphorous acid
Description: 52297.png
S(IV)
H2S2O5 di- or pyrosulphurous acid
Description: 51446.png
S(V)
H2S2O4 dithionous acid
Description: 51455.png
S(III)

Sulphuric acid series

H2SO4 sulphuric acid
Description: 51466.png
S(VI)
H2S2O3 thiosulphuric acid
Description: 51476.png
S(VI), S(II)
H2S2O7 di- or pyrosulphuric acid
Description: 51483.png
S(VI)

Thionic acid series

H2S2O6 dithionic acid
Description: 51490.png
S(V)
H2SnO6 polythionic acid (n = 1–12)
Description: 51500.png
S(V), S(0)

Peroxoacid series

H2SO5 peroxomonosulphuric acid
Description: 51509.png
S(VI)
H2S2O8 peroxodisulphuric acid
Description: 51519.png
S(VI)




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