B.sc 1st year Book
(Page 14)
Group VIA (16) Elements: O, S, Se, Te, Po Oxygen family
The VIA group of the periodic table consist of five elements viz. oxygen, sulphur, selenium Oxygen family, tellurium and polonium. Oxygen and sulphur are typical elements of this group. Except, for polonium, all elements of this group i.e. from oxygen to selenium are collectively called as ‘CHALCOGEN’.
The name chalcogen is derived from the Greek words (Chalkos = ore) + (gonnan = to form) which means ore-forming and applies particularly to the first two elements of group VI A because oxides and sulphides are major components of ores. Polonium is a radioactive element and derives its name from Poland, the home country of Marie Curie who discovered this element in 1898.
1- Electronic configuration of Oxygen family :
The valence shell (outermost shell) electronic configuration of the group VIA elements is ns2np4 with each element requiring two electrons more to attain the next stable noble gas configuration. The electronic configuration of group VIA elements are given in table (4.12) shown below:
Table 4.12: The electronic configuration of group VIA elements
| Element |
Symbol |
Atomic No. |
Electronic configuration with an inert gas core |
| Oxygen |
0 |
8 |
[He]2s2,2p4 |
| Sulphur |
S |
16 |
[Ne]3 s2,3p4 |
| Selenium |
Se |
34 |
[Ar]3 d10,4 s2,4p4 |
| Tellurium |
Te |
52 |
[Kr]4 d10,5 s2,5p4 |
| Polonium |
Po |
B4 |
[Xe]4f14,5 d10,6 s2,6p4 |
2- Atomic and ionic size of Oxygen Family:
As expected, atomic and ionic size increases with an increase in atomic number. Thus, this property increases as we move down the group from O to Te as shown in table (4.12).
3- Molecular structure (atomicity) and physical state of Oxygen Family:
Oxygen may exist as diatomic (O2) or triatomic (O3) molecules and are therefore a gas. While other elements exist in a polyatomic form which has a complex structure and hence are solids. e.g. S and Se exit as SE and Se8 at room temperature and have the structures in which the atoms are linked by normal covalent bonds leading to the formation of either puckered ring structure or chain polymers(See fig. 4.18). This ability of these elements to form long chains is called catenation.
4- Metallic and Non-metallic character:
The first four elements such as O, S Se and Te are nonmetals but the nonmetallic character of oxygen is stronger as compared to S, Se and Te. Se and Te have some metallic character and are therefore may be regarded as semimetals. On the other hand, the last element Po is distinctly metal. However, this element is radioactive and short-lived. Thus, on moving down. a group, metallic property increases with the increase of the atomic number.
5- Melting point and Boiling point :
With the increase in the atomic number of the elements the intramolecular force increases so the melting and boiling points of the elements of group VIA are in the order of:
O < S < Se < Te.
6- Electron affinity:
The electron affinity value of oxygen is less than that of sulphur because due to the small atomic size of an oxygen atom, the addition of an extra electron in its 2p-orbital is difficult due to electron-electron repulsion. On the other hand, moving down the group from $ to Po electron affinity values decrease. Thus the actual order of electron affinity values for the elements of group V∣A is :
7- Ionization potential and electronegativity :
As expected, ionization potential and electronegativity values of the elements of group VIA decrease gradually as we move down the group from O to Te. This is because the increase: in the atomic size of the elements leads to the lesser attraction of the nucleus with the electrons.
8- Allotropy :
Oxygen exists only in two allotropes: diatomic (O2) and triatomic (O3) molecules. Sulphur exists in several allotropic forms: red (nonmetallic) and grey (metallic).
The common 368.46 K(95.5∘C) sulphur transforms to the high-temperature b-form. Monoclinic sulphur, (SM) has also the same molecular formula Sg.
S8 molecules are packed up to give different crystal structures for the two forms. The S2 ring in both forms is puckered and has a crown shape. In addition, several other modifications of sulphur containing 6-20 sulphur atoms per ring are also known. In cyclo SB1 the ring adopts the chair form. The structure of some allotropic forms of sulphur is shown in the figure (4.18). The compact arrangement of S-atoms (the hole in the middle of the ring is very small) causes the S6 molecule to be the densest of all sulphur modifications so far isolated.
Selenium exists in eight allotropic forms of which three are red monoclinic forms containing Se8 rings. The thermodynamically stable form is grey hexagonal metallic selenium, which consists of polymeric helical chains. Grey selenium can conduct electricity.
Tellurium has only one crystalline form with a chain structure similar to that of grey Se.
9- Oxidation states and Valency:
Except in O2 F2 and OF2 where oxygen has oxidation numbers +1 and +2 respectively, In other compounds, oxygen has a negative oxidation state, In H2O2 and its derivatives the oxidation number is −1 and in other compounds, it is −2. After forming two bonds all the valency orbitals in oxygen are completely filled and no empty orbital is then available to receive more electrons. This is why oxygen can not form more than two covalent bonds. Oxygen shows oxidation numbers −2 to +2 only, because due to the absence of empty d. orbitals no expansion of electron occur in valency shell in oxygen. On the other hand, S, Se and Te exhibit the oxidation numbers: −2,+2,+4 and +6. Due to the presence of empty nd-orbitals expansion of electrons occur from ns and np orbitals to nd-orbitals when necessary. Consequently, the element is able to form tetravalent bonds if one electron from np orbitals is transferred to the nd-orbital. If ns electron is also transferred along with p-electrons to the nd-orbitals then the central atom will be able to form a hexavalent bond.
Since the electronegativity decreases as we move from S to Po the tendency of these elements to show-2 an oxidation state decreases and that to show +2 oxidation state increases down the group. This causes Po which is the last element of group VIA not to show a −2 oxidation state. II shows only +2 and +4 oxidation states due to the inert pair effect.
10- Hydrides: All the elements form two types of hydrides.
(i) H2X type hydride:
O, S, Se and Te form this type of hydrides such as H2O, H2 S, H2Se and H2Te. In these hydrides, H2O is liquid while H2 S, H2Se and H2 Te are gases at room temperature. Thus, the stability of these hydrides decreases in the order:
H2O > H2S > H2Se > H2Te
Due to the successive decrease in thermal stability the ease with which H2X molecule can lose H-atoms increases from H2O to H2 Te. Therefore, the reducing power of the given molecule also increases in the same order as :
H2O < H2S< H2Se < H2Te
