Noble-gas configuration

Since 1916 it has been discovered that some noble gases (originally called the inert gases) do form compounds and also there are many reactions known in which elements do not achieve a noble gas configuration. Nevertheless, the theory was a considerable advance towards modem ideas and provides a good basis for discussion. 

Energy must be supplied if these elements are to attain a noble gas configuration. 

Loss of one electron gives the noble gas configuration the very large difference between the first and second ionisation energies implies that an outer electronic configuration of a noble gas is indeed very stable. 

Group II elements can be seen to follow a pattern very like that found in Group I. Note, however, that the energy required to attain a noble gas configuration is considerably higher indicating that the elements will be less metallic or electropositive in their chemistry (Chapter 6). 

Typical elements in Groups V. VI and VII would be expected to achieve a noble gas configuration more easily by gaining electrons rather than losing them. Electron affinity is a measure of the energy change when an atom accepts an extra electron. It is difficult to measure directly and this has only been achieved in a few cases more often it is obtained from enthalpy cycle calculations (p. 74). 

The two kinds of covalent bond are not identical, one being a simple covalent bond, a sigma (ct) bond, the other being a stronger (but more reactive) bond called a n bond (p. 56). As in the formation of methane both elements attain noble gas configurations. We can consider the formation of ethene as the linking of two tetrahedral carbon atoms to form the molecule C2H4 represented as ... 

In each of the examples given so far each element has achieved a noble gas configuration as a result of electron sharing. There are. however, many examples of stable covalent compounds in which noble gas configurations are not achieved, or are exceeded. In the compounds of aluminium, phosphorus and sulphur, shown below, the central atoms have 6. 10 and 12 electrons respectively involved in bondinc... 

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Boron achieves a covalency of three by sharing its three outer electrons, for example BFj (p. 153). By accepting an electron pair from a donor molecule or ion, boron can achieve a noble gas configuration whilst increasing its covalency to four, for example H3N->BCl3. K BF4. This is the maximum for boron and the second quantum level is now complete these 4-coordinate species are tetrahedral (p. 38). 

Aluminium also has a strong tendency to achieve a noble gas configuration by electron pair acceptance as shown in dimeric aluminium chloride. 

The elements in this group have six electrons in their outer quantum level, and can thus achieve a noble gas configuration by acquiring two electrons. 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

The O oxidation state is known in vanadium hexacarbonyl. V(CO)(,. a blue-green, sublimable solid. In the molecule VfCO), if each CO molecule is assumed to donate two electrons to the vanadium atom, the latter is still one electron short of the next noble gas configuration (krypton) the compound is therefore paramagnetic, and is easily reduced to form [VfCO, )]. giving it the... 

The number of covalent bonds an atom forms depends on how many additional valence electrons it needs to reach a noble-gas configuration. Hydrogen has one valence electron (Is) and needs one more to reach the helium configuration (Is2), so it forms one bond. Carbon has four valence electrons (2s2 2p2) and needs four more to reach the neon configuration (2s2 2p6), so it forms four bonds. Nitrogen has five valence electrons (2s2 2p3), needs three more, and forms three bonds oxygen has six valence electrons (2s2 2p4), needs two more, and forms two bonds and the halogens have seven valence electrons, need one more, and form one bond. 

Cations with noble gas configurations. The alkali metals, alkaline earths and aluminium belong to this group which exhibit Class A acceptor properties. Electrostatic forces predominate in complex formation, so interactions... 

A number of tertiary phosphine ligands have been synthesized that also contain an alkene linkage capable of coordinating to a metal. A good example of this kind of coordination is formed in the complex of (tri-o-vinyl-phenyl)phosphine (Figure 2.29) with each alkene acting as a two-electron donor, a noble gas configuration is achieved [67],... 

To predict the electron configuration of a monatomic cation, remove outermost electrons in the order np, ns, and (n — l)d fora monatomic anion, add electrons until the next noble-gas configuration has been reached. The transfer of electrons results in the formation of an octet (or duplet) of electrons in the valence shell on each of the atoms metals achieve an octet (or duplet) by electron loss and nonmetals achieve it by electron gain. 

When ionic bonds form, the atoms of one element lose electrons and the atoms of the second element gain them until both types of atoms have reached a noble-gas configuration. The same idea can be extended to covalent bonds. However, when a covalent bond forms, atoms share electrons until they reach a noble-gas configuration. Lewis called this principle the octet rule ... 

B In fluorine (Group 17), an additional electron fills the single vacancy in the valence shell the shell now has the noble-gas configuration of neon and is complete. In neon, an additional electron would have to enter a new shell, where it would be farther from the attraction of the nucleus. 

The description of the first 10 electrons in the configuration of aluminum is identical to that of neon, so we can represent that portion as [Ne]. With this notation, the configuration of A1 becomes [Ne] 3 5" 3 p The element at the end of each row of the periodic table has a noble gas configuration. These configurations can be written in the following shorthand notation ... 

To write the configuration of any other element, we first consult the periodic table to find its location relative to the noble gases. Then we specify the noble gas configuration and build the remaining portion of the configuration according to the aufbau principle. Example applies this procedure to indium. 

The structures used to show the bonding in covalent molecules are called Lewis structures. When bonding, atoms tend to achieve a noble gas configuration. By sharing electrons, individual atoms can complete the outer energy level. In a covalent bond, an octet of electrons is formed around each atom (except hydrogen.)... 

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