Triatomic molecules valence electrons

Triatomic species can be linear, like CO2, or bent, like O3. The principles of orbital overlap do not depend on the identity of the atoms involved, so all second-row triatomic species with 16 valence electrons have the same bonding scheme as CO2 and are linear. For example, dinitrogen oxide (N2 O) has 16 valence electrons, so it has an orbital configuration identical to that of CO2. Each molecule is linear with an inner atom whose steric number is 2. As in CO2, the bonding framework of N2 O can be represented with sp hybrid orbitals. Both molecules have two perpendicular sets of three tt molecular orbitals. The resonance structures of N2 O, described... 

A similar situation arises in the ir-v excited states of ethylene and acetylene where the n interactions become antibonding the excited states are consequently twisted (11) and bent (12) respectively. Another example is provided by triatomic molecules formed by second row atoms ifthe total number of valence electrons exceeds 16, the molecules are bent 33) since there are now antibonding electrons present and bending reduces the resulting unfavourable 7r interaction by replacing p AOs by hybrid AOs (cf. ozone, 13). 

The applications of valence bond theory, the valence shell electron pair repulsion theory, to the bonding and bond angles of triatomic molecules... 

Fig. 1.13 The Pettifor structure map for sp-valent AB2 triatomic molecules with N 16 where N is the total number of valence electrons. The full triangles and circles correspond to bent and linear molecules respectively whose shape is well established from experiment or self-consistent quantum mechanical calculations. The open symbols correspond to ambiguous evidence. The data base has been taken from Andreoni et a/. (1985).

Triatomic fragments involving boron and carbon, such as those found in many borocarbide structures,16 show typical valence electron counts ranging between 12 and 16 electrons. These counts are unusual for isolated triatomic molecules, which usually have 16 or more electrons, unless one of the terminal atoms is FI (12 electrons).17 Within the boron-carbon family, four of six possible arrangements for the trimeric fragments have been observed (1) C—C-C, (2) C-B-C, (3) B-B—C, and (4) B-C-B. Coincident-ally, only those with D,h point symmetry occur in metal borocarbides. They include (1) CCC units in Sc3C4 18 (2) CBC units in Si BCj 19,20... 

Rabalais, J.W., McDonald, J.M., Scherr, V., and McGlynn, S.P. (1971). Electronic spectroscopy of isoelectronic molecules. II Linear triatomic groupings containing sixteen valence electrons, Chem. Rev. 71, 73-108. 

Are all triatomic molecules bent like the water molecule To find out, you can model carbon dioxide, CO2, as you did water. Begin by drawing the electron dot diagrams for the two atoms. Carbon has four valence electrons, and each oxygen atom has six. 

The emerging picture is one in which the quantum-mechanical equivalents of the constants of motion for the two valence electrons in these atoms are like those associated with the near-rigid rotations, bending vibrations, and stretching vibrations we normally associate with linear triatomic molecules. These new results bring into question the range of validity of the nearly-independent-particle model, the quantum-mechanical counterpart of Bohr s planetary model, for atoms with more than one valence electron. 

This problem has been somewhat ameliorated by collapsing coordinates, i.e., using functions that combine coordinates. For example, Kong has plotted molecular data on the total numbers of valence electrons with great success. For another example, the SAU three-dimensional system for acyclic triatomic molecules (Carlson et al. 1997 Wohlers et al. 1998) uses axes (7 i7 2 + R2R3), (Ci + C2 + C3), and C2, thus collapsing coordinates of both rows and columns and producing a three-dimensional system from one of six dimensions. 

In a rough-and-ready way the result can be seen to be of this kind by the following argument. In the molecule two electrons are paired, that is to say they have opposite spins. The electron of the third atom, which is approaching, must have a spin parallel to that of one of those already paired in the molecule. The Panh principle disallows the inclusion of this extra electron in the group of valency electrons, and therefore the triatomic combination is not permissible. 

The question addressed in this paper is, in its most general terms To what extent is it appropriate to describe a few-body system in terms of one quantization scheme or another Slightly more precisely, we ask Which simple model, among the choices we can invent, is the best starting approximation for any designated state of a given system As our vehicle to study this problem, we use the system of the two valence electrons in the helium and alkaline earth atoms. The models we consider and compare are the Hartree-Fock, independent-particle model and the collective model like that for a linear triatomic molecule. 

In Table 20.1 we compare the valence angles of some representative triatomic molecules or ions formed from elements in Groups 14 to 18. It is seen that the shapes of the molecules are determined by the numbCT of valence electrons carbon dioxide and aU other 16 valence electron species are linear, and species with 17 to 20 valence electrons are angular. When the number of valence electrons is increased to 22, the shape reverts to linearity. The author is not aware of any exception to this trend. 

Table 20.1. Some triatomic molecules ABC formed from elements in Groups 14 to 18 the total number of valence electrons and ABC valence angles.

Consider the triatomic molecule beryllium chloride (BeCl2). Experimental evidence shows that the BeCl2 molecule in the gas phase is linear with two equivalent Be—Cl bonds, consistent with the VSEPR model prediction. The construction of a valence bond picture of bonding in linear BeCl2 requires the existence of unpaired electrons in Be atomic orbitals that overlap with the half-filled 3p orbital on each Cl to form two equivalent bonds that are 180° apart. However, the ground state electron configuration for Be is ls 2s, which contains no unpaired electrons. 

A (a) Why is trifluoromethane (HCF,) polar and tetrafluoromethane (CF4) nonpolar (b) Give an example of a hnear triatomic molecule that is polar, (c) What can be said about the distribution of valence electrons in a polar molecule ... 

Nitrogen dioxide with 17 valence-shell electrons is perhaps the most familiar triatomic molecule for which Pauling 3-electron bond theory is appropriate. Electron spin resonance measurements indicate that the odd electron is delocalized over the three atomic centres estimates of the nitrogen and oxygen odd-electron charges are 0.52 and 0.24 (Table 6-1), respectively. 

The 03 molecule is isoelectronic with S02, N02 , and other triatomic species containing 18 valence-shell electrons. As a result, the structure can be represented by the resonance structures (having C2 symmetry)... 

---