Constructing a molecular geometry table1/13/2024 ![]() ![]() ![]() Mulliken 15 presents a discussion on NH 3 which considers this molecule in the geometries pyramidal and plane trigonal. Other works deal with the same subject we present in this article. These gradual modifications on the energy of the molecular orbitals, however, in some cases, are not easy to forecast, making difficult the discussion in class. 8-14 These Walsh diagrams describe how the energy of each molecular orbital changes as the molecular geometry is modified between two possibilities as, for example, linear and bent geometries. This kind of discussion is based on qualitative Walsh diagrams. 4-6 More specialized books on spectroscopy, 7 present a discussion based on the change of the energy of molecular orbitals as the geometry of a molecule is gradually changed, for example, in the case of AH 2 species, changing from linear to bent geometry. However, as far we know, these textbooks do not present a discussion of what happen with the molecular energy if we consider a molecule with a different geometry. Several inorganic chemistry textbooks present a very detailed molecular orbital energy-level diagrams for several simple molecules, for example, BeH 2, CO 2, and H 2O, justifying their geometries based on the occupancy of the molecular orbitals. On the other hand, in the inorganic chemistry courses, when part of the time is dedicated to the application of symmetry to build the symmetry-adapted linear combinations of atomic orbitals, the stability of these molecules can be discussed more properly based on the molecular orbital energy-level diagrams for these molecules. In this case some sp 3 hybrid orbitals contain lone pair of electrons, resulting in the observed geometry for the cited molecules. When using hybrid orbitals, the central atom in all these molecules is considered to use sp 3 orbitals, which corresponds to the tetrahedral geometry. In the case of the molecules indicated above, all of them have four electron groups but some of these have lone pairs resulting in the H 2O bent, NH 3 pyramidal, and CH 4 tetrahedral geometry, as it is well known. It is considered that electrons groups around the central atom will be as far as possible, producing the observed molecular geometry (linear for two electrons groups, triangular for three electrons groups, tetrahedral for four electrons groups, and so on). 1-6 In the case of the VSEPR the starting point is the Lewis structure of each compound. The geometries of molecules like H 2O, NH 3 and CH 4 are usually explained, at undergraduate level, in general and inorganic chemistry courses, using the Valence Shell Electron-Pair Repulsion theory (VSEPR theory) and hybrid orbitals. Palavras-chave: symmetry group theory molecular orbitals molecular geometry This discussion is more appropriate to inorganic chemistry courses where symmetry is a common topic. This simple energy principle does not need to consider the Valence Shell Electron-Pair Repulsion theory (VSEPR theory) neither hybrid orbitals to explain the geometry of simple molecules. The diagrams are built using simple symmetry principles and explain, on basis of the number of nonbonding electrons, for example, why the molecule of water is bent and not linear and ammonia is pyramidal and not planar. The built of qualitative energy-level molecular diagrams for different geometries of simple molecules allow to explain the preferred geometry. Publicado na web em Endereço para correspondência Explaining the geometry of simple molecules using molecular orbital energy-level diagrams built by using symmetry principlesĭepartamento de Química Inorgânica, Instituto de Química, Universidade Federal do Rio de Janeiro, 21941-909 Rio de Janeiro - RJ, Brasil ![]()
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