EXPLAINING THE GEOMETRY OF SIMPLE MOLECULES USING MOLECULAR ORBITAL ENERGY-LEVEL DIAGRAMS BUILT BY USING SYMMETRY PRINCIPLES - Article

Oct 26, 2026

Explains molecular geometry of simple molecules using qualitative molecular orbital energy-level diagrams constructed from symmetry principles. The method accounts for preferred geometries such as bent H2O, pyramidal NH3, and tetrahedral CH4 by focusing on the number of electrons occupying nonbonding molecular orbitals for each geometry. The approach avoids reliance on VSEPR theory and hybrid orbitals, and is positioned as suitable for inorganic chemistry courses where symmetry and group theory are taught.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages7

Reading Time18 min

AccessFree to download

Introduction

Conventional explanations (VSEPR and hybrid orbitals)

Symmetry-based molecular orbital energy-level diagrams

Related work and motivation

Approach and educational application

Quim. Nova, Vol. 41, No. 5, 587-593, 2018 [http://dx.doi.org/10.21577/0100-4042.20170198](http://dx.doi.org/10.21577/0100-4042.20170198) | EXPLAINING THE GEOMETRY OF SIMPLE MOLECULES USING MOLECULAR ORBITAL ENERGY-LEVEL DIAGRAMS BUILT BY USING SYMMETRY PRINCIPLES | | --- | | Sérgio P. Machado and Roberto B. Faria*
Departamento de Química Inorgânica, Instituto de Química, Universidade Federal do Rio de Janeiro, 21941-909 Rio de Janeiro – RJ, Brasil
Recebido em 27/08/2017; aceito em 08/01/2018; publicado na web em 08/02/2018 | The built of qualitative energy-level molecular diagrams for different geometries of simple molecules allow to explain the preferred geometry. 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. 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. This discussion is more appropriate to inorganic chemistry courses where symmetry is a common topic. Keywords: symmetry; group theory; molecular orbitals; molecular geometry Educação INTRODUCTION The geometries of molecules like H2O, NH3 and CH4 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.1-6 In the case of the VSEPR the starting point is the Lewis structure of each compound. 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) . 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 H2O bent, NH3 pyramidal, and CH4 tetrahedral geometry, as it is well known. When using hybrid orbitals, the central atom in all these molecules is considered to use sp3 orbitals, which corresponds to the tetrahedral geometry. In this case some sp3 hybrid orbitals contain lone pair of electrons, resulting in the observed geometry for the cited molecules. 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. Several inorganic chemistry textbooks present a very detailed molecular orbital energy-level diagrams for several simple molecules, for example, BeH2, CO2, and H2O, justifying their geometries based on the occupancy of the molecular orbitals. 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.4-6 More specialized books on spectroscopy,7 present a discussion based on the change of the energy of molecular orbitals asthe geometry of a molecule is gradually changed, for example, in the case ofAH2 species, changing from linear to bent geometry. This kind of discussion is based on qualitative Walsh diagrams.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. These gradual modificationson the energy of the molecular orbitals, however, in some cases, are not easy to forecast, making difficult the discussion in class. *e-mail: [faria@iq.ufrj.br](faria@iq.ufrj.br) Other works deal with the same subject we present in this article. Mulliken15 presents a discussion on NH3 which considers this molecule in the geometries pyramidal and plane trig

FAQs

What traditional theories does the article aim to avoid when explaining molecular geometry?

The discussion is presented as not requiring VSEPR theory or hybrid orbitals, relying instead on symmetry-based molecular orbital reasoning.

Oct 26, 2026

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