Chemistry Outcomes Review: Chapter 5

Structure of Molecules

In this chapter, we used electron wave mechanics to further develop the bonding models introduced in Chapter 1. We found that the atomic cores in all molecules are held together by attractions between their positive charges and negatively-charged two-electron sigma bonding (σ) molecular orbitals located directly between them. Atomic cores in molecules may also have sigma nonbonding (σ_n) molecular orbitals. There are also molecules with multiple bonds between atoms, a σ bond and a pi (∏) or two ∏ bonds. In some cases, orbitals may be delocalized over three or more atomic centers in a molecule.

The purposes for developing the bonding model are to understand and predict the shape and reactivity of molecules. The geometry (shape) of a molecule is determined by its framework of σ and σ_n electrons and we discussed several ways, including using molecular models, to represent shapes. Isomers, both structural isomers and stereoisomers, are distinguished from one another by different properties and different molecular shapes. The reactions of molecules generally involve attraction of a positive site in one molecule to a negative site in another. Models that help us visualize the electron distribution in molecules show the location of these sites, which are the functional groups in the molecule.

Check your understanding of the ideas in the chapter by reviewing these expected outcomes of your study.

You should be able to:

• Write appropriate Lewis structures for molecules whose atomic composition you know, including multiple Lewis structures representing possible isomers [Sections 5.2 and 5.5].
• Build molecular models based on the connectivities shown in Lewis structures of the molecules [Sections 5.2, 5.5, 5.8 and 5.9].
• For a series of compounds, correlate patterns of chemical behavior with their Lewis structures and molecular models and, based on these patterns, predict the behavior of other compounds [Sections 5.1, 5.2, 5.5, and 5.9].
• Use Lewis structures to predict multiple bonding in the molecules of a compound [Section 5.5].
• Draw and/or describe the σ, σ_n, and p orbitals for simple molecules containing elemental atoms from periods one, two, and three [Sections 5.3, 5.4, 5.6, and 5.7].
• Draw and/or describe the sigma framework of a molecule and predict the shape of the molecule [Sections 5.4 and 5.6].
• Use Lewis structures to predict whether a molecule is likely to have delocalized ∏ orbitals [Sections 5.7 and 5.8].
• Beginning with any one of the following representations of relatively simple molecules, write, draw, or build all the other representations: condensed formula, Lewis structure, 3-d structure, condensed structure, skeletal structure, molecular model [Sections 5.2, 5.5, and 5.8].
• Use appropriate molecular representations to show whether a pair of structures are unrelated to one another, are identical to one another, are structural isomers, or are stereoisomers (cis/trans and/or optical isomers) [Sections 5.1, 5.2, 5.8, and 5.9].
• Use appropriate molecular representations to show whether structural isomers and/or stereoisomers (cis/trans and/or optical isomers) are possible for a given molecular formula [Sections 5.2, 5.8, and 5.9].
• Identify the functional group(s) present in a molecular structure [Section 5.10].
• Use the polarities of functional groups (from electronegativities and bond polarities and/or calculated electron distributions) to predict where molecules will be likely to interact with one.