14. Valence Bond Theory and Hybridization

The paragraph introduces the educational context, emphasizing the importance of various theories in chemistry, such as valence bond theory and hybridization, in predicting molecular properties. It highlights the strengths and weaknesses of these theories, particularly molecular orbital theory for energy levels and valence bond theory for molecular shapes. The speaker, Catherine Drennan, also mentions her personal preference for valence bond theory due to its utility in her work determining molecular shapes. Additionally, she humorously references dreams about atomic orbitals as a segue to the upcoming thermodynamics lessons.

This paragraph delves into the specifics of valence bond theory, explaining how bonds form from the pairing of unpaired electrons. It introduces sigma and pi bonds, detailing their characteristics and how they relate to molecular orbitals. The concept of hybridization is explored, particularly focusing on carbon and its role in organic chemistry. The process of electron promotion and the formation of sp3 hybrid orbitals in methane are described, illustrating how carbon forms four bonds with hydrogen, leading to a stable methane molecule.

The paragraph discusses the geometric implications of hybrid orbitals, specifically sp3 hybridization, which results in a tetrahedral shape. It uses ethane as an example to explain how carbon atoms with sp3 hybridization form single sigma bonds with hydrogen atoms, maintaining the tetrahedral geometry. The paragraph also touches on the naming conventions for these sigma bonds, emphasizing the importance of understanding the type of orbitals involved in the bonding.

This section extends the discussion of hybridization to include nitrogen and oxygen, which have lone pairs that affect molecular geometry. The paragraph explains how nitrogen, with its five valence electrons, forms three bonds using sp3 hybridization, as seen in NH3, leading to a trigonal pyramidal shape. Similarly, oxygen, with its six valence electrons, forms two bonds and has two lone pairs, resulting in a bent molecular geometry when forming water, H2O.

The focus shifts to sp2 hybridization, which involves the combination of one s orbital and two p orbitals, leaving one p orbital unhybridized. The paragraph discusses the use of sp2 hybridization in boron, which has three unpaired electrons and requires electron promotion to form three sp2 hybrid orbitals, resulting in a trigonal planar geometry. It also mentions the versatility of carbon, which can exhibit sp2 hybridization in addition to sp3.

The paragraph explores carbon's ability to form sp2 hybrid orbitals, which leads to trigonal planar geometry. It uses C2H4 as an example, explaining the formation of a double bond consisting of one sigma and one pi bond. The pi bond is formed by the unhybridized 2py orbitals, while the sigma bonds are formed by the sp2 hybrid orbitals. The paragraph also notes the rigidity of molecules with double bonds, which is important for organic chemists.

This section discusses the importance of double bonds in creating rigid molecular structures, as seen in the example of a molecule used to treat schizophrenia in the 1950s. The paragraph explains how the double bond restricts rotation, fixing the orientation of atoms and contributing to the molecule's activity as a pharmaceutical. It also reviews the types of bonds present in such a molecule, including sigma and pi bonds formed by hybridized and non-hybridized orbitals, respectively.

The paragraph serves as a review of hybridization concepts, focusing on the importance of understanding the types of bonds and geometries for exam preparation. It provides a cheat sheet for carbon hybridization in various molecules, such as C2H6, C2H2, and C2H4, detailing the hybridization type, bond type, and geometry for each. The speaker also emphasizes the need to pay attention to electron promotion and the conditions under which it occurs.

The final paragraph encourages students to practice applying hybridization concepts to determine the types of bonds in given molecules. It includes a clicker question about a specific molecule with a central carbon atom bonded to various other atoms, guiding students through the process of identifying the correct bond types based on the hybridization of each atom. The paragraph concludes with a reminder of the importance of practice for the upcoming exam.