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Cover Elements of Physical Chemistry

Valence bond theory  

This chapter discusses the procedure for writing wavefunction for a shared electron pair and it gives an account for the structures of a wide variety of molecules. It explains how the valence bond theory (VB theory) introduces the concepts of σ bonds, π bonds, promotion, and hybridization. It also points out how a bond forms when an electron in an atomic orbital on one atom pairs its spin with that of an electron in an atomic orbital on another atom. The chapter analyzes the covalence-shell electron pair repulsion (VSEPR) model, in which it is supposed that the shape of a molecule is determined by the repulsions between electron pairs in the valence shell. It extends the general concepts of the VB theory with arguments grounded in quantum theory.


Cover Making the Transition to University Chemistry

Bonding and Molecular Shape  

This chapter discusses bonding and molecular shape of electrons. A covalent bond occurs when atoms share a pair of similar electrons. Modern theories on covalent bonding are dominated by molecular orbital theory. Polar covalent bonds occur when different atoms share the electron pair unequally due to electronegativity. With benzene being the most familiar molecule of the bonding, delocalization happens when more than two atoms are involved in the bonding. The valence-shell electron-pair repulsion (VSEPR) theory can help us to visualize the shapes of simple molecules. Additionally, ionic bonding occurs when an atom transfers an electron to another atom and the ions formed a crystal lattice electrostatically.


Cover Chemistry for the Biosciences

Molecular shape and structure: life in three dimensions  

This chapter describes the factors that influence the shape and structure of atoms and molecules, asking what it is that gives different chemical substances their particular three-dimensional shapes. The bond length is the distance between two covalently bonded atoms; the atomic radius is half the distance between two covalently bonded atoms of the same element. As the number of electrons shared by two atoms increases—as we move from atoms being joined by single to double to triple bonds—the bond length decreases. The bond angle is the angle between two bonds that are joined by a central atom. Valence shell electron pair repulsion (VSEPR) theory allows the prediction of the arrangement of valence electron pairs around a central atom. The chapter then looks at the geometry of atoms in larger molecules, before considering hybridization and bond rotation, which both place limitations on molecular shape.


Cover Periodicity and the s- and p-Block Elements


This chapter assesses periodic trends in relation to structure, looking at electron counting rules including the octet rule. The octet rule was first formulated by Gilbert Lewis and Irving Langmuir in the early twentieth century. Octet refers to 8, of course, and this is an important number especially for the s- and p-block elements. The importance of the number 8 arises since, for these elements, we have one valence s and three valence p orbitals which, when involved in covalent bonding, will result in four bonding orbitals and four antibonding orbitals (or a reduced number corresponding to the number of lone pairs or non-bonding orbitals). Eight electrons will therefore fill the bonding/non-bonding orbitals completely. The chapter then considers the valence shell electron-pair repulsion (VSEPR) theory; the trivalent compounds of Group 15; and the Zintl principle.


Cover Inorganic Chemistry

Molecular structure and bonding  

This chapter examines the development of models of molecular structure in terms of the concepts of valence bond and molecular orbital theory. It reviews the methods for predicting the shapes of molecules. The discussion covers the Lewis structures, octet rule, and VSEPR model. The chapter introduces the concepts of bond order and correlations. Then, it looks at bond length, bond strength, and reaction enthalpies. Lastly, it explains the basic principles of catalysis, discussing energetics, catalytic cycles, catalytic efficiency and lifetime, selective catalysts, as well as homogeneous and heterogeneous catalysts. The chapter illustrates the importance of the interplay between qualitative models, experiments, and calculations.