Chapter 9 Skieletons - Chapter 9 Molecular Geometry and...

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Chapter 9. Molecular Geometry and Bonding Theories Lecture Outline 9.1 Molecular Shapes Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which atoms. The shape of a molecule is determined by its bond angles . The angles made by the lines joining the nuclei of the atoms in a molecule are the bond angles. Consider CCl 4 : Experimentally we find all Cl C Cl bond angles are 109.5 . Therefore, the molecule cannot be planar. All Cl atoms are located at the vertices of a tetrahedron with the C at its center. In order to predict molecular shape, we assume that the valence electrons repel each other. Therefore, the molecule adopts the three-dimensional geometry that minimizes this repulsion. We call this model the V alence S hell E lectron P air R epulsion ( VSEPR ) model. 9.2 The VSEPR Model A covalent bond forms between two atoms when a pair of electrons occupies the space between the atoms. This is a bonding pair of electrons. Such a region is an electron domain . A nonbonding pair or lone pair of electrons defines an electron domain located principally on one atom. Example: NH 3 has three bonding pairs and one lone pair. VSEPR predicts that the best arrangement of electron domains is the one that minimizes the repulsions among them. The arrangement of electron domains about the central atom of an AB n molecule is its electron- domain geometry . There are five different electron-domain geometries: linear (two electron domains), trigonal planar (three domains), tetrahedral (four domains), trigonal bipyramidal (five domains) and octahedral (six domains). The molecular geometry is the arrangement of the atoms in space. To determine the shape of a molecule we must distinguish between lone pairs and bonding pairs. We use the electron-domain geometry to help us predict the molecular geometry. Draw the Lewis structure. Count the total number of electron domains around the central atom. Arrange the electron domains in one of the above geometries to minimize electron-electron repulsion. Next, determine the three-dimensional structure of the molecule. We ignore lone pairs in the molecular geometry. Describe the molecular geometry in terms of the angular arrangement of the bonded atoms. Multiple bonds are counted as one electron domain. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles We refine VSEPR to predict and explain slight distortions from “ideal” geometries. Consider three molecules with tetrahedral electron domain geometries: CH 4 , NH 3 , and H 2 O.
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Molecular Geometry and Bonding Theories 103 By experiment, the H X H bond angle decreases from C (109.5 in CH 4 ) to N (107 in NH 3 ) to O (104.5 in H 2 O).
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern