When there is a difference in electronegativity between two atoms, then the bond between them is polar. We assign the geometry about each central atom separately. Shapes of Larger Molecules In acetic acid, CH 3 COOH, there are three central atoms. Molecules with Expanded Valence Shells VSEPR Model Molecules with Expanded Valence Shells To minimize e   e  repulsion, lone pairs are always placed in equatorial positions. Similarly, the fifth and sixth electron pairs are located above and below this plane. For octahedral structures, there is a plane containing four electron pairs. The fourth and fifth electron pairs are located above and below this plane.

For trigonal bipyramidal structures there is a plane containing three electrons pairs. Molecules with Expanded Valence Shells Atoms that have expanded octets have AB 5 (trigonal bipyramidal) or AB 6 (octahedral) electron pair geometries. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles Similarly, electrons in multiple bonds repel more than electrons in single bonds. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles VSEPR Model Therefore, the bond angle decreases as the number of lone pairs increase. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles By experiment, the H-X-H bond angle decreases on moving from C to N to O: Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. All the atoms that obey the octet rule have tetrahedral electron pair geometries. We ignore lone pairs in the molecular geometry. We name the molecular geometry by the positions of atoms. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles We determine the electron pair geometry only looking at electrons. To determine the electron pair geometry: draw the Lewis structure, count the total number of electron pairs around the central atom, arrange the electron pairs in one of the above geometries to minimize e  -e  repulsion, and count multiple bonds as one bonding pair. The electrons adopt an arrangement in space to minimize e  -e  repulsion. We define the electron domain geometry by the positions in 3D space of ALL electron pairs (bonding or non-bonding). To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms). When naming the molecular geometry, we focus only on the positions of the atoms. When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs). There are five fundamental geometries for molecular shape: Molecular Shapes There are simple shapes for AB 2 and AB 3 molecules. We call this process V alence S hell E lectron P air R epulsion ( VSEPR ) theory. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion. In order to predict molecular shape, we assume the valence electrons repel each other. All Cl atoms are located at the vertices of a tetrahedron with the C at its center. Therefore, the molecule cannot be planar.

Consider CCl 4 : experimentally we find all Cl-C-Cl bond angles are 109.5 . The shape of a molecule is determined by its bond angles. Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which. Chapter 9 Molecular Geometry and Bonding Theories CHEMISTRY The Central Science 9th Edition David P.

Molecular geometry