Resonance Structures Two completely equivalent Lewis structures can be drawn for ozone, O 3. Experiment, however, shows that both bonds in ozone are equivalent.  The reality is a blend of the two Lewis structures.

The actual ozone molecule with its equal bond distances is a blend of the two Lewis structures. Each of these structures is called a resonance structure. Resonance Structures

Three resonance structures can be drawn for the nitrate ion, NO 3 -. Again, reality is a blend of the three. There are no double bonds in the nitrate ion, but each bond is more stable than just a single bond. Resonance Structures

Resonance in Benzene and Other Aromatic Molecules Benzene is a cyclic compound, C 6 H 6, and is very stable. Often, this resonance is represented by: where the circle shows the blending of the resonance structures. Other compounds containing this ring structure are called aromatic compounds.

Lewis Structures that are Exceptions to the Octet Rule Exceptions to the Octet rule are: 1. Molecules in which an atom has more than an octet (this can occur with period 3 and higher elements…NEVER with period 2 elements). 2. Molecules in which an atom has less than an octet: H, Be, B. 3. Molecules with an odd number of electrons. An example is NO, but we are not going to dwell on this category.

Exceptions to the Octet rule are: Molecules in which an atom has more than an octet (this can occur with period 3 and higher elements…NEVER with period 2 elements). Lewis Structures that are Exceptions to the Octet Rule

Exceptions to the Octet rule are: 2. Molecules in which an atom has less than an octet (generally occurs with B or Be and always with H). Lewis Structures that are Exceptions to the Octet Rule

Electron Domains Being able to draw the correct Lewis structure is the first step in determining the geometry of the covalent compound. This allows the identification of the electron domains of the molecule. Domains are the regions in the molecules where it is most likely to find electrons.

Domains are the regions in the molecules where it is most likely to find electrons. For a bond (single, double, or triple), the electron domain is between the two atoms in the bond and consists of all the electrons involved in the bond. For nonbonding pairs of electrons, the domain is the nonbonding pair and is centered on a single atom. Electron Domains

Identifying Electron Domains It is important to differentiate between bonding domains and nonbonding domains. Xenon difluoride has five electron domains, two bonding, three nonbonding. The nitrate ion has three electron domains, all bonding.

VSEPR Theory Valence Shell Electron Pair Repulsion Electrons in each domain are subject to electrostatic repulsion from the electrons in the other domains. The domains will orient themselves so as to minimize this repulsion. The orientation of these domains is a function of the number of domains around the central atom and is one of several simple geometric figures. Memorize these geometries and angles

Trigonal Bipyramid Geometry 90° 120° B B C C CA angles BAC = 90° angles CAC = 120° angle BAB = 180°

Electron Domain Geometries Trigonal planar Bond angles all 120° Trigonal bipyramid Bond angle is 180°

Different Ways to Represent Molecular Geometry Ball and stick model Space-filling model a tetrahedron

Molecular Geometries for Molecules with No Nonbonding Electron Domains Memorize both the geometries and the bond angles. CO 2 BCl 3 CCl 4 PCl 5 SF 6

Molecular Geometries for Molecules with Nonbonding Electron Pairs An important thing to remember is that the shape of ANY molecule is based on the location of its atoms (NOT the location of its electrons).

The shape of ANY molecule is based on the location of its atoms (NOT the location of its electrons). However, the nonbonding electron pairs take up more space than bonding pairs and have the effect of squeezing (decreasing) the bond angles among the atoms. Molecular Geometries for Molecules with Nonbonding Electron Pairs

The shape of ANY molecule is based on the location of its atoms (NOT the location of its electrons). Example: CH 4 Bond angles 109.5° Example: NH 3 Bond angles <109.5° The e - pair here takes up extra space. Example: H 2 0 Bond angles <109.5° The two e - pairs take up extra space. Molecular Geometries for Molecules with Nonbonding Electron Pairs

Going from the Molecular Formula to the Molecular geometry NH 3 molecular formula H—N—H | H Lewis structure electron domain geometry: tetrahedral.. molecular geometry: trigonal pyramid

The nonbonding electron pairs go to the equatorial positions to minimize repulsions. Trigonal Bipyramid Geometry

Octahedral Geometry

Electron Domain Geometries are also Molecular Geometries if there are no nonbonding electron domains linear trigonal planar tetrahedral octahedral trigonal bipyramidal 1 nb domain 2 nb domains 3 nb domains linear bent seesaw T-shaped square planar trigonal pyramidal square pyramidal