Ch 115 Final Exam

Resonance

A molecule can be drawn in different ways and this provides stability; the electrons in these molecular are delocalized meaning they can move among the atoms... when writing resomace structures, the same atoms must be bonded to each other in all structures so that the only differences are the arrangements of electrons Ex: Ozone (O3): do to resonance both O—O bonds are the same length(the length of the bond is an average b/w a single and a double bond) and both outer oxygens have a charge of −1/2.

Fewer Than Eight Electrons (lewis structure)

Consider BF3: -Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine...This would not be an accurate picture of the distribution of electrons in BF3.... Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons. • The lesson is: If filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don't fill the octet of the central atom.

Exceptions to the Octet Rule

There are three types of ions or molecules that do not follow the octet rule: 1) ions or molecules with an odd number of electrons... these are radicals (ex: ClO2, NO, NO2, and O2^-) 2) ions or molecules with less than an octet (occurs with boron and beryllium) 3) ions or molecules with more than eight valence electrons... (an expanded octet) (Ex: PCl5, PF5, SF4, AsF6^-, ICl4^-)

Electron-Domain Geometries

• Geometry for 2 electron domains: Linear, 180 degrees • Geometry for 3 electron domains: Trigonal Planar, 120 deg • Geometry for 4 electron domains: Tetrahedral, 109.5 deg • Geometry for 5 electron domains: Trigonal Bipyramidal, 120 deg b/w the equatorial positions and 90 deg b/w the equatorial and axial positions, axial positions are 180 deg fro each other • Geometry for 6 electron domains: Octahedral, 90 deg

Nonbonding Pairs and Bond Angle

• Nonbonding pairs(lone pairs) are physically larger than bonding pairs....Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule. •A bonding pair of electrons is attracted by both nuclei of the bonded atoms, but the nonbonding pair is attracted primarily by only one nucleus... b/c a nonbonding pair experiences less nuclear attraction, its electron domain is spread out more in space than is the elctron domain for bonding pairs. Nonbonding electrons pairs therefore take up more space than bonding electrons... as a result electron domains for nonbonding electron pairs exert greater replusive forces on adjacent electron domains and tend to compress bond angles

What Determines the Shape of a Molecule?

• Simply put, electron pairs, whether they be bonding or nonbonding, repel each other. • By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule. •The shape of a molecule is determined by its bond angles and the bond lengths

Molecular/Structural Geometries

• The electron-domain geometry is often not the shape of the molecule, however. •The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs(does not include lone pairs) Ex: the electron domain geometry of NH3 is a tetrahedral but the molecular geometry is trigonal pyramidal

Tetrahedral Electron Domain

• There are four molecular geometries: 1) Tetrahedral, if all are bonding pairs (Ex: CH4) 2) Trigonal pyramidal, if one is a nonbonding pair (Ex: NH3... the H-N-H angle is 107 deg instead of 109.5 deg do to the lone pair) 3) Bent, if there are two nonbonding pairs (Ex: H2O... the H-O-H angle is 104.5 deg instead of 109.5 deg) 4) Linear, if there are three nonbonding pairs(Ex: HF, the H-F angle is still 180 deg) going from #1- #3 the bond angles are decreaseing do to an increase in the number of nonbonding pairs

Electron Domains

• We can refer to the electron pairs as electron domains. • In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. • To determine the electron domain geometry, All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to the number of electron domains.

Octahedral Electron Domain

•All positions are equivalent in the octahedral domain. • Least stable of the structures •max number of nonbonding electron pairs is 2, so there are only 3 posssible molecular shapes •There are three molecular geometries: 1) Octahedral, all 6 are bonding domains 2) Square pyramidal, if there is 1 nonbonding pair (angles will be a little less than 90 deg 3)Square planar, if there are 2 nonbonding pairs (when there are 2 nonbonding pairs you point them toward opposite sides of the octahedron to reduce repulsion)

Multiple Bonds and Bond Angles

•Double and triple bonds place greater electron density on one side of the central atom than do single bonds...Therefore, they also affect bond angles. (ex: in COCl2 the double bond b/w C and O will cause the angle b/w C and the Chlorines to be less than 120 deg and the O-C-Cl angle to be greater than 120 deg

Linear Electron Domain

•In the linear domain, there is only one molecular geometry= linear. (Ex: CO2) • There are 2 bonding domains and 0 nonbonding domains •NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. (Ex: HF)

Overlap and Bonding

•Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron- electron repulsion. •However, if atoms get too close, the internuclear repulsion greatly raises the energy.

Trigonal Bipyramidal Electron Domain

•There are two distinct positions in this geometry: -Axial(bond length in the axial position is stretched b/c of the electron repulsion from the equatorial positions -Equatorial • Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry • There are four distinct molecular geometries in this domain: 1) Trigonal bipyramidal, if all 5 are bonding domains 2) Seesaw (aka disphenoid), if one is a nonbonding pair 3) T-shaped, if there are two nonbonding pairs 4) Linear, if there are 3 nonbonding pairs ***lone pairs add to the equatorial positions

Trigonal Planar Electron Domain

•There are two molecular geometries: 1) Trigonal planar, if all the electron domains are bonding (Ex: BF3) 2) Bent, if one of the domains is a nonbonding pair.(Ex: NO2) -Lone pairs repel more than bonds... so the angle b/w the lone pair and the oxygen, in NO2, will be greater than 120 deg and the angle b/w the oxygen atoms and nitrogen will be less than 120 deg

More Than Eight Electrons (lewis structure)

•These are called hypervalent molecules and are only formed for central atoms from period 3 and below becuase they are large enough to accomodate all the bonding and electrons... becuase size is a factor, hypervalent molecules occur most often when a central atom is bonded to the smallest and most EN atoms such as F, Cl, and O • The only way PCl5 can exist is if phosphorus has 10 electrons around it. • It is allowed to expand the octet of atoms on the third row or below; Presumably d orbitals in these atoms participate in bonding. •The lesson is: When the central atom is on the third row or below and expanding its octet eliminates some formal charges, do so.