2 IntroductionMolecules have shapes and sizes that are defined by the angles and distances between nuclei of atoms.Shape, size, and strength and polarity of bonds determine properties of a substance.We start with Lewis structures to determine the number and types of bonds between atoms.
3 Electron DomainElectron domain: a region in which e-’s will most likely be found.Bonding pair: e- domain between two atoms.Non-bonding pair (lone pair): e- domain located mainly on one atom.An e- domain consists of a nonbonding pair, a single bond, or a multiple bond.
5 VSEPR Model = “Valence Shell Electron Pair Repulsion” Model e-’s are negatively charged, so they repel each other.VSEPR Model says that the best arrangement of a given number of e- domains is the one that minimizes the repulsions among them.
6 VSEPR Model Using Balloons Electron domain geometry: arrangement of e- domains around a central atom.
8 Molecular Geometry The actual spacial arrangement of atoms. Molecular geometry is determined from e--domain geometry.To predict molecular shapes with VSEPR model, draw Lewis structure, count e--domains, then use the arrangement to determine molecular geometry.
9 Molecular Shapes from Electron-Domain Geometries
10 More Molecular Shapes from Electron-Domain Geometries
16 Effect of Non-bonding Electrons & Multiple Bonds Non-bonding pairs and multiple bonds exert greater repulsive forces on adjacent domains and tend to compress bond angles.Lone pair > triple bond > double bond > single bond
17 Bond Angle of WaterWater’s H-O-H bond angle is always 104.5º due to two lone electron pairs.
18 Bond Angle of Methane and Ammonia NH3 has a smaller bond angle than methane due to the lone pair of electrons.
19 Bond DipolesBond dipoles and dipole moments are vectors (magnitude + direction).Overall dipole moment of a molecule is the sum of its bond dipoles.
20 Polarity of WaterIf the water molecule were linear, water would NOT be polar.
22 Polar or Nonpolar?BrClYes. All diatomics with polar bonds are polar molecules.SO2Yes. Bent molecule. O’s more neg.NF3Yes. Trigonal bipyramidal geometry.BCl3No. Symmetry in trigonal planar geometrySF6No. Symmetry in octahedral arrangement.
23 Molecular Shape & Molecular Polarity Molecular polarity has a significant effect on physical and chemical properties.For a molecule with more than two atoms, dipole moment depends on polarities of individual bonds and the molecular geometry.
24 Valence-Bond TheoryCovalent bonds form when a valence atomic orbital of one atom merges with that of another atom.The orbitals overlap (share a region of space).The overlap allows two e-’s of opposite spin to share common space.
25 Formation of Bonds in H2The bond in H2 forms from the overlap of two 1s orbitals from two hydrogen atoms.
35 Sigma () BondSigma () bond: e- density concentrated symmetricaly around the line connecting the nuclei (internuclear axis).Single bonds are bonds.Can be made from s- or p-orbitals.Allows rotation at bond.
36 Pi (∏) Bond∏ bond: covalent bond in which there is a side-to-side overlap of p-orbitals.p-orbitals are perpendicular to internuclear axis.Overlap regions lie above and below internuclear axis.Less orbital overlap than in bonds, so ∏ bonds are weaker.Does NOT allow rotation around bond.
37 Double & Triple BondsDouble bonds consists of one bond and one ∏ bond.Triple bonds consist of one bond and two ∏ bonds.In a double bond, one set of p-orbitals overlap above and below the internuclear axis.In a triple bond, the second set of p-orbitals overlap in front of and behind the internuclear axis.