Nonpolar Covalent Bonds If the electrons are shared equally, it is called a nonpolar covalent bond. (This type of bond only occurs if the electrons are shared between atoms having similar electronegativities and results in no net charges)

Polar Covalent Bonds If the electrons are shared unequally, it is called a polar covalent bond. The unequal sharing results in a partial positive charge and a partial negative charge called dipoles. The atom having the greater electronegativity will have the partial negative charge.

Review

Nonpolar Molecules Molecules consisting of nonpolar bonds are also nonpolar. Examples of nonpolar molecules include all of the diatomic molecules (H2, N2, O2, Cl2, Br2, I2, F2)

Molecules: Polar or Nonpolar Just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. If the dipoles cancel (the dipoles are arranged symmetrically), the molecule is nonpolar. If the dipoles do not cancel (the dipoles are arranged asymmetrically), the molecule is polar. By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

Review of Polarity

“Like Dissolves Like” Nonpolar molecules will dissolve in other nonpolar substances due to their similar structure. Polar molecules will dissolve in other polar molecules due to the force of attraction between the positive end of one polar molecule and negative end of another polar molecule. Ionic compounds will also dissolve in polar compounds. Corn oil does not dissolve in water. Is corn oil polar or nonpolar?

Molecular Geometries Chemistry, The Central Science, 10th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten Molecular Geometries  2006, Prentice-Hall, Inc.

Molecular Shapes The shape of a molecule plays an important role in its reactivity. By noting the number of bonding and nonbonding electron pairs we can easily predict the shape and polarity of the molecule.

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.

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. This molecule has four electron domains.

Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

Electron-Domain Geometries These are the electron-domain geometries for two through six electron domains around a central atom.

Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to that number of electron domains.

Molecular 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.

Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry.

Linear Electron Domain In this domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

Trigonal Planar Electron Domain There are two molecular geometries: Trigonal planar, if all the electron domains are bonding Bent, if one of the domains is a nonbonding pair. *Note: Boron is an exception to the octet rule and tends to form compounds in which boron has fewer than eight electrons around it (an incomplete octet).

Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

Tetrahedral Electron Domain There are three molecular geometries: Tetrahedral, if all are bonding pairs Trigonal pyramidal if one is a nonbonding pair Bent if there are two nonbonding pairs

Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: Trigonal bipyramidal, if all are bonding pairs Seesaw if one is a nonbonding pair T-shaped if two are nonbonding pairs Linear if three are nonbonding pairs *Note: The central atoms in this domain are able to exceed the octet rule. This is only observed in those atoms in period 3 of the periodic table and beyond. The presence of an unfilled “d” sublevel in these atoms allows for this behavior.

Octahedral Electron Domain There are three molecular geometries: Octahedral, if all are bonding pairs Square pyramidal if one is a nonbonding pair Square planar if two are nonbonding pairs

Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

Valence Bond Theory There are two ways orbitals can overlap to form bonds between atoms.

Sigma () Bonds Sigma bonds are characterized by end-to-end overlap.

Pi () Bonds Pi bonds are characterized by Side-to-side overlap.

Single Bonds Single bonds are always  bonds, because  overlap is greater, resulting in a stronger bond and more energy lowering.

Multiple Bonds In a multiple bond one of the bonds is a  bond and the rest are  bonds.

Multiple Bonds In a molecule like formaldehyde (shown at left) one orbital on carbon overlaps in  fashion with the corresponding orbital on the oxygen. The other orbital overlap in  fashion.

Multiple Bonds In triple bonds, as in acetylene, one orbital forms a  bond between the carbons, and two orbitals overlap in  fashion to form the two  bonds.