Chemical Bonding - Part II

Gap-Fill Exercise
Content © 2006 Melanie Cecere; Authors of Activity: Melanie Cecere & Tami Maloney; All rights reserved. No commercial, for-profit use of this material is allowed. E-mail comments and questions to Tami Maloney.

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
Electron-dot notation is an electron configuration notation in which only the electrons of an atom of a particular element are shown, indicated by placed around the element's symbol. Electron-dot can also be used to represent . structures are formulas in which atomic symbols represent nuclei and inner-shell electrons, dot-pairs or dashes between two atomic symbols represent electron pairs in covalent bonds, and dots adjacent to only one atomic symbol represent unshared electrons. A structural indicates the kind, number, arrangement, and bonds but not the unshared pairs of the atoms in a molecule.

bonds occur when 2 or 3 pairs of electrons are shared. Those sharing two are bonds; those sharing three are bonds. Double bonds in general have bond energies and are shorter than single bonds. Triple bonds are even stronger and .

Some molecules and ions cannot be represented adequately by a single Lewis structure. One such molecule is (O3), which can be represented by either of the following Lewis structures:

O = O - O : or : O - O = O

Experiments revealed that the oxygen-oxygen bonds in ozone are . Scientists now say that ozone has a single structure that is the average of these two structures. Together the structures are referred to as structures or resonance . Resonance refers to bonding in molecules or ions that cannot be correctly represented by a single structure. To indicate resonance, a double-headed is placed between a molecule's resonance structures.

Molecular Geometry. The properties of molecules depend not only on the bonding of atoms but also on molecular - the 3-dimensional arrangement of a molecule's atoms in space. The of each bond, along with the of the molecule, determines molecular polarity, or the uneven distribution of molecular charge.

-VSEPR Theory. The model (valence shell electrons pair repulsion) states that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as apart as possible. To determine shapes: (1) Draw a dot diagram. Start with the atom. The electrons of the central atoms are either a shared or pair. (2) According to the VSEPR model, each pair of electrons surrounding the central atom is considered to all other electron pairs around that atom. (3) The repulsion causes the electron pair to take a position about the central atom as far away as possible from the other electron .

- Molecules With No Unshared Valence Electrons. For molecules with the general formula AB2, the molecule is linear. The shared pairs are oriented as far away from each other as possible. The angle bond is . For molecules with the general formula AB3, the bonds stay farthest apart by pointing to the corners of an equilateral . The bond angle is 120°. For molecules with the general formula AB4, the mutual repulsion of electron pairs produces a molecule with the shape of a regular (4 faces all are equilateral triangles, having the same area). The bond angle is 109.47°. The 4sp3 hybrid orbitals account for this angle.

- B can represent a single type of , a group of identical atoms, or a group of different atoms of the same . The shape of the molecule will still be based on the forms given in Table 6-5 on page 186. However, different sizes of B groups can distort bond angles (due to differences in , for example).

- VSPER and Unshared Electrons. The general formula for such as ammonia is AB3E, where E represents the electron pair. A has 4 pairs of valence electrons in the molecule, but only 3 are . The unshared pair has a greater repulsive effect than the shared pairs because there is no nucleus on one side of the unshared electron pair to help dissipate the negative charge of the electrons. The greater effect of the unshared pair tends to push the shared electrons closer together. The bond angle is 107°. The shape is . Molecules like water have unshared pairs of electrons with a formula of AB2E2. Only of the four pairs of electrons are shared. The 2 unshared pairs have a greater repulsive effect than the 2 pairs. The combined repulsive effect of the two unshared pairs give the molecule a shape and make the bond angle 105°.

- Multiple Bonds. Double and triple bonds are treated in the same way as bonds. In the case that a central atom is bonded to another atom by a double or triple bond, the second and third shared pairs of electrons in the bond are counted because they do not affect the shape of the molecule. For example, in acetylene (C2H2), each C is considered to be a central atom and since only 1 of the triple bonds between these atoms is counted, each C is considered as sharing two pairs of electrons (1 with H and 1 with the other C). Since they share 2 pairs of electrons, the shape is .

orbitals are orbitals of equal energy produced by the combination of two or more orbitals on the same atom. Methane is a good example. of carbon's valence electrons occupy the 2s orbital and occupy the 2p orbitals. To achieve four equivalent bonds, carbon's 2s (spherical) and three 2p (dumbbell shaped) hybridize to form four new, identical orbitals called sp3 ( shaped). The number of hybrid orbitals produced equals the number of that have combined.

Intermolecular forces: As a liquid is heated, the kinetic energy of its particles . At the point, the energy is sufficient to overcome the force of attraction between the liquid's particles. The particles pull away form each other and enter the phase. Boiling point is therefore a good measure of the force of between particles. The higher the boiling point, the the forces between particles. The forces of attraction between molecules are known as forces.

The strongest intermolecular forces exist between molecules. Polar molecules act as tiny dipoles because of uneven charge distribution. A is created by equal but opposite charges that are separated by a short distance.

- Polar Molecules. When one end of a molecule behaves as if it were and the other positive (like a magnet), you have a dipolar molecule. The uneven distribution of electrons in the molecule is caused by an uneven distribution of one or more polar bonds. For example, in HCl, the shared electron pair is attracted toward the highly electronegative Cl atom and away form the less electronegative H atom. The resulting concentration of negative charge is closer to the chlorine. The end of the molecule containing the Cl atom will be slightly and the H slightly . The direction of a dipole is from the dipole's + pole to the - pole. The - region in one polar molecule attracts the + region in adjacent molecules, and so on throughout the liquid or solid. forces are forces of attraction between polar molecules.

- Nonpolar molecules. Nonpolar molecules result from an uneven distribution of with molecules made of more than atoms. Geometry determines polarity. Nonpolar molecules consist of either all nonpolar bonds or evenly distributed bonds. Polar molecules, on the other hand, have unsymmetrical polar bonds. (See figure 6-26 on page 191).

- Hydrogen Bonding. bonds are formed due to strong electrostatic attraction between the hydrogen in one compound and a strongly electronegative element in a neighboring molecule (F, O, N). Molecules that form hydrogen bonds are highly because the H only has a very small share of the electron pair. The size of the positive charge on the H end is much greater than on an average dipole. Each H acts like an exposed proton. The bond is more than just an electrostatic attraction between opposite charges-it actually has some character.

Normally, as you increase the molecular weight, you the boiling point. Take the compounds H2O, H2S, H2Se, and H2Te. You would think that water would have the lowest boiling point, but it actually has the due to hydrogen bonding. You have to break the hydrogen bonds before the molecules can boil. It takes additional to break the bonds. Hydrogen bonding explains why some substances have low vapor pressures, high heats of vaporization, and/or high melting points. Hydrogen bonding has an effect on the structure of ice. Ice has a lot of hexagonal openings. This accounts for ice's density. When ice is melted, the hydrogen bonds are and the open structure is destroyed and the molecules move closer together.

- Dispersion Forces. These forces are the result of attraction of molecules for nearby molecules caused by the constant of the electron cloud. It is a weak attractive force between (weaker than hydrogen bonds or chemical bonds). Weak forces arise as a result of shifts in the positions of the within the molecule. The shifting produces an uneven distribution of charge. One portion becomes temporarily negative and, by repelling electrons in a neighboring molecule, makes the near end of the neighboring molecule temporarily . The attraction of the opposite charges acts to hold molecules together. It can start a chain reaction. The strength of the London forces is directly related to the number of ; therefore, the larger the molecule (the more electrons), the greater the attraction.
London forces cause heavier molecules to boil at temperatures. They are important only when molecules are together (liquids and solids, or gases at high pressure and low temperature). They are the only intermolecular force acting among noble-gas atoms, molecules, and slightly polar molecules. London dispersion forces are the intermolecular attractions resulting from the motion of electrons and the creation of instantaneous dipoles.