AP+Chem+Lab+6


 * Shapes of Molecules and Polyatomic Ions **

One of our objectives in chemistry is to explain the properties of macroscopic samples in terms of the nature and behavior of the molecules that make up the sample. As you might expect, there are wide variations in the properties of individual molecular substances. Properties such as melting points and boiling points depend on attractions between molecules. Such properties are influenced by the polarity of the individual molecules. We would expect the properties of substances composed of polar covalent molecules to differ from those composed of nonpolar molecules. Polarity, in turn, depends on the shape of the molecule and the relative electronegativities of the atoms involved. Shapes of molecules, however, also influence the properties of nonpolar substances. thus, structural isomers whose chemical compositions are identical, differ in structure (shape) and in properties. The shape of a molecule depends on the angle between atoms bonded to a central atom. This angle, known as the bond angle, is the angle formed by two imaginary lines which join, respectively, the nuclei of each of two outlying atoms to the nucleus of the central atom. The distance between the nuclei of two bonded atoms is known as the bond length. The shape of the molecule is outlined by imaginary lines which join the nucleus of each of the outlying atoms to the nuclei of adjacent atoms. One simplified theory of molecular geometry, called the “valence-shell, electron-pair repulsion theory,” assumes that the shapes of molecules (and other species) are related to the potions of the valence-electron clouds on the central atom of the molecule. Each negative charge cloud, which contains two electrons of opposing spins, tends to repel all other charge clouds in the vicinity. To achieve a condition of minimum potential energy, it is necessary that the charge clouds be located so that they will be as far apart as possible. In this position, the electrostatic repulsion between the clouds is at a minimum. The spatial orientation of the charge clouds depends on the number that are present and on their size. The number is equal to the total number of electron-pairs (bonded and unbonded) in the valence level of the central atom as indicated by the Lewis structure of the species. A double or triple bond involving, respectively, two or three electron pairs is counted as or considered equivalent to a single electron cloud for purposes of determining spatial orientation. The relative size depends on whether the electron-pair is bonded or a lone (unbonded) pair. We would expect clouds associate with bonded electrons to be rather localized between nuclei and to take up less space than those associated with unbonded electrons. In this activity, you will be given a set of atomic models and asked to identify structural features of a chemical species associated with the model. Electron-pairs are represented by a stem that does not have an atom attached to it. You should be aware, of course, that the shapes and bond angles you identify are idealized. you are to use the symmetry of the molecular models as a guide to the polarity of the species. Assume that the “atoms” attached to the central “atom” are identical. As a follow-up activity you are to construct a model for a species whose formula is furnished by your instructor.
 * __Introduction:__**


 * __Objectives:__**
 * 1) To relate the shapes and bond angles of chemical species to the number of bonded and unbonded electron pairs in the valence level of the central atom in the species.
 * 2) To relate the polarity of molecules to the shape and symmetry of the species.

This list may help: linear, trigonal planar, angular, tetrahedral, pyramidal, trigonal bipyramidal, seesaw, T-shaped octahedral, square planar, square-bases pyramid, pentagonal bipyramidal
 * __Procedure:__**
 * 1) Examine each model and identify: (a) the number of electron-pairs in the valence level of the central atom (assume no multiple bonds), (b) the number of bonded atoms (do not count the central atom), (c) the number of lone pairs (un-bonded electron pairs), and (d) the bond angles. Record your observations in Table 1.
 * 2) Write in Table 1 the shape of the species represented by each of the models in your set.
 * 1) Assuming all the models represent neutral molecules, indicate whether each species is polar or nonpolar.
 * 2) (a) Write the general formula for a chemical species with each of the shapteslisted in the table. Use the symbol X to represent the central atom and the symbol Y to represent the atoms (all identical to each other) bonded to the central atom. (b) Write a formula showing all atoms and the number of lone pairs. Represent the lone pairs of electrons with the symbol E. For example, the formula for a species composed of two atoms of Y bonded to an X atom which still has two unbonded pairs of electrons in its valence level would be XY2E2.

In this activity you used the number of atoms bonded to a given atom and the number of lone-pair valence electrons as a guide to the shape of a molecule. You found that the shapes of species in which there are lone pairs of valence electrons are related to the shapes of species in which all pairs of valence electrons are involved in bond formation. For example, the shapes of all the species in which the central atom has five pairs of valence electrons can, in theory, be related to the trigonal bipyramid structure. Removal of one atom from a vertex of the equilateral triangle in the central plane of a bipyramid leaves a lone pair of electrons and yields a seesaw structure. It should be noted that the angles between the atoms in the central plane of the bipyramid are identical but differ from the angles between the atoms at the apices of the pyramid and the atoms in the central plane. It can be shown that minimum repulsion exits when lone pairs of electrons in the valence level of the central atom occupy positions in the central plane of the bipyramid. For the purpose of this experiment we assumed that some of the models represented double bonded structures. It is possible to predict the presence of a double bond in the Lewis structure of the species by comparing the total number of valence electrons in the central atom with the sum obtained by adding the number of lone pairs to the number of atoms bonded to the central atom. For example, if there are four pairs of valence electrons, three atoms bonded to a central atom and no lone pairs, then the Lewis structure for the species must contain a double bond, be trigonal planar and nonpolar. Another assumption we made in this activity was the identical atoms were bonded to a central atom. With this assumption, four atoms bonded to a central atom having no lone pairs gives rise to a symmetrical tetrahedral species which is non-polar. If the attached atoms were not identical, then the symmetry would be destroyed, the molecule would be polar, and its shape would resemble a distorted tetrahedron. You should also keep in mind that the geometry of polyatomic ions can be described in the same terms as those for neutral molecules. Polarity, however, is a term reserved for describing the electric nature of neutral molecules, not ions. Although partial charges are associated with molecular dipoles, ions carry a unit charge or a multiple of the unit charge. For example, sulfate ions (SO42-) although tetrahedrally shaped with no lone electron pairs on the sulfur atom, carry a net charge of 2-. As noted earlier, the degree of molecular polarity has an important bearing on the behavior of substances. Quantitatively, the degree of molecular polarity may be expressed in terms of a dipole moment. Dipole moments can be experimentally determined by means of electric measurements and then used to calculate the percent ionic character in a bond and to help determine the geometry of a molecule. For example, knowing that carbon dioxide has a zero dipole moment enables us to predict that the molecule is linear.
 * __Post lab Discussion:__**

Divide your lab book page into 12 quadrants. In each square write the question number and letter, the formula of each molecule given below, the Lewis structure, the shape, the electron geometry, the hybridization and draw the arrow indicating the dipole moment if one is present. BE NEAT. g) PH3 h) BCl3 i) SO2 j) TeCl4
 * __Post Lab Questions:__**
 * 1) a) CH4 b)CH3Cl c) CCl4 d)C2H4 e) SO3 f)SCl2

a) CO32- b) SO42- c) SO3 d) CO2 e) SiO32- f)SO32-
 * 1) Which of these involve double bonds in their Lewis structures? Assume that all atoms follow the octet rule.

a) CS2 b) BO33- c) TiCl4 d) TeO2 e) IO- f)SCl2 g) SiO32- h) SF4 i) AsF5 k) XeF2 l) MoF6 m) ICl4- n) IF5
 * 1) What is the shape of the following species?

Hydrogen peroxide (H2O2) and ozone (O3) have a dipole moment. Are these symmetrical molecules? Explain. Are they polar or non polar?