Chemical Bonds, Molecular Models and Shapes

Transcription

1 Chem 100 Section Experiment 6 Chemical Bonds, Molecular Models and Shapes Introduction The properties of chemical compounds are directly related to the ways in which atoms are bonded together into molecules. Chapter 2 in Chemistry in Context 5th Ed. presents the basic ideas of chemical bonding, while Chapter 3 shows how the three-dimensional shapes of molecules are related to the bonding. In this exercise, you will have the opportunity to apply your knowledge from those two chapters by constructing simple ball-and-stick models for some common molecules. The models should help your understanding of electron arrangements in molecules and the resulting shapes of the molecules. You will investigate a number of small molecules containing carbon, nitrogen, oxygen, and hydrogen, as well as a few molecules containing fluorine, chlorine, or sulfur. These are mostly substances that are important in the atmosphere and in polluted air, as discussed in Chapters 1, 2, and 3 of Chemistry in Context. In the process of doing this exercise, you will see how "models" become very useful to chemists in understanding and predicting chemical properties. Background Information The existence of chemical compounds with fixed (or constant) composition implies that the atoms in compounds must be connected in characteristic patterns. Early models showed the atoms hooked together like links on a chain. Modem representations are a good deal more abstract and often mathematical in nature. Nevertheless, it is possible to represent molecular structures with reasonable accuracy by using relatively simple models. The models serve as a three-dimensional representation of an abstract idea. Molecular model building has proven so useful that it is rare to find a chemist who does not have a model kit close at hand. The chemical bonds that hold atoms together in molecules generally consist of pairs of electrons shared between two atoms. Atoms tend to share outer electrons in such a way that each atom in the union has a share in an octet of electrons in its outermost shell. This generalization has come to be known as the octet rule. (You should review the discussion of Lewis structures and the octet rule in Chapter 2 of the text.) The location of each element in the periodic table provides information about the number of electrons in the outermost level of the atoms. Carbon, for example, is in Group 4A and has four outer electrons; thus, it must share four additional electrons from other atoms in order to achieve a share in eight outer electrons (an octet). This is summarized in the table at the top of the next page. Oxygen, in Group 6A, has six outer electrons and shares two electrons from other atoms in order to achieve an octet. Hydrogen is a special case, needing to share its one electron with only one electron from another atom in order to achieve the stable outer electron configuration of the nonreactive element helium (He). A single bond consists of one shared pair of electrons; a double bond is two shared pairs (i.e., 4 electrons), and a triple bond is three shared pairs (6 electrons). On paper, the bonds are represented by single, double, or triple lines, respectively (-, =, ). In model kits, straight sticks represent single bonds, while double and triple bonds are represented by pairs or triplets of curved sticks or springs. Electrons not involved in bonding are termed unshared electrons. 6-1

2 Electron Configurations in Atoms and Molecules Atom Outer electrons Electrons needed from another atom Carbon Nitrogen Oxygen Fluorine & Chlorine Hydrogen Sulfur Electrons shared (no. of bonds) An important part of this exercise involves identifying the three-dimensional shapes of molecules. (Molecular shapes are discussed in the text in Chapter 3.) Molecules have certain shapes depending on their component atoms and the ways in which they are bonded to each other. The important shapes encountered in this exercise are linear, bent, triangular, pyramidal, or tetrahedral. Several factors contribute to determining molecular shape: (1) Electron pairs (both shared and unshared) try to keep as far away from each other as possible, while still remaining "attached" to atoms. (After all, they are all negatively charged, and electrical charges of the same type will repel each other.) (2) Electron pairs tend to be symmetrically arranged around each atom in a three-dimensional manner. (3) Electron pairs not involved in the bonding ("unshared pairs" or "lone pairs") are equally as important as bonding electron pairs (shared pairs) in determining the overall molecular shape and arrangement of atoms. Model Building Basics Molecular model kits vary; therefore, your instructor will explain the particular models that you will use. The kit probably contains balls (used for atoms), sticks (used for single bonds and unshared electron pairs), and springs or curved sticks (used for double and triple bonds). Each stick or spring represents two electrons. Hydrogen atoms are usually represented by small, light-colored balls (yellow, white, or pale blue) that have only one hole. The color code for other atoms will vary. A common set of colors is shown on the table below. Typical Color Code for Molecular Model Sets Atom Hydrogen Carbon Nitrogen Oxygen Fluorine or Chlorine Sulfur Color Yellow or white Black Blue Red Green or purple Yellow Note: There is one disadvantage to using the colored balls provided in most model sets. They usually have only enough holes for the correct number of bond pairs, and thus you will not be able to see the unshared electron pairs. An alternate approach is to use balls with four holes in them for all atoms other than hydrogen so that the octet (four pairs of electrons) will always be visible. 6-2

3 Model Building Basics (continued) 1. Assemble the atoms required. (For example, to make the CH 4 molecule, you will need one black or blue sphere and four yellow ones.) Next, note the group in the periodic table to which each element belongs. The number of the group is also the number of outer electrons in an atom of the element. 2. To determine how many sticks (pairs of electrons) you will need, divide the total number of outer electrons by 2. For example, H 2 O has one outer electron from each hydrogen and six from oxygen, for a total of eight. Hence, you will need four sticks to represent all the electrons in H 2 O. Two sticks represent bonds between H and O, and two sticks represent unshared electron pairs. 3. If there is only one atom of one element in the molecule and more than one atom of another element, the single atom usually goes in the center of the molecule. This is the case in CO 2, but there are a few exceptions to this rule (such as N 2 O, which has the arrangement NNO). 4. With the collected parts, assemble the model in such a way that each atom except hydrogen has a share in an octet of electrons. If you do not appear to have enough sticks (electron pairs) to give each atom (except hydrogen) an octet, try sharing more electrons by forming double or triple bonds (replace straight sticks with curved sticks or springs). The Assignment 1. Each pair of students should have a model set. First, get acquainted with the components of the set. Note the holes in the various colored balls and their positions. If there are two lengths of sticks, the short ones are for bonds involving hydrogen, and the longer ones are for any other single bond. 2. Using the procedure outlined above, build models for each of the molecules listed on the data sheet. Then use information obtained from viewing the models to fill in the information in the last two columns. You should take time to think about (and write down in words and a diagram) the shape of each molecule before proceeding to the next one. Questions To Be Answered After Completing This Experiment In the space provided, write out answers to the following questions and turn them in along with the entire experiment (procedures and data sheets). 1. The tetrahedral shape is one of the most fundamental shapes in chemical compounds. How would you describe it in words to someone who has never seen it? 6-3

4 2. The octet rule appears to be a very important rule governing the structures of molecules. In light of your work with models, provide a simple explanation for the importance of eight electrons. 3. Explain in your own words why nonbonded electron pairs help determine the shapes of molecules. 4. Do all of the assigned molecules obey the octet rule? If not, why (or in what way) did the octet rule fail? 5. As a test of what you have learned, predict the shapes of (a) NF 3, (b) H 2 S, (c) Cl 2 O. 6. Models do not necessarily have to be physical objects. They can be two-dimensional drawings or even mental constructs. Cite one or more examples of such models encountered outside of chemistry. Can you think of models that are used in your own field of study or that you will use in your future career? 6-4

5 Experiment 6 Date Chem 100 Section Chemical Bonds, Molecular Models and Shapes Molecule Total outer electrons Lewis Structure Geometry of the atoms (Sketch the structural formula) Methane CH 4 Ammonia NH 3 Water H 2 O Fluorine F 2 Oxygen O 2 Nitrogen N 2 Carbon dioxide CO 2 Ozone O 3 Sulfur Difluoride SF 2 Dichloroethylene C 2 H 2 Cl 2 Hydrazine N 2 H 4 Hydrogen Peroxide H 2 O 2 6 5

6 Experiment 6 Date Chem 100 Section Data Sheet page 2 Molecule Total outer electrons Lewis structure Geometry of the atoms (Sketch the structural formula) CF 2 Cl 2 (CFC-12) CHF 2 Cl (CFC-22) Sulfur dioxide SO 2 Suflur trioxide SO 3 Carbon monoxide CO Formaldehyde H 2 CO Additional Challenge Nitric Oxide NO Additional Challenge Nitrogen dioxide NO 2 Additional Challenge Thionyl chloride SOCl 2 6 6

7 Experiment 6 Date Chem 100 Section Data Sheet page 3 EXTRA CREDIT (5): The following article was taken from Chemical & Engineering News in It purports to discuss the N 5 + cation. In the space below, propose a Lewis Structure for this ion. To receive maximum credit, you must show all work leading up to your proposed structure. 6 7