OBJECTIVE THE GEOMETRICAL STRUCTURE OF MOLECULES Draw the Lewis structures of covalently bonded molecules and use these structures to predict their geometrical shapes using VSEPR theory. DISCUSSION Lewis structures give basic information in two-dimensional representations that can be used to predict the three dimensional geometrical shapes of molecules. models can be constructed from ball and stick sets or a combination of sticks, springs, and balls. The balls represent atoms and the sticks or springs represent bonds. In a model kit, each different ball color represents a different element. For each element, the ball has the correct number of holes drilled at proper angles so that, when used to make a complete model, will give proper representation of the correct shape of the molecule. Thus we can learn a lot about the shape of the molecules by building models. SHAPES OF MOLECULES: Molecules come in a number of shapes. Among these shapes predicted by VSEPR theory are: Linear: Two pairs of electrons around atom in the center. Bond angles are 180. Trigonal Planar: Three pairs of electrons around the central atom. Bond angles are 120. Tetrahedral: Four pairs of electrons around atom in the center. Bond angles are 109.5. Bent: Molecules consist of three atoms not arranged in a straight line. Typically observed if two unshared pairs of electrons surround the central atom. Trigonal Pyramidal: Four atoms arranged in the shape of a pyramid. Four atoms with one not in the plane of the other three. One unshared electron pair on the atom in the center. Hauser 11/09 1
MOLECULAR DIPOLE: An understanding of the concept of "symmetry" is important when considering molecular geometry. In simple terms, a symmetric molecule is "the same" on both side of the middle of a structure. However, "the same" might refer to equivalent forces rather than just bonds and atoms. If the molecule is symmetrical, then the compound is considered to have no dipole moment (nonpolar) and it may have a shape like linear, trigonal planar, or tetrahedral. If the molecule is unsymmetrical, then the compound has a dipole moment (polar) and the shape of the molecule may be bent, linear, or several other types of geometries. PROCEDURE 1. Obtain a model kit from the supply area. Use the following color scheme to identify each type: Atom Color # of Holes hydrogen yellow one carbon black four oxygen red two nitrogen blue three halogens purple / green / orange one 2. Sticks are used to represent single bonds, and springs are used for representing each bond of double bonds and triple bonds. Each stick or spring represents a shared pair of electrons. 3. Determine the molecular formula for each compound listed on the DATA SHEET, count the valence electrons, and draw the Lewis structure for each compound. Do not build a ball-stick model until you are certain the Lewis structure is correctly configured. If not, you will be building with a bad "blueprint." 4. Construct a model for the compound using the balls, sticks, and (possibly) springs. Make sure that all holes are filled with either a stick or a spring (An exception is the "blue" ball - it CAN have left-over holes.) Make sure that if your Lewis structure indicates a double bond that you use two springs; use three springs for a triple bond. 5. Determine the molecular geometry of the molecule. You may wish to refer to handouts or your text that describe molecular geometries. The compounds on the DATA SHEET with a "*" will not fit any of our defined geometries. For these Hauser 11/09 2
compounds, please make up a descriptive name that illustrates the molecule's shape. For example, one of these structures will resemble a "French poodle."! 6. Examine the model you have made for symmetry, then decide whether it has a dipole ("D") or no dipole ("ND"). List the appropriate answer in the dipole column. IF YOU HAVE NOT YET RECEIVED INSTRUCTION ABOUT HYBRIDIZATION, YOU MAY OMIT THE FOLLOWING STEP. ASK YOUR INSTRUCTOR. 7. Look at your Lewis structure (not the model) and determine the hybridization of each interior atom. For example, in the structure shown below, H.... N N H you would need an answer for the left-side N and an answer for the right-side N because these are both interior atoms. 8. Disassemble the structure (you will need the parts) and start the entire process for the next compound on the DATA SHEET. Complete all structures. FINAL THOUGHTS before you build all these structures Lewis structures show bonds and lone pairs, but not shapes. Models show shapes, but not lone pairs. However, the models DO show the result of the influence of the lone pairs. When both Lewis structures and models are considered at the same time, a good representation is achieved. A symmetric geometry where all bonds or forces are offset by equal and opposite bonds or forces usually causes a molecule to have NO dipole moment. Hauser 11/09 3
SUBMIT ONLY PAGES 4 through 6 FOR GRADING NAME: SEC: Valence e- Total PARTNER'S NAME: Name hydrogen fluoride Formula Lewis Structure * = make up a descriptive name Geometry Dipole D or ND Hybridiz ation not applicable for HF water ammonia NH 3 methane CH 4 ethylene* C 2 H 4 acetylene C 2 H 2 Hauser 11/09 4
Valence e- Total Name Formula Lewis Structure Geometry Dipole D or ND Hybridiz ation hydrogen cyanide HCN ethanol* C 2 H 5 OH Use C-C-O as basic framework O = carbon dioxide carbon tetrachloride dichloro monoxide CL 2 O formaldehyde CH 2 O Hauser 11/09 5
PROBLEMS 1) TRUE or FALSE: A bent geometry will always have a dipole moment. (HINT: visualize this geometry or build a model.) 2) Complete the following three pieces of information for each of the compounds shown below: Lewis Structure Geometry Dip or No Dip silicon hydride SiH4 : nitrogen trichloride NCL3: germanium oxide GeO2: 3) Suppose a new compound was synthesized with the formula KrF4. Use the boxes below to draw a proper Lewis structure, predict the molecular geometry, and indicate whether or not it would have a dipole moment. Lewis Structure Geometry Dipole or NO Dipole?! This is a 60 carbon molecule named 'BUCKMINSTERFULLERENE". Can you build it? (Just kidding!) Hauser 11/09 6