# EXPERIMENT 14: COMPARISONS OF THE SHAPES OF MOLECULES AND IONS USING MODELS

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1 EXPERIMENT 14: CMPARISNS F TE SAPES F MLECULES AND INS USING MDELS PURPSE Models of various molecules and ions will be constructed and their shapes and geometries will be compared. BACKGRUND LEWIS STRUCTURES The Lewis Structure of a single atom, often called a dot diagram, consists of the chemical symbol for an element and a dot for each valence electron. The number of valence electrons of an element is the same as its column number on the periodic table. When Lewis Structures are drawn for molecules, the total number of valence electrons for the molecule must be known. The total number of valence electrons is found by taking the sum of the valence electrons in all of the atoms involved. Next, the molecule is drawn in the order in which the atoms are connected. A pair of electrons is inserted between the connected atoms. This bond can either be written as two dots or as a line. Finally, electrons are added to each atom to until all atoms obtain or share eight electrons. There are some exceptions to this rule: for example, when writing Lewis Structures, hydrogen only ever shares two electrons. Example: The Lewis Structure for hydrogen is The Lewis Structure for oxygen is The Lewis Structure for water is The oxygen now has eight electrons. Each hydrogen has two electrons. GEMETRY The Lewis Structure represents the shape of the molecule in two dimensions. The actual arrangement of the atoms in space involves three dimensions: this can be approximated by considering that arrangement which maximizes the distance between regions of negative charge, or valence shell electron pairs, on the central atom. After determining the number of electron pairs around an atom VALENCE SELL ELECTRN PAIR REPULSIN TERY is used to predict the geometry. The Valence Shell Electron Pair Repulsion (VSEPR) Theory states that the geometric arrangement of atoms or groups of atoms around a central atom is determined solely by the repulsion between electron pairs present in the valence shell of the central atom.

2 There are only five possible geometries: # of electron pair domains Geometry 2 linear 3 trigonal planar 4 tetrahedral 5 trigonal bipyramidal 6 octahedral In general, the geometry can be predicted from the number of electron pairs around the central atom. owever, the possibility of double and triple bonds must be accounted for. Rather than count the pairs of electrons around the central atom, the number of electron pair domains around the central atom are counted. Thus a double bond with two pairs of electrons contributing to the bond counts as just one electron pair domain. The three pairs of electrons in a triple bond also count as just one electron pair domain. SAPE After the geometric arrangement of the electron groups has been described, it is possible to describe the shape. The shape describes the three-dimensional arrangement of the atoms in a molecule. If the molecule has no lone pairs, the shape is the same as the geometry. For molecules with lone pairs, the unshared pairs are used to determine the geometry but ignored when describing the shape. There are several different shapes within each geometry. Example: 2 has tetrahedral geometry but the shape is described as bent. Lewis Structure: Geometry: tetrahedral Shape: bent

3 PLARITY nce the shape of a molecule or ion has been determined, it is possible to predict whether the molecule or ion is polar. A covalent bond in a molecule is polar when two atoms with different electronegativities share the pair of electrons in the bond. The atom with greater electronegativity will be partially negative, while the other atom will be partially positive. A molecule or ion with a polar bond will be polar unless there is a bond (or sum of bonds) that is equally polar in the opposite direction. Any asymmetric molecule will be polar if it has polar bonds and any symmetric molecule will be non-polar whether or not it has polar bonds. If a molecule is polar, the positive and negative areas of the molecule should be shown. There are two methods of showing this described below. Example: Water is a polar molecule: δ + BND ANGLES The bond angles are an important part of the description of the geometry and shape of a molecule in three dimensions. It is possible to predict the approximate bond angles from the geometric arrangement of the atoms in a molecule or ion. Geometry Bond Angles Linear 180 Trigonal planar 120 Tetrahedral Trigonal bipyramidal 90, 120, 180 ctahedral 90, 180 δ δ +

4 YBRIDIZATIN F TE CENTRAL ATM In atoms, the areas where electrons are likely to be found are called orbitals. In molecules, these atomic orbitals become hybridized so that the bonding electrons can be located between the atoms. The hybridization about the central can be predicted if the number of electron domains is counted. The number of electron domains is always the same as the number of orbitals that make up the hybridization. # of electron pair domains hybridization of the central atom 2 sp 3 sp 2 4 sp 3 5 sp 3 d 6 sp 3 d 2

5 MATERIALS ne molecular model kit per pair of students. MDEL KIT ATMS Part White Black (4 holes) Red or black (4 holes) Blue Green Brown Gray Atom ydrogen Carbon xygen Nitrogen alogens Expanded ctets (5 domains) Expanded ctets (6 domains) MDEL KIT ELECTRN PAIRS Part Short straight Long straight Long curved Electron Pair 2 electrons in single bond 2 electrons as a pair but not involved in a bond (lone pair) 2 electrons as part of a double or triple bond MDEL KIT GEMETRIES 1-hole piece 4-hole piece (many colors, most black pieces) 5-hole piece (brown pieces) linear (only used for hydrogen) tetrahedral geometry trigonal planar geometry (using 3 holes) trigonal-bipyramidal (using all 5 holes) 6-hole gray piece tetrahedral geometry (using correct 4 holes) octahedral geometry (using all 6 holes)

6 Name: Date: PRE-LAB QUESTINS 1. When drawing the Lewis Structures (dot diagrams), it is important to know the octet rule. a. State the octet rule. b. Which atoms are allowed to take less than eight electrons when drawing Lewis Structures? c. Which atoms are allowed to obtain more than eight electrons when drawing Lewis Structures? 2. Complete the table below to show the all possible shapes within each of the 3 given geometries. Consult your textbook if necessary. The Five Geometries: linear trigonal planar tetrahedral trigonal bipyramidal octahedral Tetrahedral geometry # of lone pairs 0 Example: Tetrahedral shape 1 Trigonal planar geometry Linear geometry 2 3

7 Name: MDELS Date: PRCEDURE Begin by making a LARGE 10 by 11 table. For each of the molecules below, draw the Lewis Structure in your table (A), build the model, and then answer questions B-I in your table. Keep each model until you have answered all of the questions! Model No. 1: BF 3 (boron trifluoride) Model No. 2: C 4 (methane) Model No. 3: N 3 (ammonia) Model No. 4: 2 (water) Model No. 5: + N 4 (ammonium) Model No. 6: PCl 5 (phosphorus pentachloride) Model No. 7: SF 6 (sulfur hexafluoride) Model No. 8: C 2 (carbon dioxide) Model No. 9: 2 C 3 (carbonate ion) Model No. 10: C 3 Cl (chloromethane, C is the central atom) RESULTS A. Draw the Lewis Structure. B. ow many electron pair domains are present around the central atom? C. ow many of the electron pair domains are bonded to other atoms? D. ow many of the electron pair domains are lone pairs? E. What is the geometry of the molecule? F. What is the shape of the molecule? G. Identify any polar bonds. Is the whole molecule polar or non-polar?. List all bond angles 180 or less. I. What is the hybridization around the central atom?

8 DATA Model A B C D E F G I

9 PST-LAB QUESTINS Do all of the following comparisons before leaving the laboratory if possible. Be sure to consider your answers to the questions on the previous page when you re thinking about the comparisons. Always use the formula or name of the compounds, not the model numbers, to make the comparisons. Write a complete sentence to answer each question. 1. Compare models for C 4, N 3 and 2. Are there similarities in the number of electron pair domains, the geometry, the shape, the hybridization, the polarity, etc? Differences? 2. Compare models for C 4 and N 4 +. Are there similarities in the number of electron pair domains, the geometry, the shape, the hybridization, etc? Differences. 3. Compare models for N 3 and N 4 +. Are similarities in the number of electron pair domains, the geometry, the shape, the hybridization, etc? Differences? 4. Compare models for PCl 5 and SF 6. What do they have in common?

10 5. Compare models for C 4 and C 2. Are there similarities in the number of electron pair domains, the geometry, the shape, the hybridization, etc? Differences? 6. Compare models for 2 and C 2. Are there similarities in the number of electron pair domains, the geometry, the shape, the hybridization, etc? Differences? 7 The model for C 3 2 represents one resonance structure of the carbonate ion. What do the other resonance structures look like? Draw them. 8. Compare models for BF 3 and C 3 2. Are there similarities in the number of electron pair domains, the geometry, the shape, the hybridization, etc? Differences?

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