Lab 6 - Molecular Geometry - Lab 6 M olecular Geometr y...

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4/15/13 Lab 6 - Molecular Geometry www.webassign.net/ebooks/wsugencheml1/lab_6/manual.html 1/8 Contents > Lab 6 - Molecular Geometry Lab 6 - Molecular Geometry Purpose A To explore some simple molecular structures. B To explore the relationship between bond order and bond length. C To explore resonance structures. Goals To compare Lewis structures to three-dimensional models. To visualize the three-dimensional structures of some common molecules. To obtain bond angle, bond length, and hybridization data for molecules. To rationalize differences in predicted and measured values. To learn how to use computer modeling software. Introduction The chemical and physical properties of covalently bonded materials are related to the spatial arrangement of the atoms and other electrons not involved in the actual bond formation. There are many ways to depict the spatial arrangement in both two and three dimensions. A Lewis structure is a two-dimensional representation of the arrangement of the atoms, bonding electrons and non-bonding (lone pair) electrons in a covalent material. In a Lewis structure, the nucleus is represented by the atomic symbol with a line between the atoms in a bond depicting each pair of shared bonding electrons in the structure. Non-bonding electrons around the atoms are depicted as dots. The steps to building a Lewis structure representation of a molecule are shown below. The Lewis structure of the formate ion, CHO 2 - , will be used as an example. 1 Calculate the electrons required (ER) = the minimum number of electrons necessary to satisfy the octet rule for the non-hydrogen atoms and the duet rule for hydrogen. For CHO 2 - , this would be (2 electrons × 1 hydrogen
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4/15/13 Lab 6 - Molecular Geometry www.webassign.net/ebooks/wsugencheml1/lab_6/manual.html 2/8 atom) + (8 electrons × 3 non-hydrogen atoms) = 2 + 24 = 26 electrons required. 2 Calculate the number of available valence electrons (VE) = the total number of electrons available for the molecule. For example, in CHO 2 - , this would be (1 C atom × 4 electrons) + (1 H atom × 1 electron) + (2 O atoms × 6 electrons) + (1 electron as the ion has a charge of -1) = 4 + 1 + 12 + 1 = 18 valence electrons. Note: For ions, the charge must be included in this by adding the charge on an anion or subtracting the charge on a cation. 3 Calculate the Shared Pairs (SP) = the number of electrons to be shared in bonds. The SP is 1/2(ER - VE); for CHO 2 - , this would be 1/2(26 - 18) = 4 shared pairs or four bonds. 4 Calculate the Lone Pairs (LP) = the number of electron pairs belonging to only one atom. The LP is 1/2(VE - (2 × SP)); for CHO 2 - , this would be 1/2(18 - (2 × 4)) = 5 lone pairs. Notice that VE = 2 × (SP + LP). 5 Place the first atom in the molecular formula as the central atom, surrounded by the other atoms in the compound.
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