For the purposes of calculating formal charge the atom is considered to have

For the purposes of calculating formal charge the

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bonded state. For the purposes of calculating formal charge the atom is considered to have full ownership of all lone pair electrons, and half-ownership of all bonded electrons (equal sharing). In terms of numbers of electrons (not electron pairs): Z = N valence – N LP – 1/2 N BP where N valence is the number of valence electrons in the free neutral atom, N LP is the number of lone pair electrons in the bonded state, and N BP is the number of bonded electrons in the bonded state. For example, in the structure of the carbonate ion given above, we have three different kinds of atom: C (always has 4 bonds): Z = 4 – 1/2 x 8 = 0 O (doubly-bonded): Z = 6 – 4 – 1/2 x 4 = 0 O (singly-bonded): Z = 6 – 6 – 1/2 x 2 = -1 The formal charges add up to the true total charge of the ion, which is -2. Rules of thumb for formal charge: 1. Formal charges should be small for covalent molecules. 2. More electronegative atoms should have more negative formal charges. 3. Molecules should not contain adjacent atoms with like charges. Exercises: Pick one of the lists below (each contains 10 molecules) and work through it in the spaces provided on the following pages. Unless otherwise stated, these compounds all contain a central atom bonded to several surrounding atoms. List A: (1) SiCl 4 , (2) PCl 3 , (3) NO 3 - , (4) SF 6 , (5) PCl 5 , (6) IF 3 , (7) XeF 5 + , (8) SO 4 2- , (9) C 4 H 8 O, (10) C 5 H 12 . ((9) and (10) contain chains or rings of atoms. More than one structure is possible for each of these – such molecules exhibit isomerism .) List B: (1) PO 4 3- , (2) NH 4 + , (3) ClF 5 , (4) XeF 4 , (5) SbF 5 , (6) CS 2 , (7) XeF 2 , (8) C 3 H 8 O, (9) C 6 H 12 , (10) C 6 H 6 . (Multiple structures are possible for (8), which contains a chain or ring of atoms. (9) and (10) should contain rings.) Show: Number of valence electrons, Lewis structure, 3D geometry, formal charges on all atoms. Lab report: Include an Introduction, Lewis structures and 3D shapes for all molecules in your list (which may be scanned or photographed from your worksheet), and a brief discussion of your findings.
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CHE151Lab8_MolecularStructure.doc Dr . Roderick M. Macrae CHE 151 1. Formula: Number of valence electrons: Molecular geometry: 2. Formula: Number of valence electrons: Molecular geometry:
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CHE151Lab8_MolecularStructure.doc Dr . Roderick M. Macrae CHE 151 3. Formula: Number of valence electrons: Molecular geometry: 4. Formula: Number of valence electrons: Molecular geometry:
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CHE151Lab8_MolecularStructure.doc Dr . Roderick M. Macrae CHE 151 5. Formula: Number of valence electrons: Molecular geometry: 6. Formula: Number of valence electrons: Molecular geometry:
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CHE151Lab8_MolecularStructure.doc Dr . Roderick M. Macrae CHE 151 7. Formula: Number of valence electrons: Molecular geometry: 8. Formula: Number of valence electrons: Molecular geometry:
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CHE151Lab8_MolecularStructure.doc Dr . Roderick M. Macrae CHE 151 9. Formula: Number of valence electrons: Molecular geometry: 10. Formula: Number of valence electrons: Molecular geometry:
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