FINAL REVIEW

FINAL REVIEW - Click to edit Master subtitle style 12/20/09...

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Unformatted text preview: Click to edit Master subtitle style 12/20/09 11 1 0 .4 Le w is St ruct ure s: A M e t hodology Pt . 1 1 . Count total number of valence electrons (add electrons for anions, remove for cations). CH 4 2 . Draw a skeletal structure (2 electrons per covalent bond) 3 . Identify terminal atoms; complete octets (duets for H) using remaining electrons added as lone pairs [4 + (4 l 1)] = 8 valence electrons * 8 valence electrons used * 12/20/09 22 10.4 Lewis Structures: A Methodology Pt. 2 4. Are there any electrons left over ? Place remaining electrons on central atom as lone pairs Do all atoms have octets (duets H) ? YES NO YES NO A satisfactory Lewis structure is obtained Form multiple bonds as necessary to complete octets 12/20/09 33 10.4 Writing Lewis Structures: Hints/Tips EXAMPLES Hydrogen atoms are terminal atoms (bonded to only one other atom). The central atom of a structure usually has the lowest electronegativity In oxoacids (HClO4, HNO3, etc.) hydrogen atoms are usually bonded to oxygen atoms. Molecules and polyatomic ions usually have compact , symmetrical structures (clusters rather than chains). 12/20/09 44 Usually, the most plausible Lewis structure is one with no formal charges . When formal charges are required, they should be as small as possible . Negative formal charges should appear on the most electronegative atoms . Adjacent atoms in a structure should not carry formal charges of the same sign. Sum of formal charges = overall molecular charge 10.4 Lewis Structures: Formal Charge FC = #valence e- #lone pair e #bonding pair e- 2 1 12/20/09 55 10.4 Lewis Structures: Formal Charge EXAMPLE LP + BP LP + BP 12/20/09 66 10.6 Exceptions to the Octet Rule: Odd Electrons Molecules with an odd number of valence electrons have at least one of them unpaired and are called free radicals HCH H OH N=O Nitrogen Monoxide: 11 electrons Hydroxyl Radical: 7 electrons Methyl Radical: 7 electrons Check formal charges to find where to put the electron 12/20/09 77 10.6 Exceptions to the Octet Rule: Expanded Octets expanded valence shell means that the central atom has more than eight electrons around it A central atom can have expanded valence if it is in the third period or higher (i.e., S, Cl, P) P Cl Cl Cl P C l C l C l C l C l S F F F F F F EXAMPLE 12/20/09 88 10.7 VSEPR electron groups around the central atom will be most stable when they are as far apart as possible we call this valence shell electron pair repulsion theory H2 O O H H 12/20/09 99 10.7 The Shapes of Molecules We designate formulae for our compounds in terms of electron...
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FINAL REVIEW - Click to edit Master subtitle style 12/20/09...

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