CHEM 150 1nd Edition Chapter 8: Chemical Bonding and Climate Change Lecture 27Outline of Last Lecture • Lewis dot symbols: The atomic symbol surrounded by a dot for each valence electron present in the atom.• Octet Rule: Main group elements gain, lose or share electrons to achieve a set of eight valence electrons.• Lewis Structures for covalently bonded (molecular) compounds:• Bonds are shown as lines. Lone pairs of electrons (non-bonding) are shown as pairs of dots.• General Rules:• All atoms must have a set of eight valence e- in the final molecule. [Except: H, Be, B] • An atom will usually form one bond for each electron it requires. [One bond per unpaired e- in the Lewis dot symbol.]Outline of Current Lecture • Formal charge: • Total the electrons around the atom (bonding electrons are split evenly between the bound atoms). • Subtract this from the atoms normal compliment of valence electrons.• Formal charge can help us decide between different possible structures. The best structure minimizes the formal charges. (Ideally, all formal charges will be zero.)Current LectureWe wrote COCl2 with the structure on the left: Q: Draw Lewis structure for hydrocyanic acid:N2OQ: On our structures for F2O, NH3, CCl4, C3H6, and H2O2, all the formal charges arezero. This was not true for BF4-, PH4+, and CH3COO-, why?!A: -Resonance structures depict the possible electron arrangements for the species. If more than one resonance structure is possible, all equivalent (by formal charge) resonance forms must be shown linked by double-headed arrows. Q: How many resonance structures must be drawn for each of the following? (Draw them)nitrous acidnitric acidnitrite ionnitrate ioncarbonate ion-Resonance Hybrid: The ‘average’ of the resonance structures, an attempt to show the molecule as it really exists.Q: How many resonance structures does sulfate ion have? How many share the best formal charge
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