Chem 112 Exam AID Course Pack

Therefore oh2 has a much smaller bond length than ch4

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Unformatted text preview: COHCO2H, CH2N(CH3)2, SF6, CO2, H2SO4 If structures are charged, put brackets around them and then write the charge (ex +, 2+) in the top right. - to find out which atoms the charges are on, count and see if any of the atoms have greater or fewer bonds than they normally make - for example, oxygen (normally makes two bonds) is negative if it has only one, and positive if it has three - also, the total charges of the atoms in a molecule must add to the total charge on the molecule - other than ionic compounds, no individual atom is ever likely to have a charge greater than +1 or - 1 - it is also unlikely for a molecule to have both positive and negative charges (though this is permissible under certain circumstances, if no other options are available) More precisely, we calculate formal charge: count the number of electrons around an atom (counting only one of any pairs of electrons within covalent bonds) and see if this is more or less than the atom’s group number. - for example, oxygen (group VI) is positive if it has five electrons and negative if it has seven - note that an atom which is not a noble gas may still have a valence shell of eight while still having zero formal charge, because some of those might be shared: formal charge counts only one of the electrons in a covalent bond, while a valence shell counts both - for example, oxygen with two bonds has a formal charge of zero (six electrons) but a valence of eight – while oxygen on its own with a valency of eight (unlikely to exist) would have all eight electrons entirely to itself and would have a formal charge of –2. Atoms which have the same numbers of bonds and lone pairs as each other are termed isoelectronic. - ex any carbon atom is isoelectronic with the N in NH4+ (both have four bonds, no lone pairs) Molecules may also exhibit resonance - occurs when there are multiple valid Lewis structures - the actual molecule is a mix of the individual structures - if the resonance structures are not equally stable, the most stable one (ex one with fewer charges) will dominate Ethan Newton & Barry Zhang for SOS Winter 2012 10 - only electrons may move; atoms may not - especially common when a molecule is charged also more common around electronegative atoms, like oxygen and nitrogen 3.0 Atomic Structure Molecular Geometries - electron pairs assume orientations to minimize electron repulsions Electron Repulsion...
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This note was uploaded on 10/02/2012 for the course CHEM 112 taught by Professor Carran during the Winter '08 term at Queens University.

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