W2-Lewis Structures.pptx

PY4040 Lecture 2 Lewis Structures Professor Fawaz Aldabbagh Methane is the simplest Organic Compound Covalent Bonding – Atoms Share Electrons Drawing Lewis Structures Carbon dioxide, CO2 Total Number of valence electrons = 4 + (2 x 6) = 16 Bromoform: CHBr3 Total Number of valence electrons = 4...

PY4040 Lecture 2 Lewis Structures Professor Fawaz Aldabbagh Methane is the simplest Organic Compound Covalent Bonding – Atoms Share Electrons Drawing Lewis Structures Carbon dioxide, CO2 Total Number of valence electrons = 4 + (2 x 6) = 16 Bromoform: CHBr3 Total Number of valence electrons = 4 + 1 + (3 x 7) = 26 Exceptions to the Octet Rule 1. Molecules with an odd number of electrons (Radicals) 2. Other Natural Radicals (e.g. O2), which do not obey Lewis Structures 3. Molecules in which an atom has less than an octet 4. Molecules in which an atom has more than an octet NO Number of valence electrons = 11 NO2 Number of valence electrons = 17 (the middle structure is wrong) Exceptions to the Octet Rule: O2 Oxygen & Group 3 2S + 1 Rule, where S = spin To get Singlet and Triplet States of Oxygen 2. Less than an Octet However, Lewis acids “accept” a pair of electrons readily from Lewis bases to establish a stable octet More than an Octet Elements from the third Period and beyond, have ns, np and unfilled nd orbitals which can be used in bonding PCl5 P : (Ne) 3s2 3p3 3d0 Number of valence electrons = 5 + (5 x 7) = 40 10 electrons around the phosphorus SF4 S : (Ne) 3s2 3p4 3d0 Number of valence electrons = 6 + (4 x 7) = 34 The Larger the central atom, the more atoms you can bond to it – usually small atoms such as F, Cl and O allow central atoms such as P and S to expand their valency. Formal Charges Formal Charges: the number of valence electrons minus the number of electrons assigned to the atom in the Lewis structure. These are not real charges but help with keeping count of electrons in Lewis structures. E.g., CN- Number of valence electrons = 9 + 1 =10 Question: Draw the Lewis structures of NO+ and determine the formal charges of the atoms. Which Lewis structure is the preferred one? Number of valence electrons = 11 - 1 = 10 1 Structure 1 is preferred because the positive charge is on the least electronegative atom. Learning Outcomes ow how to represent Lewis structures using the total number of valence electrons derstand that resonance is a redistribution of valence electrons. ow that a compound with an odd total number of valence electrons will give a ra n define the term “radical” or “free radical”. n draw the two resonance forms of oxygen, and I can identify the form we breath n define and identify Lewis acids and Lewis bases. n draw a curly arrow mechanism for a Lewis acid and Lewis base reaction to give derstand that S and P can form compounds that hold more than an octet. n assign formal charges on atoms within any compound. n draw Lewis structures of charged molecules and ions, and assign the charge co