Ch 8 Bonding and Molecular Structure 8 1 Chemical Bond Formation 2 general types of bonds 1 Ionic forms when one or more valence electrons is transferred from one atom to another creating negative and positive ions occurs between a metal and a nonmetal 2 Covalent involves sharing of valence electrons between atoms occurs between 2 nonmetals 1 Ionic bonding Examples 2 Covalent bonding Examples 3 In most chemical compounds electrons are shared unequally with the extent of sharing varying widely from very little sharing largely ionic to considerable sharing largely covalent 4 8 2 Covalent Bonding and Lewis Structures Valence Electrons and Lewis Symbols for Atoms Valence electrons the electrons found in the outermost shell usually the highest n number Core Electrons the electrons in the inner shells usually the last noble gas configuration Chemical reactions result in the loss gain or rearrangement of valence electrons Core electrons are not involved in bonding or chemical reactions 5 Useful guideline for main group elements is that the number of valence electrons is equal to the group number Elements in a periodic group family have the same number of valence electrons which accounts for the similarity of chemical properties in a family Valence e for transition elements include the electrons in the ns and n 1 d orbitals 6 Examples of valence e Clicker Q How many valence e does Iodine have 7 Lewis dot structure Named after American chemistry Gilbert Lewis Octet of electrons 8 dots around element most stable just like noble gases 8 Where is the following element found What is the group number Draw a Lewis dot structure with dots for e 9 Lewis Electron Dot Structures and the Octet Rule Covalent bonding a bond results when one or more electron pairs are shared between 2 atoms 10 Bond pair the electrons in the bond Single bond shares 2 e CH4 Double bond shares 4 e CH2CH2 O2 CO2 Triple bond shares 6 e N2 C2H2 Lone pairs nonbonding electrons not involved in bonding 11 Octet rule the tendency of molecules and polyatomic ions to have structures in which eight electrons surround each atom just like the noble gases ethane ethene ethyne 12 Drawing Lewis Electron Dot Structures 1 Determine the arrangement of atoms within a molecule central atom is usually the one with the lowest affinity for electrons least EN 2 Determine total number of valence e3 Place one pair of electrons between each pair of bonded atoms to form a single bond 4 Use any remaining pairs as lone pairs around each terminal atom except H can only have 2 e so that each terminal atoms is surrounded by 8 e 13 5 If the central atom doesn t have eight e change one or more lone pairs on the terminal atoms into a bonding pair to make double or triple bonds 14 Write Lewis dot formula for C3H8 Write Lewis dot formula for NO3 pretend that was a superscript 15 Write a Lewis dot structure for dimethyl ether CH3 O CH3 Write a Lewis dot structure for n propylamine CH3CH2CH2NH2 16 Lewis formulas can also be drawn for polyatomic ions One example is the ammonium ion NH4 17 Predicting Lewis Structures Hydrogen Compounds 18 19 Oxoacids and Their Anions All covalent compounds but since they are acidic they must loose H 20 Write Lewis dot formula for the sulfite ion SO32 21 Isoelectronic Species Reminder isoelectronic means same of electrons 22 8 3 Atom Formal Charge in Covalent Molecules and Ions Formal charge charge that would reside on an atom in a molecule or polyatomic ion if we assume that all bonding e are shared equally not a real charge FC group number of unshared e of bonds 23 Formal Charge Calculation of a formal charge on a molecule is a mechanism for determining correct Lewis structures The formal charge is the hypothetical charge on an atom in a molecule or polyatomic ion The best Lewis structures will have formal charges on the atoms that are zero or nearly zero 24 Rules for Assigning Formal Charge FC 1 FC grp number of bonds of unshared e 2 An atom that has the same number of bonds as its periodic group number has a formal charge of 0 3 a The formal charges of all atoms must sum to 0 in molecules b The formal charges must sum to the ion s charge for a polyatomic ion PO43 25 Consider nitrosyl chloride NOCl Cl N O Cl N O 26 Do Lewis structures for CO2 do formal charges Which one is better 27 Cyanite ion do Lewis structures OCN page 374 28 8 4 Resonance Resonance is a means of representing the bonding in a molecule or polyatomic ion when a single Lewis structure fails to give an accurate picture 29 Resonance There are three possible structures for SO3 The double bond can be placed in one of three places When two or more Lewis formulas are necessary to show the bonding in a molecule we must use equivalent resonance structures to show the molecule s structure Double headed arrows are used to indicate resonance formulas 30 Resonance is a flawed method of representing molecules There are no single or double bonds in SO3 In fact all of the bonds in SO3 are equivalent The best Lewis formula of SO3 that can be drawn is O Delocalization e are shared among more than 2 atoms Resonance structures cannot differ in atom positions only in the arrangement of e pairs Only move e The actual structure is the avg or composite of its resonance structure S O O 31 32 Draw a Lewis structure for ozone How many resonance structures can you draw Clicker Q Draw a Lewis structure for How many resonance structures can you draw 33 8 5 Exceptions to the Octet Rule There are some molecules that violate the octet rule 1 The covalent compounds of Be Only needs 4 e 2 The covalent compounds of the IIIA Group especially B Need 6 e 3 Species which contain an odd number of electrons example NO and NO2 4 Species in which the central element must have a share of more than 8 valence electrons to accommodate all of the substituents Expanded valence shell Hypervalent an atom that exhibits an expanded valence shell 5 Compounds of the d and f transition metals 34 In those cases where the octet rule does not apply the substituents attached to the central atom nearly always attain noble gas configurations The central atom does not have a noble gas configuration but may have fewer than 8 exceptions 1 2 3 or more than 8 exceptions 4 5 Look at the examples in the book 35 Compounds in which an Atom Has Fewer than Eight Valence Electrons B and Be mostly Coordinate covalent bond formation Lewis base nucleophile attacks Lewis acid electrophile 36 37 p 360 Compounds in Which an Atom Has