What’s coming up??? Oct 25The atmosphere, part 1Ch. 8 Oct 27Midterm … No lecture Oct 29The atmosphere, part 2Ch. 8 Nov 1Light, blackbodies, BohrCh. 9 Nov 3,5Postulates of QM, p-in-a-boxCh. 9 Nov 8,10Hydrogen and multi – e atoms Ch. 9 Nov 12Multi-electron atomsCh.9,10 Nov 15Periodic propertiesCh. 10 Nov 17Periodic propertiesCh. 10 Nov 19Valence-bond; Lewis structures Ch. 11 Nov 22VSEPRCh. 11 Nov 24Hybrid orbitals; VSEPRCh. 11, 12 Nov 26MO theoryCh. 12 Nov 29MO theoryCh. 12 Dec 1bonding wrapupCh. 11,12 Dec 2Review for exam

Atoms combine to form compounds in an attempt to obtain a stable noble gas electron configuration with 8 electrons in the valence shell THE OCTET RULE A stable electronic configuration can be attained in two ways……. ELECTRON SHARING ELECTRON TRANSFER IONIC BONDING COVALENT BONDING LEWIS MODEL OF BONDING

electron transfer IONIC BONDING Lewis Symbol symbol for element and a dot for each valence electron 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 3p 5 Cl Na Na +  1s22s22p61s22s22p6 1s 2 2s 2 2p 6 3s 2 3p 6 Cl Na Cl

electron sharing FF COVALENT BONDING Atoms go as far as possible toward completing their octets by sharing electron pairs Lewis Symbols….. Consider F 2 FF The electronic configuration of F is 1s 2 2s 2 2p 5 These are combined to form F 2

electron sharing F 1s 2 2s 2 2p 5 + FF + FF or FF non-bonding, or lone pair of electrons bonding pair of electrons COVALENT BONDING Atoms go as far as possible toward completing their octets by sharing electron pairs The simplest molecule

The type of bond that forms depends on the electronegativity difference between the two atoms involved in the bond IONIC OR COVALENT?

IONIC VERSUS COVALENT BONDS Compounds composed of elements with a large difference in ELECTRONEGATIVITY tend to have significant ionic character in their bonding A B B has a greater share

H + + FFH HYDROGEN FLUORIDE Fluorine is more electronegative than hydrogen.

LEWIS BONDING MODEL Lewis structures are based on a localized electron model Electrons are always localized in one of two ways: As lone pairs on a specific atom OR As bonding pairs between two specific atoms

Drawing Lewis Structures OHH non-bonding, or lone pair of electrons bonding pair of electrons Lewis structures consider only valence electrons Water How do we draw these????? H2OH2O

Building Lewis (ELECTRON DOT) Structures of Molecules. HCN as an example... Step 1. Count the total number of valence electrons C has 4N has 5 H has 1Total of 10 Step 2. Place one e - pair between each BONDED atom HCN We have 6 e - left All atoms must have an octet or duet Add electrons to terminal atoms first Step 3. to get an octet or duet.

Building Lewis (ELECTRON DOT) Structures of Molecules. Add 6 electrons in pairs to give the N an octet. Step 3. Add remaining electrons to terminal atoms first HC N Step 4.Add any electrons left over to central atom Do all atoms have an octet? We have none left! Step 5.Check for an acceptable Lewis Structure IN THIS CASE

Building Lewis (ELECTRON DOT) Structures of Molecules. HCN Step 5.Check for an acceptable Lewis Structure bring electron pairs from outer N atom to form shared pairs to give C its octet!!! HCN Still no octet on CDo it again!!!! HCN three electron pairs between the C and N………...

Another possible structure is…. HCN Building Lewis (ELECTRON DOT) Structures of Molecules. HCN HNC How can we choose? FORMAL CHARGE

Structures with the lowest formal charges are likely to have the lowest energy. Each atom is assigned all of its lone electrons and Formal charge = #valence electrons _ #unshared electrons _ 1/2#shared electrons { } {} gives an indication of the extent to which atoms FORMAL CHARGE have gained or lost electrons in the process of covalent bond formation. half of the electrons bonded to it.

All possible Lewis structures with stable electronic configurations for HCN and HNC. Calculate formal charge for this one FC on C= /2 (8) = 0 _ 1/2#shared electrons {} Formal charge = #valence electrons _ #unshared electrons { } FC on N = /2 (6)= 0 Hydrogen is zero HCN.. HNCHCN

All possible Lewis structures with stable electronic configurations for HCN and HNC WE CHOOSE THE STRUCTURE WITH THE FORMAL CHARGES CLOSEST TO ZERO AND ANY NEGATIVE FORMAL CHARGES ON THE MOST ELECTRONEGATIVE ELEMENTS HNCHCN

Example: H 2 CO Total number of valence electrons = =12 Put carbon in the middle….. Place electrons between atoms Now add remaining electrons to O Now O has an octet but C does not…. Share a pair between C and O Now we have a double bond between C and O H H OC

Example: H 2 CO Total number of valence electrons = =12 Give C an octet We can write this…. H H OC H H OC C H H O

N has five1s 2 2s 2 2p 3 O has six 1s 2 2s 2 2p 4 Count up valence electrons Let’s look at the nitrate anion NO 3 - Plus one extra for negative charge Valence electrons =5 + 3 x = 24

Put a pair between each atom Nitrate anion NO 3 - N O OO nitrogen does not have noble gas structure!!! form a double bond by sharing a pair from one of the oxygen atoms……….

FORM A DOUBLE BOND BETWEEN O AND N Here is one Here is another! N O OO - N O OO - N O OO -

Experiment shows all three bonds are the same. Any one of the structures suggests one is different! OO O N All bond angles All bond lengths 128 pm N O OO - Double Bond Single Bond Should be different! So…….

We use a double headed arrow between the structures.. The electrons involved are said to be DELOCALIZED over the structure. The blended structure is a RESONANCE HYBRID RESONANCE N OO O OO O NN OO O

S OO S OO SO 2 We say that the real SO 2 molecule is a hybrid of the two resonance forms. Experiment shows that both S-O bonds are equivalent. LEWIS STRUCTURE

EXCEPTIONS TO THE OCTET RULE……. Molecules with more than 8 electrons around central atom. Molecules with unpaired electrons. Molecules with less than an octet around central atom Lets do SF 6 ……..

Elements in rows 3 and following can exceed the octet rule: F S F F F F F SF 6 I II - I3I3 When it is necessary to exceed the octet rule the extra electrons go on the central third row element. S … 12 Central I … 10

Molecules which have unpaired electrons. NO 2 Is a free radical Total number of valence electrons = = 17 ONO FREE RADICALS Form double bond to get N close to octet ONOONO RESONANCE