Chemical Bonding and Molecular Structure (Chapter 9) Ionic vs. covalent bonding Molecular orbitals and the covalent bond (Ch. 10) Valence electron Lewis dot structures octet vs. non-octet resonance structures formal charges VSEPR - predicting shapes of molecules Bond properties polarity, bond order, bond strength 18, 20 Oct 97 Bonding and Structure

Chemical Bonding Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties? 18, 20 Oct 97 Bonding and Structure

Forms of Chemical Bonds There are 2 extreme forms of connecting or bonding atoms: Ionic—complete transfer of electrons from one atom to another Covalent—electrons shared between atoms Most bonds are somewhere in between. 18, 20 Oct 97 Bonding and Structure

- NON-DIRECTIONAL bonding via Coulomb (charge) interaction Ionic compounds - essentially complete electron transfer from an element of low IE (metal) to an element of high electron affinity (EA) (nonmetal) Na(s) + 1/2 Cl2(g)  Na+ + Cl-  NaCl (s) Ionic Bonds - primarily between metals (Grps 1A, 2A and transition metals) and nonmetals (esp O and halogens) - NON-DIRECTIONAL bonding via Coulomb (charge) interaction 18, 20 Oct 97 Bonding and Structure

Covalent Bonding ELECTRONS of each atom in a bond Br Br Covalent bond is the sharing of the VALENCE ELECTRONS of each atom in a bond Recall: Electrons are divided between core and valence electrons. ATOM core valence Na 1s2 2s2 2p6 3s1 [Ne] 3s1 Br [Ar] 3d10 4s2 4p5 [Ar] 3d10 4s2 4p5 Br Br 18, 20 Oct 97 Bonding and Structure

8A Valence Electrons 1A 2A 3A 4A 5A 6A 7A Number of valence electrons is equal to the Group number. 18, 20 Oct 97 Bonding and Structure

Covalent Bonding The bond arises from the mutual attraction of 2 nuclei for the same electrons. A covalent bond is a balance of attractive and repulsive forces. 6_H2bond.mov 18, 20 Oct 97 Bonding and Structure

Bond Formation H Cl MOLECULAR ORBITAL along the line between A bond can result from a “head-to-head” overlap of atomic orbitals on neighboring atoms. H Cl •• • + Overlap of H (1s) and Cl (2p) This type of overlap places bonding electrons in a MOLECULAR ORBITAL along the line between the two atoms and forms a SIGMA BOND (). 18, 20 Oct 97 Bonding and Structure

Sigma Bond Formation by Orbital Overlap Two s Atomic Orbitals (A.O.s) overlap to form an s (sigma) Molecular Orbital (M.O.) 6_H2pot.mov 18, 20 Oct 97 Bonding and Structure

Sigma Bond Formation by Orbital Overlap Two s A.O.s overlap to from an s  M.O. Similarly, two p A.O.s can overlap end-on to from a p M.O. e.g. F2 18, 20 Oct 97 Bonding and Structure

Electron Distribution in Molecules Electron distribution is depicted with Lewis electron dot structures Electrons are distributed as: shared or BOND PAIRS and unshared or LONE PAIRS. G. N. Lewis 1875 - 1946 18, 20 Oct 97 Bonding and Structure

Bond and Lone Pairs H Cl This is a LEWIS ELECTRON DOT structure. Electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. •• H Cl • Unshared or lone pair (LP) shared or bond pair This is a LEWIS ELECTRON DOT structure. 18, 20 Oct 97 Bonding and Structure

Rules of Lewis Structures • No. of valence electrons of an atom = Group number For Groups 1A-4A (Li - C), no. of BOND PAIRS = group number For Groups 5A-7A (N - F), no. of BOND PAIRS = 8 - group No. Except for H (and atoms of 3rd and higher periods), #Bond Pairs + #Lone Pairs = 4 This observation is called the OCTET RULE 18, 20 Oct 97 Bonding and Structure

Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Central atom is atom of lowest affinity for electrons. In ammonia, N is central 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons or 4 pairs 18, 20 Oct 97 Bonding and Structure

Building a Dot Structure 3 BOND PAIRS and 1 LONE PAIR. 3. Form a sigma bond between the central atom and surrounding atoms. 4. Remaining electrons form LONE PAIRS to complete octet as needed. H •• N 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 electrons), while each H shares 1 pair. 18, 20 Oct 97 Bonding and Structure

10 pairs of electrons are left. Sulfite ion, SO32- Step 1. Central atom = S Step 2. Count valence electrons S = 6 3 x O = 3 x 6 = 18 Negative charge = 2 TOTAL = 6 + 18 + 2 = 26 e- or 13 pairs Step 3. Form sigma bonds 10 pairs of electrons are left. 18, 20 Oct 97 Bonding and Structure

Sulfite ion, SO32- (2) O S Remaining pairs become lone pairs, first on outside atoms then on central atom. • O S •• Each atom is surrounded by an octet of electrons. NOTE - must add formal charges (O-, S+) for complete dot diagram 18, 20 Oct 97 Bonding and Structure

Carbon Dioxide, CO2 1. Central atom = __C____ 2. Valence electrons = _16_ or _8_ pairs 3. Form sigma bonds. This leaves __6__ pairs. 4. Place lone pairs on outer atoms. 18, 20 Oct 97 Bonding and Structure

The second bonding pair forms a pi () bond. Carbon Dioxide, CO2 (2) 4. Place lone pairs on outer atoms. 5. To give C an octet, form DOUBLE BONDS between C and O. The second bonding pair forms a pi () bond. 18, 20 Oct 97 Bonding and Structure

H2CO Double and even triple bonds are commonly observed for C, N, P, O, and S SO3 C2F4 18, 20 Oct 97 Bonding and Structure

Sulfur Dioxide, SO2 1. Central atom = S 2. Valence electrons = 6 + 2*6 = 18 electrons or 9 pairs 3. Form pi () bond so that S has an octet — note that there are two ways of doing this. 18, 20 Oct 97 Bonding and Structure

O = S = O Sulfur Dioxide, SO2 RESONANCE STRUCTURES Equivalent structures called: RESONANCE STRUCTURES The proper Lewis structure is a HYBRID of the two. A BETTER representation of SO2 is made by forming 2 double bonds Each atom has - OCTET - formal charge = 0 O = S = O 18, 20 Oct 97 Bonding and Structure

Urea (NH2)2CO 1. Number of valence electrons = 24 e- 2. Draw sigma bonds. Leaves 24 - 14 = 10 e- pairs. 3. Complete C atom octet with double bond. 4. Place remaining electron pairs on oxygen and nitrogen atoms. 18, 20 Oct 97 Bonding and Structure

Violations of the Octet Rule Usually occurs with: Boron elements of higher periods. SF4 BF3 18, 20 Oct 97 Bonding and Structure

Boron Trifluoride Central atom = B Valence electrons = 3 + 3*7 = 24 or electron pairs = 12 Assemble dot structure The B atom has a share in only 6 electrons (or 3 pairs). B atom in many molecules is electron deficient. 18, 20 Oct 97 Bonding and Structure

Sulfur Tetrafluoride, SF4 Central atom = S Valence electrons = 6 + 4*7 = 34 e- or 17 pairs. Form sigma bonds and distribute electron pairs. 5 pairs around the S atom. A common occurrence outside the 2nd period. 18, 20 Oct 97 Bonding and Structure

Formal Atom Charges Formal charge = Group no. Atoms in molecules often bear a charge (+ or -). Formal charge = Group no. - 1/2 (no. bond electrons) - (no. of LP electrons) The most important dominant resonance structure of a molecule is the one with formal charges as close to 0 as possible. 18, 20 Oct 97 Bonding and Structure

Carbon Dioxide, CO2 O C 6 - ( 1 / 2 ) 4 = 4 - ( 1 / 2 ) 8 = At OXYGEN O C • • • 4 - ( 1 / 2 ) 8 = At CARBON 18, 20 Oct 97 Bonding and Structure

O C O C + + Carbon Dioxide, CO2 (2) C atom charge is 0. + An alternate Lewis structure is: 6 - ( 1 / 2 ) = + O C • • • C atom charge is 0. 6 - ( 1 / 2 ) = + O C • • • + AND the corresponding resonance form 18, 20 Oct 97 Bonding and Structure

O C O C + Carbon Dioxide, CO2 (3) Which is the predominant resonance structure? O C • • • + O C • • • OR Answer ? Form without formal charges is BETTER - no +ve charge on O REALITY: Partial charges calculated by CAChe molecular modeling system (on CD-ROM). +1.46 -0.73 18, 20 Oct 97 Bonding and Structure

Boron Trifluoride, BF3 What if we form a B—F double bond to satisfy the B atom octet? 18, 20 Oct 97 Bonding and Structure

Boron Trifluoride, BF3 (2) + F •• B • fc = 7 - 2 - 4 = +1 Fluorine fc = 3 - 4 - 0 = -1 Boron To have +1 charge on F, with its very high electron affinity is not good. -ve charges best placed on atoms with high EA. Similarly -1 charge on B is bad NOT important Lewis structure 18, 20 Oct 97 Bonding and Structure

Thiocyanate ion, (SCN)- Which of three possible resonance structures is most important? A. S=C=N B. S=C - N C. S-C N -0.52 -0.32 -0.16 Calculated partial charges ANSWER: C > A > B 18, 20 Oct 97 Bonding and Structure