18, 20 Oct 97Bonding and Structure1 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 97Bonding and Structure2 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 97Bonding and Structure3 Most bonds are somewhere in between. 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

18, 20 Oct 97Bonding and Structure4 Ionic Bonds 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 Cl 2 (g)  Na + + Cl -  NaCl (s) - NON-DIRECTIONAL bonding via Coulomb (charge) interaction - primarily between metals (Grps 1A, 2A and transition metals) and nonmetals (esp O and halogens)

18, 20 Oct 97Bonding and Structure5 Covalent Bonding Covalent bond is the sharing of the VALENCE ELECTRONS of each atom in a bond Recall: Electrons are divided between core and valence electrons. ATOMcorevalence Na 1s 2 2s 2 2p 6 3s 1 [Ne] 3s 1 Br [Ar] 3d 10 4s 2 4p 5 [Ar] 3d 10 4s 2 4p 5 Br Br

18, 20 Oct 97Bonding and Structure6 Valence Electrons 1A 2A3A4A5A6A7A 8A Number of valence electrons is equal to the Group number.

18, 20 Oct 97Bonding and Structure7 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 97Bonding and Structure8 Bond Formation A bond can result from a “head-to-head” overlap of atomic orbitals on neighboring atoms. HH Cl 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 97Bonding and Structure9 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 97Bonding and Structure10 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. F 2

18, 20 Oct 97Bonding and Structure11 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

18, 20 Oct 97Bonding and Structure12 Bond and Lone Pairs Electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. H Cl This is a LEWIS ELECTRON DOT structure. shared or bond pair Unshared or lone pair (LP)

18, 20 Oct 97Bonding and Structure13 This observation is called the OCTET RULE Rules of Lewis Structures No. of valence electrons of an atom = Group number Except for H (and atoms of 3rd and higher periods), #Bond Pairs + #Lone Pairs = 4 For Groups 5A-7A (N - F), no. of BOND PAIRS = 8 - group No. For Groups 1A-4A (Li - C), no. of BOND PAIRS = group number

18, 20 Oct 97Bonding and Structure14 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons or 1. Decide on the central atom; never H. Central atom is atom of lowest affinity for electrons. In ammonia, N is central Building a Dot Structure Ammonia, NH 3 4 pairs

18, 20 Oct 97Bonding and Structure15 4. Remaining electrons form LONE PAIRS to complete octet as needed. 3. Form a sigma bond between the central atom and surrounding atoms. Building a Dot Structure H H 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 97Bonding and Structure16 Step 2. Count valence electrons S = 6 3 x O = 3 x 6 = 18 Negative charge = 2 TOTAL = = 26 e- or 13 pairs Step 1. Central atom = S 10 pairs of electrons are left. Sulfite ion, SO 3 2- Step 3. Form sigma bonds

18, 20 Oct 97Bonding and Structure17 Remaining pairs become lone pairs, first on outside atoms then on central atom. Sulfite ion, SO 3 2- (2) Each atom is surrounded by an octet of electrons. OO O S NOTE - must add formal charges (O -, S + ) for complete dot diagram

18, 20 Oct 97Bonding and Structure18 Carbon Dioxide, CO 2 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 97Bonding and Structure19 Carbon Dioxide, CO 2 (2) 4. Place lone pairs on outer atoms. The second bonding pair forms a pi (  ) bond. 5. To give C an octet, form DOUBLE BONDS between C and O.

18, 20 Oct 97Bonding and Structure20 SO 3 H 2 CO Double and even triple bonds are commonly observed for C, N, P, O, and S C2F4C2F4

18, 20 Oct 97Bonding and Structure21 Sulfur Dioxide, SO 2 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 97Bonding and Structure22 Sulfur Dioxide, SO 2 Equivalent structures called: RESONANCE STRUCTURES The proper Lewis structure is a HYBRID of the two. A BETTER representation of SO 2 is made by forming 2 double bonds O = S = O Each atom has - OCTET - formal charge = 0

18, 20 Oct 97Bonding and Structure23 Urea (NH 2 ) 2 CO 1. Number of valence electrons = 24 e- 2. Draw sigma bonds. 4. Place remaining electron pairs on oxygen 3. Complete C atom octet with double bond. Leaves = 10 e- pairs. and nitrogen atoms.

18, 20 Oct 97Bonding and Structure24 Violations of the Octet Rule Usually occurs with: Boron BF 3 SF 4 elements of higher periods.

18, 20 Oct 97Bonding and Structure25 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 97Bonding and Structure26 Sulfur Tetrafluoride, SF 4 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 97Bonding and Structure27 Formal charge = Group no. - 1/2 (no. bond electrons) - (no. of LP electrons) Formal Atom Charges Atoms in molecules often bear a charge (+ or -). The most important dominant resonance structure of a molecule is the one with formal charges as close to 0 as possible.

18, 20 Oct 97Bonding and Structure (1/2)(8) - 0 = 6 - (1/2)(4) - 4 = 0 Carbon Dioxide, CO 2 At OXYGEN O C O At CARBON

18, 20 Oct 97Bonding and Structure29 C atom charge is (1/2)(6) - 2 = (1/2)(2) - 6 = -1 Carbon Dioxide, CO 2 (2) O C O An alternate Lewis structure is: AND the corresponding resonance form + O C O +

18, 20 Oct 97Bonding and Structure30 REALITY: Partial charges calculated by CAChe molecular modeling system (on CD-ROM) Carbon Dioxide, CO 2 (3) Which is the predominant resonance structure? O C O OR O C O O C O + + Answer ? Form without formal charges is BETTER - no +ve charge on O

18, 20 Oct 97Bonding and Structure31 Boron Trifluoride, BF 3 What if we form a B—F double bond to satisfy the B atom octet?

18, 20 Oct 97Bonding and Structure32 Boron Trifluoride, BF 3 (2) 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 fc = = +1 Fluorine F F F B fc = = -1 Boron +

18, 20 Oct 97Bonding and Structure33 A. S=C=N Thiocyanate ion, (SCN) - Which of three possible resonance structures is most important? Calculated partial charges ANSWER: C > A > B C. S-C N B. S=C - N