Lewis Structures PDF

TOPIC 4: Molecular Structure & Bonding (~ 9h) Lewis Symbols For main group elements: Elements in the same group have: - same number of ____________in valence e similar orbitals - similar __________ chemical properties Lewis Electron-Dot Symbol...

TOPIC 4: Molecular Structure & Bonding (~ 9h) Lewis Symbols For main group elements: Elements in the same group have: - same number of ____________in valence e similar orbitals - similar __________ chemical properties Lewis Electron-Dot Symbols Element symbol = nucleus +_____________ valence electrons Dots = ______________ Valence e- H : F : Octet Rule when atoms bond, they want to achieve a full, complete, closed shell of 8 electrons (s2p6 = noble gas electron configuration) 3) draw a valid Lewis structure for simple main group compounds.  How to Draw a Lewis Structure Count the total # of valence electrons in the structure. Anion, add an electron; Cation, subtract an electron Draw the skeletal structure (basic connectivity) with most electropositive atoms as central (H atoms are an exception and are always terminal. Only make rings if told too!!! Each bond between terminal and central atoms contain 2 electrons. Subtract all used electrons. Complete the octets of terminal atoms first. (H require a duet.) Subtract further used electrons. Do any electrons remain? Place remaining on central atom to satisfy octets, and form multiple bonds as necessary to complete octets 3) draw a valid Lewis structure for simple main group compounds.  NH3 - 8 5 + 3x) = H : N : H 8 - 6 = 22 - H ⑪ lone pair H-N - H ↓  Exercise: Draw Lewis structures for the following compounds. Write any non-zero formal charges on the appropriate atoms, show all lone pairs of electrons as pairs of dots, and all bond pairs as lines. (a) CH3Cl (chloromethane) (b) [NH4]+ (ammonium ion) & NH3 a) CHzC b) NHyt 1x4 1 Se 5 = + Total - of e : Total = 4 + 1x3 + 7 = 14e- 14 -8 = 6 - [+ : [m - ) + iH H- - : H 3) draw a valid Lewis structure for simple main group compounds.  Formal Charges Formal charge is the electrical charge difference between the ____________ valence e- in an isolated atom and the number of electrons _________ to that atom in a Lewis structure. assigned It allows us to select the best Lewis structure or the best way to represent the electron distribution around a molecule if there is more than one possibility # = #valence - assigned e- e- Guidelines: 1. Magnitude of Formal Charges: should be as small as possible to be on E N atom want. 2. Sign of Formal Charges : - we 3. Sum of Formal Charges: overall charge 6) understand the concept of formal charges, and determine formal charges for atoms in Lewis structures.  How to Calculate Formal Charge Count the electrons surrounding the atom Bonds count as 1 electron, Lone Pairs count as 2 electrons Determine the number of valence electrons in the neutral atom. (This is same number of electrons in the Lewis Symbol). NH3 5 - 5 = 0 N: Valence 5 = Calculate F.C = (# valence electrons) – (surrounding electrons) 3+ 2 5 assigned = : H- N - + H Label the FC on the respective atom as the charge in a circle. 6) understand the concept of formal charges, and determine formal charges for atoms in Lewis structures.  - 4 + 6x2 = 162 CO2 (carbon dioxide) ~ :: C : 0 : : c = 0 : of FC -i o ii 646 ↓ magnitude : 0= C- : ~ 646 ve- 646 Fc000 -assigned 547 + 0 1 FC - N2O (nitrous oxide “laughing gas”) 5 x2 + 6 = 162 : N :N: i N = 0 : N-0 : - : N = : N 0 5 - : = N = - 556 56 4 5 556 7 7 - = 4 - - 5 - - - 64 - 6 - 2 + 1 + 1 0 + 1 - 1 - 1 10 + ↓ is more E N Oxygen.  Draw the best Lewis structure: 5 4 6 + 1 16e NCO- total valence e + = + Central atom C is not octet = · double/triple bond 122 ; comparing 0 ] needs to make * [ N c : > - : : : 1 is better as the ⑦ charge is on > - 3 possibilities the most election I L ② -3 [iN-c = 0 ] : atom o negative [in = 0-0 ] : [in = c 0 ] = : Magnitude of FC Ve 546 - re- 546 re- 546 - Ae - 7 - 4 - 5 is too big He A -6-4-6 - 5 - 4 - 7 - 201 - 100 00 - 1 NCS- 5 + 4+ 6 + 1 = 16 - Central - atom C is not octet [ N : : C :: ] > - needs to make double/triple bands [in-CES ] - [NEC-5 ] [in 5 ] : c possibilities : = : = - - > - 3 Ve 546 ve 546 Ve5 + 6 1 2- 2 is better Al 5 4 7 - = = Al - 6 - 4 - 6 Ae 7 - 4 - 5 * Comparing 1 EN the - the 8 is on. 00 -1 as - 100 - 20 1 the magnitude Can't be 3 - as of Fe is too big  Resonance Structures Definition: The possibility of more than one valid Lewis Structure due to electron-pair delocalization* actual structure = blend of the resonance structures 6 7 - = - 1 O - I - I - I - I 04 -4 = 0 G 8 8 6 6 - = 0 - 1 #bonds 4 = 1. 33 b 0. = 4) Understand the concept of resonance as it relates to Lewis Structures, and define the term resonance hybrid.  EXERCISE: Draw chemically reasonable structures of the following compounds. Write any non-zero formal charges on the appropriate atoms, show all lone pairs of electrons as pairs of dots, and all bond pairs as lines. (a) O3 (ozone) (b) [NO3]− (nitrate anion) (c) [C2O4]2− (oxalate anion) a)6x3 - 18 = 0 = : : : ::: : b. o = 3 = 1. 5 := - 0 : - 0 = : Ve 666 ve 666 Ae 6 5 7 - - - Ae - 7 = 5 - 6 0 + 1 - 1 - 1 + 10 4) understand the concept of resonance as it relates to Lewis structures, identify and draw resonance forms of Lewis structures, and define the term resonance hybrid  - b) [NO3] 5 + 6x3 + 1 = 24 - : -- : 0 : [ j :N : I : 7- : 0 = N-0 : I 0: : ↑ ~ 7- : : - N = 0 :..... 0. : :  Exceptions to the Octet Rule 1. Incomplete octet: Some compounds containing_______ Be and ________have B less than four pairs of electrons around them. : -B-... 2. Unpaired electrons: Fi : They have an unpaired electron and are known as (free) radicals ==>reactive. 6 lle - NO + 5 = # : N - H · i = it 3. Hypervalence (Allowed Expanded Octets): hI3 beyond periods Can only occur around a central non-metal ________. They have more than 4 pairs of electrons around them (due to larger size/lower EN of central atom and possible involvement of d-electrons) S F Br * N , O, C , , > - obey octet rule 9) recognize common exceptions to the octet rule, and recognize which elements may be capable of adopting an expanded octet structures.  Exceptions to the Octet Rule An example of hypervalence: ↓ - C- ↓ 9) recognize common exceptions to the octet rule, and recognize which elements may be capable of adopting an expanded octet structures.  EXERCISE: Draw chemically reasonable structures of the following compounds. Write any non-zero formal charges on the appropriate atoms, show all lone pairs of electrons as pairs of dots, and all bond pairs as lines. (a) [ClO3 ]−(chlorate) (b) [S2O3]2- (thiosulfate) (c) SO2F2 1 + 6x3 - 1 26 + = a) 6 - 6 = 0 8 - : : : O : : - [0 : ] S = 0 : 6 7= - 1 Fc : T - 7 = 0 7 5 - = 2 1 M : 0 : :: := =- 9) recognize common exceptions to the octet rule, and recognize which elements may be capable of adopting an expanded octet structures.  - b) [S2 03] - 6x2 + 6x3 + 2 = 32 7 1 6 = = - 2 :: like all atoms have octet but [6 , seems let's checkFo to be sure 6 7 = 1 Ec it seems like After calculating - - , > - 6 - the magnitudes are too high 7= 1 - - => Form multiple bonds - => can get 2 general 6 - 6 = 04 6 - 6= 0 1( structure I ↓ : 0 : : 0 : or 11 6 - 7 - 1 1&2 2 is better Comparing = 1 , 6S as & O is should more E land. N than s on 0 , so the : 0: & there are other resonance 6 - 7= - 1 structure - I - [I " I I &  SO j F2 6 + 6x2 + 7x2 = 32 6 - 7 = - 1 : j : ↑ - 7 - 7= 0 : Fi All atoms met octet , the F, values are quite high - 7- 7= 0 ↳ could make multiple 6 - 6 = 0 6 7 - - 1 bonds = 6 6 0 : = - : 0 : : 0 : 6-7 -111 6 6 =0 = = - -S - : 0 I j : - S 6 6 - = 0 = F7 - 6=+ ↓ 6-7-111 : F: 7-7 = 0 : F: : F : 7 7 6= + 1 T= 0 - - Fe for all value here un ↑ value are high are O  Bond Order and Bond Lengths Bond Order The number of ________________ pairs of electrons between two atoms bonding Bond Length The ______________ distance between the nuclei of the bonded atoms Nb. 0 , ↓ b. length Table 10.2 Some average Bond Lengths (pg. 433) Bond Bond Length, pm Bond Bond Length, pm Bond Bond Length, pm H-H 74.14 C-C 154 N-N 145 H-C 110 C=C 134 N=N 123 H-N 100 C C 120 N N 109.8 H-O 97 10) explain qualitative trends in bond lengths and strengths using Lewis structures  GEOMETRY AND DIPOLES Procedure for determining polarity of a chemical species: 1. Draw the Lewis structure, and count the electron domains around the central atom (one for each lone pair, single, or multiple bond) 2. Determine the electron-domain geometry by arranging the electrons so repulsions are minimized 3. Determine each bond dipole 4. Sum all dipoles like vectors GEOMETRY AND DIPOLES Adding Vectors Exercise: Add in dipole arrows and determine if the structure is polar or non-polar 3 Exercise: Add in dipole arrows and determine if the structure is polar or non-polar Geometry and Dipoles SiH4 5  EXERCISE: Determine if the following molecules are polar or non-polar. a) ClOF b) BrF5 c) SO2F2 using bond polarities and molecular geometry, predict qualitative magnitude and direction of a molecular dipole, and whether a molecule is polar or non-polar