CONSTRUCTING LEWIS DIAGRAMS: THE WHOLE STORY

DRAWING LEWIS DIAGRAMS ( H + O + H) = 8 electrons for water (H2O) 1. Count and add the number of valence electrons contributed by each atom of the molecule or ion. : . 2. Determine the arrangement of the atoms (skeleton) for the molecule or ion. This you must obtain from intuition, some rules, or information given by the instructor. H-O-H 3. Subtract the electrons used in each of the skeletal bonds ( 2 electrons each bond ) from the total. (8 - 4) = 4 remaining for H2O 4. Assign the remaining electrons following the Lewis Rules.

LEWIS RULES (a reminder) 1. OCTET RULE: In a completed Lewis Diagram, Period 2 or Period 3 atoms will have a completed octet : ( 8 electrons). Hydrogen atoms will have a “duet” : ( 2 electrons). The term duet is not formally used 2. BONDS. Bonds are made by sharing a pair of electrons between two atoms. Single Bonds ( 1 shared pair ) Double Bonds ( 2 shared pairs ) Triple Bonds ( 3 shared pairs ) are all allowed in constructing a Lewis Diagram. Hydrogen, is always singly bonded.

N: LEWIS RULES ...... continued 3. ELECTRON PAIRS. Electrons not involved in forming bonds (non-bonded or unshared electrons) are arranged in pairs. The final structure must have the correct number (total) of valence electrons. 4. CORRECTNESS.

REVIEW OF METHOD 1. Determine the total number of valence electrons in the molecule or ion. 2. Determine the arrangement of the atoms (the skeleton). 3. Subtract the electrons used in the skeletal bonds ( 2 electrons each bond ) from the total. 4. Assign the remaining electrons following the Lewis Rules (octets, duets, bonds, pairs etc.). 5. Check that the total number of electrons is correct ( count them ! ). Do CO2 or H(CO)Cl

Some Lewis Diagrams of Organic Molecules hydrogen cyanide acetone ethylene glycine FOR MANY COMPOUNDS YOU WILL BEGIN TO SEE PATTERNS

COMMON BONDING PATTERNS FOR THE ELEMENTS (NO CHARGES)

NORMAL COVALENT BONDING PATTERNS FOR COMMON ELEMENTS III IV V VI VII H monovalent no pairs monovalent three pairs trivalent no pairs divalent two pairs trivalent one pair tetravalent no pairs

Atoms use these normal bonding patterns in preference to others. Try these first when building Lewis Diagrams. However, atoms do use other bonding patterns when forming molecules and ions whenever these normal patterns are not possible for some reason. When these normal patterns are used the atoms do not carry “formal charges” (next section).

Total valence electrons = 26 Write the Lewis structure of nitrogen trifluoride (NF3). Step 1 – N is less electronegative than F, put N in center 1 x N = 5 Step 2 – Count valence electrons 3 x F = 21 Total valence electrons = 26 Step 3 – Draw single bonds between N and F atoms and complete octets on N and F atoms. Step 4 - Check, are # of e- in structure equal to number of valence e- ? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons F N 9.6

Total valence electrons = 24 Write the Lewis structure of the carbonate ion (CO3)2-. Step 1 – C is less electronegative than O, put C in center 1 x C = 4 Step 2 – Count valence electrons 3 x O = 18 2- charge = 2 Total valence electrons = 24 Step 3 – Draw single bonds between C and O atoms and complete octet on C and O atoms. Step 4 - Check, are # of e- in structure equal to number of valence e- ? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons Step 5 - Too many electrons, form double bond and re-check # of e- 2 single bonds (2x2) = 4 1 double bond = 4 8 lone pairs (8x2) = 16 Total = 24 O C 9.6

FORMAL CHARGE

FORMAL CHARGE Table 1-10 Formal Charge The charge on an atom in a molecule or ion, based on a prescribed procedure for counting which electrons may be assigned to each individual atom. Unbonded Bonded Number of All One half of = valence electrons unshared + all shared in the neutral electrons electrons atom Formal Charge . .. N : . H N : . NH2- . 5e- 6e- ( Formal Charge = 5 - 4 - 2 = -1 )

Formal charge helps us determine the shape of the molecule. Two possible skeletal structures of formaldehyde (CH2O) H C O H C O Calculate the Lewis structure and formal charge H C O -1 +1 H C O

Formal Charge and Lewis Structures For neutral molecules, a Lewis structure in which there are no formal charges is preferable to one in which formal charges are present. Lewis structures with large formal charges are less plausible than those with small formal charges. Among Lewis structures having similar distributions of formal charges, the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms. Which is the most likely Lewis structure for CH2O? H C O -1 +1 H C O 9.7

The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion.

_ S O .. : _ SULFATE (SO4)2- 2 - +2 net ionic charge Calculate formal charges

COMMON BONDING PATTERNS IN WHICH AN ATOM CARRIES A FORMAL CHARGE CATIONS ANIONS _ _ _ _ _ Table 1-14

VALENCE-BOND STRUCTURES LEWIS DIAGRAMS ARE A TYPE OF VALENCE-BOND STRUCTURE BUT ARE NOT THE ONLY TYPE

Valence-Bond Structures : BE SURE TO DISTINGUISH AMONG THE VARIOUS TYPES OF VALENCE BOND STRUCTURES Valence-Bond Structures : 1. Lewis Diagrams 2. Expanded Octet Structures 3. Incomplete Octet Structures AND TO NOTE THAT LEWIS DIAGRAMS ALWAYS FOLLOW THE LEWIS RULES, BUT THE OTHER TYPES DO NOT

Usually occurs with B and elements of higher periods and most nonmetals. Common exceptions are: Be, B, P, S, Xe, Cl, Br, and As.

INCOMPLETE-OCTET STRUCTURES

. . . B INCOMPLETE OCTET STRUCTURES SOME ATOMS DO NOT FOLLOW THE OCTET RULE GROUP THREE ELEMENTS OFTEN FORM INCOMPLETE OCTET STRUCTURES . Boron often makes structures with an incomplete octet. . . B It can only form three bonds! BF3 = How can it do this?

PROMOTION OF ELECTRONS IN THE VALENCE SHELL

. . . B . . . B VALENCE SHELL OF BORON [1s2]2s22p1 2s 2p “promotion” ONE BOND . . . B paired 2s 2p unpaired “promotion” . THREE BONDS . . B 2s 2p only three bonds can be formed in the normal fashion GROUP 3A

. . . C . . . . C . CARBON FORMS 4 BONDS ALSO DUE TO PROMOTION BY THE WAY …….. CARBON FORMS 4 BONDS ALSO DUE TO PROMOTION TWO BONDS . . [1s2]2s22p2 . C . 2s 2p “promotion” . FOUR BONDS . . C 2s 2p . GROUP 4A

PROMOTION . . : : : . . . . . . . . . . VALENCE SHELL 2A 3A 4A When forming covalent bonds, Be B C . all of the elements in groups 2A to 4A promote an electron so they can form more bonds. . . . . . . . . Be B C . LEWIS DOT SYMBOLS LEWIS DOT SYMBOLS are always drawn showing the promotion already accomplished.

EXPANDED-OCTET STRUCTURES

P S EXPANDED OCTET STRUCTURES GROUP 5A OR 6A ELEMENTS OFTEN FORM Phosphorous can form up to 5 bonds P PCl5 = GROUP 5A Sulfur can form up to 6 bonds S SF6 = 3d orbitals are available GROUP 6A How can it do this?

VALENCE SHELL OF SULFUR Sulfur can use 3d orbitals TWO BONDS 3s 3d 3p promotion (x2) SIX BONDS 3s 3d 3p both ways of bonding are common

EXPANDED OCTET STRUCTURES ARE NOT LEWIS DIAGRAMS ..... THEY DO NOT FOLLOW THE OCTET RULE ! They are specifically designated: EXPANDED OCTET STRUCTURES A designation often used for Phosphorous and Sulfur structures.

Some structures can be drawn both ways: .. - .. : : : .. .. .. .. +2 .. .. .. .. : : : .. .. - LEWIS DIAGRAM EXPANDED-OCTET note charges note lack of charges Be very clear on what is asked for.

VALENCE-BOND STRUCTURES

VALENCE BOND STRUCTURES This term is broad enough to include Lewis Structures and incomplete octet or expanded octet structures.

THE IMPORTANCE OF LEWIS DIAGRAMS

A COMMON LANGUAGE FOR CHEMISTS Lewis Diagrams are one of the principal methods chemists have for communicating structural information to one another. If you ask a chemist for the structure of a molecule, you will likely get a drawing of a Lewis Diagram. Handbooks use these diagrams to communicate structural information. When you look in the CRC handbook or the Merck Index you will find Lewis Diagrams. In addition, Lewis Diagrams will help you understand the chemistry of molecules and help to identify reactive sites. We use them to explain reactions. Learning all that is coming soon!

LEWIS DIAGRAMS SHOW IT ALL ! When drawing a Lewis Diagram remember these rules. LEWIS DIAGRAMS SHOW IT ALL ! - all atoms including hydrogens - all bonds ( lines not dots ) - all unshared pairs ( dots ) - all formal charges - all atoms with octets ( except H ) - the correct number of electrons ( count! ) All of these rules (except for the octet rule) apply to all other valence-bond structures as well. Be sure you include everything specified here in your answers.