The following step-by step instructions are designed to produce a correct Lewis Structure for any molecular assembly (neutral molecule or polyatomic ion) which contains only Main Group elements joined by two-atom shared-pair covalent bonds.
- VSE - count valence shell electrons
- Connectivity - arrange bonded atoms
- BP - assign bond pairs
- PLP - assign lone pairs to peripheral atoms
- CLP - assign lone pairs to central atoms
- Rearrange - find best Lewis Structure(s) [formal charge, resonance]
- Extra - assigning oxidation numbers
- Summary (pdf)
These rules are not appropriate for free radicals or molecular assemblies which contain multicenter bonds or transition metals.
the number of valence shell electrons for an atom is equal to its group number
HCN = 1+4+5-0 = 10 VSE = 5 VSE pairs
CommentarySome general rules and definitions:
H = 1, C = 4, F = 1.
O = 2 (sometimes 3), B, N = 3 (sometimes 4).
HCN = H bonded to C, C bonded to N, H and N are not bonded.Otherwise, as a general rule, the least electronegative elements (if not monovalent) are central, the most electronegative elements are peripheral. For example:
CO32- = 3 peripheral O bonded to central CNote that the order or geometric arrangement of symbols written on the page is irrelevant as long as bonded atom pairs are contiguous. The final Lewis Structure represents only the approximate electronic disribution, and it is not meant to show the actual 3-dimensional arrangement of the atoms.
CommentaryConnect each contiguous pair of atoms with one of the VSE pairs; each Bond Pair is shown as a line. Examples:
HCN = 5 VSE pairs - 2 BP = 3 pairs remaining
CommentaryDistribute the VSE pairs remaining after Step 3 among the peripheral atoms as Lone Pairs(H cannot accept lone pairs; see the Rule of Orbitals below). At this stage, no peripheral atom may have more than 4 VSE pairs (1 BP + 3 LP). Examples
HCN : of 3 VSE pairs, all are assigned to N; no VSE pairs remain.
CommentaryThe Rule of Orbitals: the total number of lone pairs and bond pairs (LP+BP) associated with an atom cannot exceed the number of Valence Shell Orbitals (VSO = n2, where n is the row of the Periodic Table in which that atom resides).
n = 1 (H): maximum VSE pairs (LP+BP) = VSO = 1;
C2H6O = 2 pairs, both assigned to O since each C already has 4 BP.While C,N,O and F always fill their four valence shell orbitals with Lone Pairs and/or Bond Pairs, B often does not. Thus, C, N, O and F always obey the "octet rule" (the only atoms on the periodic chart which always do!), but some compounds with B have an empty valence shell orbital and are called "electron deficient". Furthermore, third row elements (e.g., Al, Si, P, S, Cl) often havemore than four valence shell orbitals filled with Lone Pairs and/or Bond Pairs; this is called (illogically) "expanded valence". Obviously, elements from the fourth and higher rows can also exhibit "expanded valence". The tendency of most main group elements (except H) is to form molecular assemblies which fill at least the first four orbitals (s and p subshells); this tendency, plus the ubiquity of assemblies which contain C, N and O, has led to the (over) emphasis on the "octet rule".
CommentaryPrinciple of Electroneutrality: each atom in a covalent molecular assembly has a formal charge close to zero.
Formal Charge: FC = (Group Number) - (Bond Pairs) - 2(Lone Pairs)
Electron Pushing: formally changing a lone pair into a bond pair, or vice versa, while retaining association with the atom.
Original Lewis Structure
C2H6O (both isomers)Original Lewis Structures
CO32-Original Lewis Structure
PO43-O (all): FC = 6-1-2(3) = -1;
H3O+H (all): FC = 0;
Examples: He(g), H2(g), Br2(l), P4(g), S8(s), Fe(s)
Examples: Na2SO3, SO3, SO42-; but in Na2O2 and H2O2, ON(O) = -1
Examples: NaH, CaH2
Examples: CH4, H2O, HPO42-
Examples: C (+4), P (+5), Se (+6), Br (+7), Xe (+8)
Examples: C (-4), P (-3), Se (-2), Br (-1), Xe (0)
The oxidation number for an individual atom in a Lewis Diagram is calculated as follows:
|HCN: C is more electronegative than H, so it is assigned the H-C bond pair; N is more electronegative than C, so it is assigned all threeC-N bond pairs. Then|
|CH3OCH3: O is more electronegative than C, so it is assigned both C-O bond pairs; C is more electronegative than H, so both C(1) (the left-most C atom) and C(2) (the right-most C atom) are assigned every C-H bond pair (both C atoms are identical). Then|
|CH3CH2OH: O is more electronegative than either C or H, so O is assigned both the C-O and O-H bond pairs; C is more electronegative than H, so both C(1) (the left-most C atom) and C(2) are assigned every C-H bond pair. The C-C bond pair is split between the two C atoms. Then|
|CO32-: O is more electronegative than C, so each O is assigned the C-O bond pair. Then|
|PO43-: O is more electronegative than P, so each O is assigned the P-O bond pair. Then|
|H3O+: O is more electronegative than H, so O is assigned all of the H-O bond pairs. Then|