Lewis Structures Handout
4.7 (5.6) Drawing Lewis Structures
Rules for Writing Lewis Dot Structures for Molecules
1. Add up valence electrons from all atoms
 It does not matter which atom they come from.
 Add 1 e- for each (-) charge
 Subtract 1 e- for each (+) charge
2. Identify the central and terminal atoms and write the skeletal structure
 The central atom will usually be one of the lowest
electronegativity (least electron loving) (H excepted)
 This may take some trial and error (or be given).
 H is always terminal
 O usually terminal except for peroxides and hydroxides OH
 C is always a central atom, it always has 4 bonds
3. Draw a bond between each pair of atoms.
 Each bond uses up 2 of the available e-.
4. Assign remaining e to terminal elements first to fill octet (duet for H)
5. If there are leftover e-,
 If there are additional e- still left, place them on central atoms that
have empty d orbitals (period 3 and below).
6. If all e- are used up and the central atoms do not have completed octets,
 move one or more lone pairs to form a multiple bond.
Usually for C, N, O, X (halogen) and H
– C forms
covalent bonds and
lone pairs.
– N forms
covalent bonds and
lone pairs.
– O forms
covalent bonds and
lone pairs.
– X forms
covalent bonds and
lone pairs.
– H forms
covalent bond
Lewis Structures Handout
Examples for Rows 1 + 2 (Octet rule never exceeded)
H2 O
NH3
HCN
Problem: Draw the Lewis structure for
N2H4
and
CO2
Lewis Structures Handout
Exceptions to Octet Rule
 Boron & Al have fewer than a full octet (3 bonds)
 Period 3 & below elements can exceed the octet rule
(have 10-12 electrons)
 The octet rule is sometimes broken because of unfilled d orbitals
that are available.
Examples
SeF6
BrF3
Problem: Draw the Lewis Structure for SF4.