General rules for writing LEWIS structures for molecules
- Draw the initial "skeletal" structure with atoms connected by single
bonds. It should be the most symmetric structure you can draw. Make sure that H has only 1
bond, and that the other atoms do not exceed their normal number (2 bonds to O, 3 bonds to
N, 4 bonds to C), and that no atom in the molecule has more than 4 bonds.
- Determine TOTAL NUMBER OF OUTER SHELL ELECTRONS in the structure by adding the number of
OUTER SHELL electrons for all the atoms in an uncharged molecule. Add electrons if the
species is negatively charged, or substract electrons if is positively charged
(i.e. if they are ions).
- Subtract 2 electrons used to form each single covalent bond in your structure and
distribute the remaining electrons in pairs around the atoms. If there are not enough
electrons to form an octet around each atom except H, complete the octet around F, O, or N
first since these are the elements that most like electrons (i.e. they have the highest
- Calculate the formal charge around each atom as follows: Count the number of electrons
surrounding each atom. Unshared pairs are assigned to the atom on which the unshared pair
is found. A shared pair (i.e. a covalent bond) contains two electrons. Arbitrarily assign
one of these two electrons to each atom connected by the bond. Count the total number of
electrons assigned to each atom. Compare that number to the usual number of electrons in
the outer shell for the atom, which is equal to the group number in the periodic table.
The atom will have a +1 charge if it has one less electron assigned to it than usually
found in the outer shell of the atom, and a -1 charge if it has one additional electron.
Higher charges are also possible
- If any atom has an incomplete octet, change nonbonded electron pairs to bonded pairs to
form double and triple bonds in order to complete the octet around that atom. This does
not affect the number of electrons around the atom that donates the nonbonded pair.
Recalculate formal charges.
- If rule 5 creates additional charges, use structure 4.
- If phosphorous (P) or sulfur (S) is in your structure, the octet rule may not apply to
that atom. Those elements are in period 3 of the periodic table and can accommodate more
electrons, since they are bigger. Form additional multiple bonds to remove as many formal
charges as possible. P often has 5 bonds while S has 6.