Dr. Robert Belford & Rebekah Reinhart
1. Determine structural connectivity.
- First atom is central atom (unless Hydrogen, which can never be central)
- If more than 2 types of atoms, set up in order of formula.
- Organic compounds often contain multiple carbons connected to each other, you
will need to be able to identify functional groups.
2. Determine total number of available valence electrons.
- Determine the total number of valence electrons available, each atoms of the
representative elements contributes the number of electrons equal to it’s group number (IA – VIIA). Note, if there is an odd number of electrons, the structure will be a free radical, which has a lone electron in an orbital. This electron will exist in a nonbonding orbital or a multiple bond.
-If the compound is an ion, add one electron for each negative charge and subtract
one electron for each positive charge
3. Draw hypothetical structure with each atom surrounded by 8 electrons in pairs.
a. Place two electrons forming a single bond between bonded atoms by drawing a
line between the two atoms.
b. For each atom not surrounded by an octet of electrons (4 bonds), add lone pairs
of electrons (two dots to a lone pair) until all atoms are surrounded by an octet. Note, hydrogen can only have 2 valence electrons.
4. Count the total number of electrons in the hypothetical structure.
a. If it is equal to the number of available electrons from step 2, this is the
correct Lewis Dot Structure.
b. If it is GREATER than the number of available electrons from step 2, go to step 5.
c. If it is LESS than the available number of electrons from step 2, go to step 7.
5. Determine if any atoms can have less than an octet. If not, proceed to step 6, if so, decrease the number of loan pairs around these atoms until the total number of electrons in structure equals the number from step 2.
a. Hydrogen always has a duet (one bond – 2 electrons)
b. Group IIA can have a quartet (4 electrons)
c. Group IIIA can have a sextet (6 electrons)
6. Place multiple bonds into structure by replacing two loan pairs from two adjacent
atoms with a bonding pair. Repeat this until the structure has the same number of electrons as determined in step 2.
7. If any atoms have d orbitals available (3rd period or greater), they can have an
“expanded octet”, that is more than 8 valence electrons. Add lone pairs to these atoms until the number of electrons equals the number from step 2.
8 RESONANCE STRUCTURES: Check to see if additional electron configurations
can be drawn by changing the position of multiple bonds, these represent
resonance structures. The best configurations will be the ones where the atoms
are assigned the formal charges closest to zero. In the event that no
resonance structure has a zero formal charge, assign negative formal charges to
the most electronegative elements.
9. FORMAL CHARGES: These correlate the electron locations in the molecule to the
atom that donates them. The formal charge (FC) for each atom can be determined by the following equation.