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.
