bonded state. For the purposes of calculating formal charge the atom is considered to have full
ownership of all lone pair electrons, and halfownership of all bonded electrons (equal
sharing). In terms of numbers of
electrons
(not electron pairs):
Z = N
valence
– N
LP
– 1/2 N
BP
where N
valence
is the number of valence electrons in the free neutral atom, N
LP
is the number of
lone pair electrons in the bonded state, and N
BP
is the number of bonded electrons in the
bonded state.
For example, in the structure of the carbonate ion given above, we have three different kinds
of atom:
C (always has 4 bonds): Z = 4 – 1/2 x 8 = 0
O (doublybonded): Z = 6 – 4 – 1/2 x 4 = 0
O (singlybonded): Z = 6 – 6 – 1/2 x 2 = 1
The formal charges add up to the true total charge of the ion, which is 2.
Rules of thumb for formal charge:
1. Formal charges should be small for covalent molecules.
2. More electronegative atoms should have more negative formal charges.
3. Molecules should not contain adjacent atoms with like charges.
Exercises: Pick one of the lists below (each contains 10 molecules) and work through it in the
spaces provided on the following pages. Unless otherwise stated, these compounds all contain a
central atom bonded to several surrounding atoms.
List A: (1) SiCl
4
, (2) PCl
3
, (3) NO
3

, (4) SF
6
, (5) PCl
5
, (6) IF
3
, (7) XeF
5
+
, (8) SO
4
2
, (9) C
4
H
8
O, (10)
C
5
H
12
. ((9) and (10) contain chains or rings of atoms. More than one structure is possible for
each of these – such molecules exhibit
isomerism
.)
List B: (1) PO
4
3
, (2) NH
4
+
, (3) ClF
5
, (4) XeF
4
, (5) SbF
5
, (6) CS
2
, (7) XeF
2
, (8) C
3
H
8
O, (9) C
6
H
12
, (10)
C
6
H
6
. (Multiple structures are possible for (8), which contains a chain or ring of atoms. (9) and
(10) should contain rings.)
Show: Number of valence electrons, Lewis structure, 3D geometry, formal charges on all
atoms.
Lab report: Include an Introduction, Lewis structures and 3D shapes for all molecules in your
list (which may be scanned or photographed from your worksheet), and a brief discussion of
your findings.
CHE151Lab8_MolecularStructure.doc
Dr
.
Roderick M. Macrae
CHE 151
1.
Formula:
Number of valence electrons:
Molecular geometry:
2.
Formula:
Number of valence electrons:
Molecular geometry:
CHE151Lab8_MolecularStructure.doc
Dr
.
Roderick M. Macrae
CHE 151
3.
Formula:
Number of valence electrons:
Molecular geometry:
4.
Formula:
Number of valence electrons:
Molecular geometry:
CHE151Lab8_MolecularStructure.doc
Dr
.
Roderick M. Macrae
CHE 151
5.
Formula:
Number of valence electrons:
Molecular geometry:
6.
Formula:
Number of valence electrons:
Molecular geometry:
CHE151Lab8_MolecularStructure.doc
Dr
.
Roderick M. Macrae
CHE 151
7.
Formula:
Number of valence electrons:
Molecular geometry:
8.
Formula:
Number of valence electrons:
Molecular geometry:
CHE151Lab8_MolecularStructure.doc
Dr
.
Roderick M. Macrae
CHE 151
9.
Formula:
Number of valence electrons:
Molecular geometry:
10.
Formula:
Number of valence electrons:
Molecular geometry:
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