Download A question on formal charge from the 2001

A question on formal charge from the 2001 CHEM 13 NEWS EXAM
Carey Bissonnette, CHEM 13 NEWS Exam coordinator
Chemistry Department, University of Waterloo, Waterloo ON N2L 3G1
reasonable. Some useful guidelines concerning formal charges are as
follows:2
• structures with negative formal charges on electronegative
elements are favoured
• structures that minimize the number of formal charges, and
with formal charges between −1 and 1 are favoured
• structures with the same formal charges on adjacent atoms
are highly unfavoured
The following is an acceptable Lewis structure for the
phosphate anion, PO43- . (question #6)
3−
••
•
•
•O •
•
•
••
O
••
P
••
O
••
•
•
•O •
••
What is the formal charge on phosphorus ?
It seems to us that while most high school students are taught
the basic rules for drawing Lewis structures for molecules and
polyatomic ions, they struggle with how to assign formal charges
to the atoms in the structures they’ve drawn. Given a Lewis
structure for a polyatomic molecule or ion, one assigns a formal
charge to each atom in the structure using the following
definition.
These guidelines can be used to assess and compare the
following two structures for the H2SO4 molecule. (Note: Other
acceptable structures for H2SO4 may also be drawn!
-1
O
H
O
S
O
+2
O
⎤
⎥
⎥
⎥
⎥
⎦
For the purposes of assigning formal charges, an atom “owns”
all of its unshared electrons (i.e., its lone pairs) but only half of
its bonding electrons. The formal charge is the charge an atom
would have if all of the atoms had the same electronegativity.1
To assign a formal charge to the phosphorus atom in the
3structure shown above for PO 4 , we proceed as follows.
Phosphorus is in group 15, with a valence configuration of
3s2 3p3, and therefore, a “free” phosphorus atom has five
valence electrons. In the structure under consideration, the
phosphorus atom “owns” five electrons (i.e., it has no lone pairs
and “owns” only half of the 10 bonding electrons). Therefore, the
formal charge on the phosphorus atom is 5 − [ ½ (10) ] = 0. We
can repeat the process for the oxygen atoms to obtain the
following results:
(i)
H
H
O
S
O
H
O
-1
A
⎡ the number of valence
⎡ the number of valence ⎤ ⎢
⎢
⎥ ⎢electrons " owned" by the
formal ch arg e = ⎢electrons
⎥ − ⎢atom in the structure
⎢⎣in a " free " atom
⎥⎦ ⎢
⎣under consideration
O
B
Each of the atoms in structure B has a formal charge of zero,
and using the guidelines above, we draw the conclusion that
structure B is the favoured structure for H2SO4.
Although the definition given above for the formal charge allows
us to assign a charge to an atom in a molecule, it must be
emphasized that the formal charge is not an accurate
representation of the “true” charge on the atom. As pointed out
by Parkin,2 the concept of formal charge exaggerates the
covalent character of the bonding in a molecule.
References
1. J.E. Huheey, E.A. Keiter, and R.L. Keiter, Inorganic Chemistry –
Principles of Structure and Reactivity, Fourth Edition, Harper Collins
College Publishers, New York (1993).
2. G. Parkin, Journal of Chemical Education, 83, 791 (2006). ∎
Cartoon by Nick Kim www.nearingzero.net
For O in P=O, formal charge = 6 − [4 + ½ (4)] = 0
(ii) For O in P-O, formal charge = 6 − [6 + ½ (2)] = −1
The sum of the formal charges is equal to the total charge:
0 + 0 − 1 − 1 − 1 = −3. This should not be a surprise. As long
as we are consistent in our use of the rule for assigning formal
charge, the formal charges will always add to give the total
charge.
The formal charge concept is important in chemistry because it helps
us assess whether or not a particular Lewis structure is important or
April 2008/Chem 13 News 17