About Formal Charges
CHEM 330 handout
The valence electrons of an atom in the bonded state (= one that is part of a molecule) often cancel out its nuclear charge. Positive (nuclear) and negative (electronic) charges are then electrostatically balanced. In some bonded states, however, an atom may find itself surrounded by a greater or a fewer number of valence electrons than expected on the basis of its nuclear charge. An electrostatic imbalance then occurs.
If the atom in question finds itself with more valence electrons than expected, then it will be negatively charged. The atom will probably be inclined to behave as an electron donor, and this propensity will govern much of its reactivity.
If the atom in question finds itself with fewer valence electrons than expected, then it will be positively charged. The atom will probably be inclined to behave an electron acceptor, and again this proclivity will control much of its reactivity.
Because organic reactions are movements of electrons, and because electrons always move from electrostatically negative sites to positive ones, it is essential to know whether the bonded state of an atom has created an electrostatic imbalance. This will allow us to rationalize and predict the reactivity of such an atom, and – by extension – of the molecule containing that atom.
Enter the formal charge
The formal charge of an atom is a parameter that indicates whether the atom in question is electrostatically balanced or unbalanced.
The formal charge of an atom in a molecule is easily calculated from the complete Lewis structure of the molecule (i.e., one that shows all bonding and nonbonding electron pairs).
One simply needs to count how many valence electrons (both bonding and nonbonding) the atom in question has contributed to the molecule, i.e., how many valence electrons are formally present around the atom, then decide whether the number of such valence electrons corresponds to an electrostatic balance or an imbalance
The following examples illustrate how this is done.
CHEM 330 p. 2 formal charges
Example 1: the formal charge on the C atom in methane, CH4
Step 1: draw a complete Lewis structure of the molecule:
H H
H C H H C H
H H
Step 2: count the number of valence electrons around the atom of interest.
Each atom in a bonded pair of atoms has contributed one of its valence electrons to the electron pair that we call "bond." If we "shatter" the molecule so that each atom in a bonded pair retrieves one of the two electrons that form the bond (i.e., if we imagine the homolysis of each bond in the molecule), we will see how many valence electrons are present around each atom:
H "shatter" H C H 4 H + C
H
Conclusion: the C atom in methane is surrounded by 4 valence electrons
Step 3: determine whether valence electrons balance the nuclear charge out.
• Carbon is in group 4 of the periodic table, so it requires 4 valence electrons to balance the nuclear charge out.
• The C atom in methane has 4 valence electrons
• The C atom in methane is electrostatically balanced
Conclusion: the formal charge on C in methane is zero
Notice that the formal charge on each H atom is also zero. Indeed, H atoms in any covalent molecule have always zero formal charge, as readily determined though the above logic.
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CHEM 330 p. 3 formal charges
Example 2: the formal charge on the O atom in water, H2O
Step 1: draw a complete Lewis structure of the molecule:
H H
H O H O lone pairs
Step 2: count the number of valence electrons around the atom of interest.
H "shatter" H O 2 H + O
Step 3: determine whether valence electrons balance the nuclear charge out.
• Oxygen is in group 6 of the periodic table, so it requires 6 valence electrons to balance out the nuclear charge.
• The O atom in water has 6 valence electrons
• The O atom in water is electrostatically balanced
Conclusion: the formal charge on O in water is zero
(the H atoms in water also have zero formal charge)
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Example 3: the formal charge on the N atom in NH4
Step 1: draw a complete Lewis structure of the molecule:
H
NH4 = H N H
H
CHEM 330 p. 4 formal charges
Step 2: count the number of valence electrons around the atom of interest.
H "shatter" H N H 4 H + N
H 4 valence electrons
Step 3: determine whether valence electrons balance the nuclear charge out.
• N is in group 5: it needs 5 valence electrons to balance out the nuclear charge. • The N atom in NH4 has 4 valence electrons: 1 fewer than it should. • The N atom in NH4 is electrostatically unbalanced
Conclusion: the formal charge on N in NH4 is + 1
Important: formal charges are integral parts of a chemical structure and must be clearly indicated. This is done with encircled + or – signs. So, the correct way to draw NH4 is:
H
H N H
H
Notice that the formal positive charge on the N atom is not balanced out by a negative charge elsewhere in the molecule. Globally, therefore, the NH4 molecule is positively charged, i.e., it is a cation. This particular cation is called the ammonium ion.
The algebraic sum of formal charges of individual atoms in a molecule gives the total charge on the molecule
+ Why "formal" charge? Rigorously speaking, the + 1 charge present in NH4 is delocalized all over the molecule, i.e., each atom (N and 4 H's in this case) bears a share thereof. For simplicity, however, it is convenient to think of it as if it were localized on the N atom. That's why one calls it a formal charge: because for chemical reasoning it is best to think of it as formally residing on the N atom.
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CHEM 330 p. 5 formal charges
Example 4: the formal charge on the N atoms in hydrazoic acid, H–N=N=N:
H–N3 = H N N N
"shatter"
H N N N proper way to draw hydrazoic acid:
group 5 group 5 group 5 H–N=N=N 5 val. e– 4 val. e– 6 val. e– zero + 1 – 1 or, since it is understood that each atom would have a complete Lewis octet:
H–N=N=N
Notice that +1 and –1 formal charges in HN3 balance each other out. Overall, the molecule is electrostatically neutral.
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Example 5: the formal charge on the C atom in the following four molecules:
CH CH 3 3 CH3 CH2 vacant orbital
H H H
C H C H C H C H
H H H H
C C C C
+ 1 zero – 1 zero
CH CH CH 3 3 CH3 2
Overall: cationic neutral anionic neutral
Called a "carbocation" a "radical" a "carbanion" a "carbene"