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Formal charge is valence electrons less non-bonding electrons less bonds; it says which atom carries a species' charge, the atoms add up to the whole, and it is not the oxidation number.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will calculate the formal charge on an atom from its bonds and non-bonding electrons, check that a species' atoms add up to its charge, and tell formal charge from oxidation number.
You can count an atom's bonds and lone pairs and say how it is hybridized. Formal charge uses the same count to say where a charge sits.
Valence electrons are the outer electrons a free atom brings: carbon 4, nitrogen 5, oxygen 6, boron 3. Non-bonding electrons are the ones an atom keeps as lone pairs or single unpaired electrons. Formal charge is the charge an atom would carry if every bond were shared exactly evenly.
Share every bond evenly, so an atom owns one electron of each of its bonds, plus all its non-bonding electrons. Compare what it owns with what the free atom brought:
$$\text{formal charge} = V - N - B$$
where $V$ is valence electrons, $N$ non-bonding electrons and $B$ bonds (a double bond is two).
The pattern is worth knowing by sight: neutral carbon makes four bonds, nitrogen three with a lone pair, oxygen two with two lone pairs. An atom with one bond more than that, and a lone pair fewer, is positive; one bond fewer and a lone pair more, negative.
Another way: table
Neutral, positive and negative forms.
| Atom | Neutral | Positive | Negative |
|---|---|---|---|
| carbon | 4 bonds | 3 bonds, no lone pair | 3 bonds, 1 lone pair |
| nitrogen | 3 bonds, 1 lone pair | 4 bonds | 2 bonds, 2 lone pairs |
| oxygen | 2 bonds, 2 lone pairs | 3 bonds, 1 lone pair | 1 bond, 3 lone pairs |
The formal charges of all the atoms add up to the charge of the whole species. That is the check: in nitromethane the nitrogen is $+1$ and one oxygen $-1$, adding to zero for a neutral molecule with charge separated inside it.
Formal charge is not the oxidation number. Formal charge shares each bond evenly; oxidation number gives each bond entirely to the more electronegative atom. Methanol's carbon has formal charge 0 and oxidation number −2. Nor is formal charge the real distribution of electron density: the carbon of carbon monoxide is formally −1 though oxygen is more electronegative. It is a bookkeeping tool that tells you which atom has too few or too many electrons to call its own — which, in unit 5, is where reactions start.
Formal charge is the same as an atom's oxidation number. It splits bonds evenly; oxidation number does not.
Count a double bond once. It is two bonds.
Count lone pairs, not electrons. Two electrons per lone pair.
The charge sits on the most electronegative atom. Formal charge can sit anywhere the count puts it.
The nitrogen bonds to one carbon and three hydrogens: four bonds, no lone pair.
Write the surroundings first.
$5 - 0 - 4 = +1$.
Brought, less owned.
Every other atom is neutral, so the ion's charge is $+1$, and it sits on nitrogen.
The atoms add up to the species.
The oxygen makes one bond. How many lone pairs does it need to be negative?
Three.
Its formal charge?
$6 - 6 - 1 = -1$, matching the ion's charge.
In a structure, an atom of carbon (which brings $4$ valence electrons) makes $3$ bonds and holds $1$ non-bonding electrons. What is its formal charge? (Give a negative charge as a negative number.)
Answer:
An atom brings $V$ valence electrons, keeps $N$ non-bonding electrons in the structure, and makes $B$ bonds. Write its formal charge $F$.
Answer:
In one structure of nitromethane, CH3NO2, the nitrogen makes four bonds (one to carbon, a double bond to one oxygen and a single bond to the other) and has no lone pair. The double-bonded oxygen has two lone pairs; the single-bonded oxygen has three. Fill in each atom's formal charge, and the charge of the whole molecule.
| formal charge | |
|---|---|
| the nitrogen | |
| the double-bonded oxygen | |
| the single-bonded oxygen | |
| the whole molecule |
The carbon in methanol, CH3OH, has a formal charge of 0 and an oxidation number of −2. Which statement explains the difference?
Carbon monoxide is written C≡O, with one lone pair on the carbon and one on the oxygen. What is the formal charge on the carbon?
Answer:
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
What is the formal charge on the oxygen of the methoxide ion, CH3O−? (Give a negative charge as a negative number.)
The formal charge on that atom is a.
You can say which atom of a structure carries its charge. Tell someone why the carbon of carbon monoxide is formally negative. Next: resonance, when one structure is not enough.
8. Your turn: the ethoxide ion, CH3CH2O−, step 3