SME
General Chemistry I · Problem type
Formal charge
Use this for “In one resonance form of nitrate, NO₃⁻, nitrogen has one double bond, two single bonds and no lone pairs. Find its formal charge.”
Diagram
NO₃⁻: 5 + 3 × 6 + 1 = 24 valence electrons; 24 drawn in each form.
N:
FC
=
v
−
N
−
B
2
=
5
−
0
−
8
2
=
+1.
The formal charges add to −1, the ion’s charge, in every form.
3 resonance forms: the ion is their blend (↔), not a switch between them.
This is an example. Type your own values to start a new problem.
FC
=
v
−
N
−
B
2
Formulas
FC
=
v
−
N
−
B
2
+1
=
5
−
0
−
8
2
Assumptions
•
An atom owns all its nonbonding electrons and half of each bonding pair. A single bond is 2 bonding electrons, a double bond 4 and a triple bond 6.
•
The formal charges in one structure add to the ion’s charge, or to 0 for a molecule. In nitrate: +1 on N, −1 on each single-bonded O and 0 on the double-bonded O.
•
The best structure has formal charges nearest zero, with any negative one on the more electronegative atom.
•
Resonance forms move electrons, never atoms. Nitrate’s three forms differ only in which O holds the double bond.
Step-by-step
WE KNOW
v = 5, N = 0, B = 8
FIND
formal charge (FC)
Step 1 · Find formal charge (FC)
Use FC = v − N − B ÷ 2
Start from the electrons the free atom brings. Take off the ones it keeps as lone pairs and half of the ones it shares.
FC
=
5
−
0
−
8
2
FC = 5 − 4
FC = +1
Check
Put the numbers back in. Both sides should match.
+1
=
5
−
0
−
8
2
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Related lessons
Bonding and molecular geometry
Main lesson
Lattice energy from a Born–Haber cycle
Use this for “Find the lattice energy of NaCl from these values in kJ/mol: sublimation of Na 107, first ionization energy of Na 496, half the Cl–Cl bond energy 121, electron affinity of Cl −349 and ΔH°f of NaCl −411.”
Bond order and magnetism from an MO diagram
Use this for “Draw the MO diagram of O₂ from its 12 valence electrons. Find the bond order and the unpaired electrons: is O₂ paramagnetic?”