Lewis Structures & Bond Energy

the idea

Atoms bond to reach a full outer shell, and a Lewis structure is the electron seating chart.

Only valence electrons are drawn. A line is a shared pair (a bond) and dots are unshared pairs (lone pairs). The goal for most atoms is the octet: 8 electrons around them, counting both bonds and lone pairs (H only needs 2). When more than one valid drawing exists, formal charge breaks the tie.

FC = valence e⁻ − nonbonding e⁻ − ½(bonding e⁻)

Same as: valence − (dots) − (lines). The best structure has FCs closest to zero, with any negative FC on the more electronegative atom.

Plant version: formal charge is a shift-coverage audit. Each atom "owns" a certain number of electrons (its valence), like each line has a set crew size. In the structure, it gets credit for all its lone pairs plus half of each shared bond. Over-crewed means a negative FC and under-crewed means a positive one. The best schedule keeps everyone as close to their normal crew as possible.

explore

Structures, resonance, and formal charges

calculate

Formal charge, one atom at a time

Valence electrons Nonbonding (dot) electrons Bonds (lines)

ionic bonding

Lattice energy: charge first, size second

Lattice energy is the energy needed to pull a solid ionic compound apart into gaseous ions. Coulomb's law sets it: E ∝ Q₁Q₂ / d. Doubling the charges quadruples the attraction, but shrinking the ions only nudges it. So compare charges first, then size.

Compound A Compound B

Predict which is larger first.

bond enthalpy

ΔH ≈ Σ(bonds broken) − Σ(bonds formed)

Breaking bonds always costs energy. Making them always releases it. If the new bonds are stronger overall than the old ones, the reaction is exothermic. Average bond enthalpies give an estimate, usually within a few percent of the true ΔH.

Bond	Enthalpy (kJ/mol)	Length (pm)
C–C / C=C / C≡C	348 / 614 / 839	154 / 134 / 120
C–O / C=O / C≡O	358 / 799* / 1072	143 / 123 / 113
N–N / N=N / N≡N	163 / 418 / 941	147 / 124 / 110
C–H · O–H · N–H	413 · 463 · 391	109 · 96 · 101
H–H · O=O · Cl–Cl · H–Cl · C–Cl · O–O	436 · 495 · 242 · 431 · 328 · 146	74 · 121 · 199 · 127 · 177 · 148

*799 is the C=O value in CO₂. Pattern: more bonds between two atoms means shorter and stronger.

practice

Formal charge and resonance

practice

Lattice energy and bond enthalpy

engineering version

Hydrogenation is a bond-enthalpy trade.

Partially hydrogenating vegetable oil (how margarine and shortening used to be made) adds H₂ across C=C double bonds. You break one C=C (614) and one H–H (436) and form one C–C (348) and two C–H (2 × 413). The estimate is about −124 kJ per double bond (measured ≈ −137), so it's exothermic and the reactors need cooling. The same process created trans fats as a side product, which is why FDA removed partially hydrogenated oils from GRAS status.

exam traps

Common mistakes

Drawing resonance forms as if the molecule flips between them. It doesn't. The real structure is one blended hybrid, and all three N–O bonds in NO₃⁻ are identical.
Moving atoms between resonance structures. Only electrons move.
Counting whole bonding pairs in formal charge. Each bond gives the atom one electron (half the pair).
Forcing an octet onto B or onto period 3+ central atoms. BF₃ is fine with 6, and SF₆ has 12.
Getting the bond enthalpy sign backwards. It's broken − formed, not formed − broken.