Bonding, Geometry, and Property Models

Driving question

How can a two-dimensional electron diagram predict a three-dimensional shape and a macroscopic property?

Three models, three jobs

A Lewis structure tracks valence electrons and connectivity. VSEPR predicts geometry from electron-domain repulsions. A structure-and-interaction model explains properties such as conductivity, melting behavior, and brittleness. No single drawing is the molecule.

Build a defensible Lewis structure

1. Count all valence electrons; add for negative charge and subtract for positive charge.
2. Choose a plausible skeleton and connect atoms with single bonds.
3. Complete terminal-atom octets, then place remaining electrons on the central atom.
4. If the central atom lacks an octet, form multiple bonds where chemically reasonable.
5. Check the electron total and formal charges.

For an atom in a Lewis structure,

where $V$ is valence electrons in the isolated atom, $N$ is nonbonding electrons, and $B$ is bonding electrons.

Formal charges must sum to the species charge. They are bookkeeping values, not measured partial charges.

Resonance: one species, several contributors

When valid structures differ only in electron placement, they are resonance contributors. The actual electron distribution is not rapidly switching drawings; it is represented by a resonance hybrid. Equivalent bonds often have equal intermediate bond orders and lengths.

From electron domains to shape

VSEPR treats each single, double, or triple bond as one electron domain; each lone pair is also one domain. Electron-domain geometry counts all domains, while molecular geometry names only atom positions.

• $\mathrm{CO_2}$: two domains, linear, $180^\circ$.
• $\mathrm{NH_3}$: four domains with one lone pair, trigonal pyramidal, about $107^\circ$.
• $\mathrm{H_2O}$: four domains with two lone pairs, bent, about $104.5^\circ$.

Lone-pair repulsions generally compress adjacent bond angles relative to the ideal tetrahedral angle.

Bond polarity is not molecular polarity

Bond dipoles depend on electronegativity differences. Molecular polarity depends on the vector sum of all bond dipoles and the geometry. Carbon dioxide contains polar bonds but is nonpolar overall because equal opposing dipoles cancel. Water is polar because its bent geometry prevents cancellation.

Structure predicts properties

• Ionic solids: extended arrays of oppositely charged ions. Strong attractions often produce high melting points. They conduct when ions can move in a melt or solution, not as rigid solids.
• Metallic solids: positive ion cores with delocalized valence electrons. Mobile electrons support electrical conductivity; nondirectional bonding helps explain malleability.
• Molecular solids: discrete molecules held together by intermolecular attractions. Their properties depend on molecular size, shape, and intermolecular forces—not on breaking covalent bonds during melting.

Worked comparison: $\mathrm{CO_2}$ and $\mathrm{H_2O}$

Both molecules contain polar bonds. $\mathrm{CO_2}$ is linear, so its two bond-dipole vectors cancel. $\mathrm{H_2O}$ is bent, so its bond dipoles produce a nonzero molecular dipole. This geometric difference helps explain why water has stronger orientation-dependent intermolecular attractions than carbon dioxide under comparable conditions.

Evidence routine

1. Conserve valence electrons and total charge.
2. Distinguish connectivity, electron-domain geometry, and molecular geometry.
3. Draw bond-dipole vectors and add them as vectors.
4. Identify whether particles are atoms, ions, or molecules.
5. Connect the available mobile charge carriers and interaction strength to the observed property.

Misconception clinic

“A double bond counts as two VSEPR domains.” Multiple bonds count as one domain around the central atom.

“A polar bond makes a polar molecule.” Geometry can make bond dipoles cancel.

“Ionic compounds contain separate molecules.” An ionic solid is modeled as an extended lattice; a formula unit gives the lowest whole-number ion ratio.

Retrieval check

1. Why must formal charges sum to the overall charge?
2. What is the difference between electron-domain and molecular geometry?
3. Why is $\mathrm{CO_2}$ nonpolar despite polar bonds?
4. Why does solid sodium chloride not conduct while molten sodium chloride does?

Key takeaway

Strong chemical reasoning moves through a model chain: electrons determine bonding, electron domains constrain geometry, geometry shapes polarity, and particle structure plus interactions explain properties.