Bonding and molecular geometry

Chemical Bonds and Lewis Structures

Atoms bond to reach a lower energy state. The type of bond depends on electronegativity difference ($\Delta\chi$).

Lewis structures map valence electrons as shared bonds and lone pairs, guided by the octet rule for main-group atoms.

$$\Delta H_{\text{rxn}} = \sum D_{\text{broken}} - \sum D_{\text{formed}}$$

A negative $\Delta H$ indicates an exothermic reaction.

VSEPR Theory and Dipoles

VSEPR theory dictates that molecular shape stems from electron-pair repulsion. Calculate the steric number: $\text{Steric no.} = \text{bonding pairs} + \text{lone pairs}$.

The total molecular dipole is the vector sum: $\vec{\mu}_{\text{total}} = \sum \vec{\mu}_i$.

Key insight: Symmetric molecules like CO$_2$ can have polar bonds but zero net dipole because vectors cancel.

Common pitfall: All electron groups set the shape, but named geometries describe only the atoms. Water has four tetrahedral groups yet is bent.