Bonding and molecular geometry

Atoms bond to reach a lower energy state. The type of bond determines molecular properties.

Bond types

Lewis structures — Count valence electrons, draw bonds (shared pairs) and lone pairs. The octet rule guides most main-group atoms.

VSEPR theory — Electron-pair geometry determines molecular shape:

$$\text{Steric number} = \text{bonding pairs} + \text{lone pairs}$$

Bond enthalpy

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

A negative $\Delta H$ means the reaction is exothermic.

Dipole moments — The molecular dipole is the vector sum of individual bond dipoles: $\vec{\mu}_{\text{total}} = \sum \vec{\mu}_i$.

Key insight: Symmetric molecules (like CO$_2$) can have polar bonds yet zero net dipole because the bond vectors cancel.

Common pitfall: Molecular shape is set by all electron groups, including invisible lone pairs — but the named geometry describes only the atoms. Water has four tetrahedral electron groups yet is called "bent".