Final answer:
You should count both lone and bonding pairs of electrons when determining electron domains in a Lewis structure, as they both contribute to the electron-pair geometry which is crucial for predicting molecular geometry.
Step-by-step explanation:
When determining the number of electron domains in a Lewis structure, you should count both lone and bonding pairs of electrons. According to Valence Shell Electron Pair Repulsion (VSEPR) theory, every pair of electrons, whether they are in a bond (single, double, or triple bonds all count as one region of electron density) or are present as lone pairs, occupies a region of electron density around the central atom.
The goal of finding these electron domains is to predict the molecular geometry of the molecule. The electron-pair geometry considers the placement of all electrons, which includes both bonding pairs and lone pairs, while molecular structure considers only the arrangement of atoms (which is the bonding-pair geometry).
To summarize, when you write the Lewis structure of a molecule or polyatomic ion, you'll need to count every single, double, or triple bond as one electron domain, in addition to each lone pair of electrons.