Chemistry CFE Practice Test

In covalent compounds, atoms share electrons to form covalent bonds, so they tend to exist as discrete groups that behave as a single unit: a molecule. That molecule is the smallest unit that retains the properties of the compound and can be counted in a sample, like H2O or CO2. An ion would be seen in ionic compounds, a formula unit describes the repeating part of an ionic lattice, and an atom would be the particle only in elemental substances or some very simple cases. While some covalent solids form extended networks with no separate molecules, the typical and most useful picture for covalent compounds is a molecule as the particle.