Electrons in atoms occupy quantized states rather than arbitrary classical orbits. For introductory chemistry, these states are organized into shells and subshells with limited capacities. The detailed arrangement of an atom's electrons is its electron configuration.

The electrons most involved in ordinary chemical bonding are the outer valence electrons. Elements with similar valence configurations often show similar chemical behavior because their outer electrons can participate in similar interactions.

An atom can also gain or lose electrons without changing its nucleus. Because electrons carry negative electric charge, losing electrons produces a positively charged ion, while gaining electrons produces a negatively charged ion.

For example, a neutral sodium atom has 11 electrons. If it loses one electron, it becomes a sodium ion with charge $+e$. A neutral chlorine atom can gain one electron to form an ion with charge $-e$.

When atoms approach one another, their electrons and nuclei interact electromagnetically. Some arrangements of the combined electrons and nuclei have lower energy than the separated atoms, producing stable chemical bonds. Valence structure therefore provides the bridge between atomic structure and ordinary chemical bonding.