- Orbit
- Valence shell
- Nucleus
- Core
The valence shell is the outermost electron shell of an atom, containing the electrons that primarily determine chemical behavior, bonding, and reactivity. These valence electrons are the most loosely held and accessible for interactions with other atoms. The number of electrons in this shell governs an element’s position in the periodic table and its chemical properties—elements with full valence shells (noble gases) are inert, while those with nearly full shells tend to gain electrons (nonmetals) and those with few valence electrons tend to lose them (metals). The valence shell concept explains periodic trends: ionization energy (energy to remove an electron) generally increases across a period as nuclear charge increases, and decreases down a group as shells become farther from nucleus. Chemical bonding—ionic, covalent, and metallic—involves valence shell interactions: sharing, transferring, or delocalizing electrons. Understanding valence shells is fundamental in chemistry, explaining compound formation, reaction mechanisms, and material properties across all of science.
