The elements in Group 18 of the periodic table—known as the noble gases—are famous for their lack of chemical engagement. They include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Noble gases do have valence electrons, but their specific arrangement is what sets them apart from every other element. This configuration explains why they are naturally stable and unreactive.
What Valence Electrons Are
Valence electrons are the electrons that occupy the outermost electron shell of an atom, referred to as the valence shell. These outer-shell electrons are the primary participants in the formation of chemical bonds, such as ionic or covalent bonds, which hold compounds together. They are the electrons that determine an atom’s chemical behavior, dictating how readily an atom will interact with others.
The number of valence electrons an atom possesses directly influences its reactivity and the type of bond it will form. Atoms strive to achieve a lower-energy, more stable state, which involves gaining, losing, or sharing their outermost electrons. For example, atoms with only one or two valence electrons tend to lose them easily, while those with six or seven tend to gain electrons to complete their outer shell.
The Full Shell Configuration
Noble gas atoms have valence electrons, but their valence shells are complete, or “full.” For nearly all noble gases, this full configuration means they have eight valence electrons, a state of maximum stability that other elements attempt to reach. This stable arrangement is described by the Octet Rule, where atoms seek to have eight electrons in their outermost shell.
Neon, Argon, Krypton, Xenon, and Radon all contain eight electrons in their outermost \(s\) and \(p\) orbitals, giving them the stable \(ns^2np^6\) configuration. This perfectly filled shell means they have no incentive to gain or lose electrons, which drives chemical bonding in other elements. Helium is the single exception, achieving stability with only two valence electrons because its first and only shell is full, following the Duet Rule.
Why Noble Gases Are Chemically Inert
The consequence of having a full valence shell is a state of chemical inertness, or non-reactivity. Since noble gases already possess the most stable electron arrangement possible, they do not need to participate in the electron-sharing or electron-transfer processes that form bonds. They exist as single, isolated atoms rather than forming molecules with themselves or other elements.
This high stability is quantified by two specific properties: ionization energy and electron affinity. Noble gases possess the highest ionization energies within their respective periods, meaning it takes significant energy to remove a valence electron. Conversely, they have near-zero electron affinities, indicating they do not attract additional electrons into their complete valence shell. This resistance to both losing and gaining electrons makes them the preferred choice for applications requiring a non-reactive environment, such as specialized welding or filling incandescent light bulbs.