Octet rule

The octet rule is the chemical principle that atoms often gain, lose, or share electrons to reach eight electrons in their outermost shell, a configuration associated with noble gases.

Lewis structure: A diagram showing atoms, bonds, and valence-electron pairs in a molecule or ion. Lewis structures make an atom’s valence electrons and shared pairs visible for octet counting.
Valence electron: An electron in an atom’s outermost occupied shell that participates in chemical bonding. Octet counting concerns the electrons available in an atom’s outer shell.
Duet rule: The tendency of hydrogen and helium to reach a stable configuration with two electrons in their first shell. First-shell atoms follow a two-electron target rather than an octet.
Chemical formula: A notation specifying the elements and relative numbers of atoms in a chemical substance. A formula constrains which atom ratios can satisfy common ionic or covalent valence patterns.
Ionic bond: An electrostatic attraction between oppositely charged ions, typically formed after electron transfer. Electron transfer can give each ion a noble-gas-like outer shell.
Electron shell: A set of atomic orbitals grouped by principal energy level around an atomic nucleus. The rule is framed in terms of filling an atom’s outer electron shell.
Odd-electron molecule: A molecule containing an odd total number of valence electrons, leaving at least one unpaired electron. An odd electron count prevents every atom from being assigned a conventional complete octet.
Chemical bonding: The interactions that hold atoms together in molecules, crystals, and other chemical structures. The octet rule is a simple heuristic for anticipating some bonding arrangements.
Covalent bond: A chemical bond formed when atoms share one or more pairs of electrons. Shared bonding pairs count toward the outer shells of both bonded atoms.
Noble gas: An element in Group 18, whose atoms generally have filled valence shells and low chemical reactivity. Their filled outer shells provide the configuration the rule treats as a common target.

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