Lewis Structure
A Lewis structure (also called a Lewis dot structure or electron dot diagram) is a two-dimensional representation of a molecule that shows the arrangement of atoms, bonding electron pairs (as lines or pairs of dots between atoms), and lone (non-bonding) electron pairs. Developed by Gilbert N. Lewis in 1916, these diagrams are essential tools for predicting molecular geometry, reactivity, and understanding bond types. Lewis structures obey the octet rule — most atoms in a molecule strive to have 8 electrons in their valence shell, with the notable exception of hydrogen (which requires only 2).
Worked Example
Problem
Draw the Lewis structure for water (H₂O). Count valence electrons and assign lone pairs.
Solution
Step 1: Count total valence electrons: O has 6, each H has 1. Total = 6 + 2(1) = 8 valence electrons. Step 2: Identify central atom: O is less electronegative than... wait — O is the central atom because H can only form 1 bond. Step 3: Connect atoms with single bonds: H–O–H uses 4 electrons (2 bonds × 2 electrons). Remaining = 8 − 4 = 4 electrons. Step 4: Place remaining 4 electrons as lone pairs on O: 2 lone pairs. Step 5: Check octets: O has 2 bonds (4e) + 2 lone pairs (4e) = 8 electrons. ✓ Each H has 1 bond = 2 electrons. ✓ Step 6: Formal charge on O = 6 − 4 − (4/2) = 6 − 4 − 2 = 0. All formal charges = 0. ✓
Answer
H₂O Lewis structure: O is central with 2 O–H single bonds and 2 lone pairs on oxygen; all formal charges = 0.
Lewis Structures and Electron Counts for Common Molecules
| Molecule | Formula | Valence Electrons | Bonding Pairs | Lone Pairs on Central Atom |
|---|---|---|---|---|
| Water | H₂O | 8 | 2 | 2 |
| Ammonia | NH₃ | 8 | 3 | 1 |
| Methane | CH₄ | 8 | 4 | 0 |
| Carbon Dioxide | CO₂ | 16 | 4 (2 double bonds) | 2 (on each O) |
| Nitrogen gas | N₂ | 10 | 3 (triple bond) | 1 (on each N) |
Interactive Tools
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Related Terms
Covalent Bond
A covalent bond is a type of chemical bond formed when two atoms share one or more pairs of electrons, resulting in a stable arrangement for both atoms. This sharing occurs most commonly between non-metal atoms that have similar electronegativities, allowing each atom to achieve a full valence shell without complete electron transfer. Covalent bonds are the foundation of organic chemistry and molecular biology, governing the structure of molecules ranging from water (H₂O) to complex proteins.
VSEPR Theory
Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the three-dimensional geometry of molecules based on the principle that electron pairs in the valence shell of a central atom repel each other and arrange themselves as far apart as possible to minimize repulsion. The theory considers both bonding pairs and lone pairs, with lone pairs exerting greater repulsive force than bonding pairs, which distorts ideal bond angles. VSEPR theory was developed by Ronald Gillespie and Ronald Nyholm in 1957 and remains one of the most useful and accessible tools for predicting molecular shape.
Molecular Geometry
Molecular geometry (or molecular shape) refers to the three-dimensional spatial arrangement of atoms within a molecule, determined by the positions of the atoms — not the lone pairs — around the central atom. The geometry is predicted using VSEPR theory or hybridization models and directly influences physical properties such as polarity, reactivity, phase of matter, colour, magnetism, and biological activity. Common geometries include linear, bent, trigonal planar, trigonal pyramidal, tetrahedral, and octahedral.
Named after Gilbert Newton Lewis (1875–1946), American physical chemist who introduced the concept of the shared electron pair bond and electron dot notation in his 1916 paper "The Atom and the Molecule."