Structural Representations

A Lewis structure (also called an electron-dot structure) shows every valence electron in a molecule. Shared pairs of electrons between two atoms form covalent bonds and are drawn as lines (one line = two electrons). Lone pairs are shown as pairs of dots on the atom. For carbon (Group 14, 4 valence electrons), four bonds must be formed to achieve an octet. For example, methane (CH₄) shows carbon at the centre with four single bonds radiating to four hydrogen atoms, each hydrogen having no lone pairs. Ethene (C₂H₄) introduces a C=C double bond — two shared pairs — leaving each carbon with two bonds to hydrogen. Lewis structures are the most informative representation but become unwieldy for molecules with more than about six carbons.

Condensed formulas list atoms in a linear sequence, grouping the hydrogens attached to each carbon immediately after that carbon symbol. For example, ethane is written CH₃CH₃, propane as CH₃CH₂CH₃, and 2-methylpropane as CH₃CH(CH₃)CH₃. Branches are shown in parentheses. Double bonds are indicated explicitly: propene is CH₃CH=CH₂ and propyne is CH₃C≡CH. Condensed formulas are compact enough for typed text while retaining every atom, making them the standard for databases and problem sets.

Bond-line notation, sometimes called skeletal or line-angle notation, represents the carbon skeleton as a zigzag of lines. Every vertex (bend) and every line terminus represents one carbon atom. Hydrogen atoms bonded to carbon are entirely omitted — you infer them by subtracting the drawn bonds from carbon's normal valency of 4. Heteroatoms (O, N, Cl, etc.) and their attached hydrogens are written explicitly. A straight line represents a single bond; two parallel lines a double bond; three parallel lines a triple bond. For example, butane is drawn as a three-segment zigzag (four carbons, no symbol written). Cyclohexane appears as a regular hexagon. This notation is used universally in advanced organic chemistry because complex ring systems and functional groups become instantly recognisable by their geometric shape alone.

To convert a Lewis structure to a condensed formula: read the connectivity from left to right (or branch to branch), group each carbon with its attached H atoms, and list branches in parentheses. To convert a bond-line structure to a molecular formula: count all vertices and line termini (each = C), count the bonds drawn to each carbon, subtract from 4 to get its hydrogen count, then add all explicit heteroatom hydrogens. To convert a condensed formula to a bond-line structure: draw a zigzag for the main chain with one vertex per carbon, add branches at the correct carbons, insert explicit heteroatoms, and draw multiple bonds as multiple lines. Always verify by computing the molecular formula from both the starting and ending representations.

• n — Number of carbon atoms (dimensionless): A positive integer representing how many carbon atoms are in the straight or branched chain of a saturated hydrocarbon.
• \ce{C_n} — Carbon skeleton (dimensionless): The n carbon atoms, each forming four single bonds (either C–C or C–H) to satisfy the octet rule.
• \ce{H_{2n+2}} — Hydrogen count (dimensionless): The maximum number of hydrogen atoms for a saturated (no double or triple bonds) acyclic hydrocarbon; adding one degree of unsaturation removes two hydrogens (H_{2n}).

Problem. Draw the bond-line structure of 2-methylbutane and determine its molecular formula by counting atoms directly from the skeletal structure.

Answer. 2-methylbutane has the molecular formula C₅H₁₂. Its bond-line structure is a four-carbon zigzag with one extra line at the second vertex.

• Valence Electrons and the Octet Rule — Lewis structures are built by distributing valence electrons so that each atom (especially carbon) achieves a full octet; understanding valence electrons is a prerequisite for drawing any structural representation correctly.
• Molecular Geometry and VSEPR Theory — Once a Lewis structure is drawn, VSEPR theory uses the arrangement of bonding and lone pairs to predict the three-dimensional shape of the molecule — the next level of structural understanding beyond 2-D representations.
• Functional Groups in Organic Chemistry — Bond-line notation makes functional groups (–OH, C=O, –NH₂, etc.) immediately recognisable by their geometric signature; identifying functional groups from skeletal structures is a core skill that builds directly on mastery of notation.
• Structural Isomers and Stereoisomers — Structural representations reveal constitutional isomers (same molecular formula, different connectivity) and are extended to wedge-dash notation for stereoisomers, making notation fluency essential for the study of isomerism.