Difficulty: Introductory | Prerequisites: General chemistry basics (atoms, bonds, valence electrons)
This is the foundation chapter for organic chemistry. It covers how molecules are represented on paper and on exams, from full structural drawings to shorthand line-angle formulas. If you cannot move fluently between these representations, every later chapter (nomenclature, reactions, stereochemistry) will be harder than it needs to be. You should already be comfortable with covalent bonding, Lewis structures, and the idea that carbon forms four bonds.
Organic chemistry uses several ways to write the same molecule on paper, each showing a different level of detail. Molecular and empirical formulas tell you what atoms are present (and in what ratio), while structural, condensed, and line-angle formulas show you how those atoms are actually connected. Being able to convert between these representations is a core skill you will use in every subsequent chapter.
Isomers
Compounds that share the same molecular formula but have different structural formulas, resulting in different physical and chemical properties.
In simple terms, isomers are molecules built from the same set of atoms but assembled in a different arrangement.
Monomer
A small molecule of identical, repeating structure that serves as the building block of a polymer.
Think of it as a single bead that, when strung together with many copies of itself, forms a chain.
Polymer (macromolecule)
A large molecule formed when monomers join together by covalent bonds, often denoted with the repeating unit in parentheses and a subscript n indicating the number of repeat units.
In simple terms, a polymer is a long chain made by linking many small identical molecules end to end. Polyethylene (plastic bags) is a common example.
Molecular formula
A formula that states the actual number of each type of atom in one molecule of a compound (e.g. ethanol = C₂H₆O).
Think of it as the ingredient count: it tells you exactly how many of each atom you have, but not how they are connected.
Empirical formula
The simplest whole-number ratio of atoms in a compound (e.g. C₈H₁₆O₂ reduces to C₄H₈O).
In simple terms, this is the molecular formula boiled down to its lowest terms. Many different compounds can share the same empirical formula.
Structural formula
A representation that shows every atom and every bond in a molecule, with covalent bonds drawn as lines (single bond: one line; double bond: two lines; triple bond: three lines).
Think of it as the full blueprint of the molecule, nothing hidden.
Condensed structural formula
A formula that shows the order of atoms and their grouping on a single line, without drawing every bond (e.g. ethanol = CH₃CH₂OH).
In simple terms, it is a shorthand version of the structural formula where you group hydrogen atoms with the carbon or heteroatom they are attached to.
Line-angle formula (skeletal formula)
A shorthand representation where carbon-carbon bonds are drawn as a zigzag line, carbon atoms are implied at each vertex and endpoint, hydrogen atoms on carbon are omitted, and all other atoms (O, N, etc.) are written out (e.g. ethanol = a two-segment zigzag ending in OH).
Think of it as the stick-figure version of a molecule. It is the most commonly used representation in organic chemistry.
Organic compounds contain at least one C–H bond.
All other compounds are classified as inorganic.
Isomers share the same molecular formula but differ in structural formula.
Different structures lead to different physical properties (boiling point, solubility) and different chemical reactivity.
This is why a molecular formula alone is often not enough to identify a compound.
A monomer is the small, repeating molecular unit.
A polymer forms when many monomers join via covalent bonds.
Notation: the repeating unit is placed in parentheses with a subscript n.
Example: ethylene (monomer) polymerises into polyethylene.
Molecular formula
States the exact number of each atom in one molecule.
Useful but can be misleading when isomers exist, because different compounds can share the same molecular formula.
Example: ethanol = C₂H₆O.
Empirical formula
The simplest whole-number ratio of atoms.
Found by dividing all subscripts by their greatest common factor.
Cannot tell you the structure, shape, or specific properties of the compound.
Multiple different compounds can reduce to the same empirical formula.
Example: C₈H₁₆O₂ divides by 2 to give C₄H₈O.
Structural formula
Shows every atom and every covalent bond explicitly.
Bond notation: single bond = one line, double bond = two lines, triple bond = three lines.
Example: ethanol drawn with all C, H, and O atoms and all bonds visible (H–C–C–O–H framework with hydrogens shown).
Condensed structural formula
Writes atoms in the order they are bonded, grouped on a single line.
Particularly useful when several hydrogen atoms attach to the same carbon or heteroatom.
Example: ethanol = CH₃CH₂OH.
Line-angle (skeletal) formula
Carbon backbone drawn as a zigzag line.
Each vertex or line endpoint represents a carbon atom.
Hydrogen atoms bonded to carbon are implied (enough H to give carbon four bonds).
Heteroatoms (O, N, halogens, etc.) are always written out.
Example: ethanol is drawn as a two-segment zigzag ending in OH.
Representation | Ethanol |
|---|---|
Molecular formula | C₂H₆O |
Empirical formula | C₂H₆O (already in simplest ratio) |
Structural formula | H–C–C–O–H with all H atoms drawn |
Condensed structural | CH₃CH₂OH |
Line-angle | Zigzag with two segments, ending in OH |
Given: C₈H₁₆O₂
Identify the subscripts: 8, 16, 2.
Find the greatest common factor: GCF = 2.
Divide each subscript by 2: C₄H₈O.
The empirical formula is C₄H₈O.
The repeating unit of a polymer is enclosed in parentheses with a subscript n. For polyethylene, the monomer ethylene (C₂H₄) is shown as the repeating –CH₂–CH₂– unit inside parentheses, and n indicates the number of times it repeats.
Line-angle formulas are the standard notation in pharmaceutical research, patent filings, and chemical databases. When a chemist sketches a drug candidate on a whiteboard, they are almost always using skeletal (line-angle) notation because it is fast and uncluttered.
Polymer notation matters in materials science and manufacturing: polyethylene, polypropylene, and PVC are all described using the repeating-unit-in-parentheses convention covered here.
Students often assume that two compounds with the same molecular formula are the same substance. They are not; isomers share a formula but differ in structure and properties.
Students confuse the empirical formula with the molecular formula. The empirical formula is a ratio, not an exact count. Glucose (C₆H₁₂O₆) and acetic acid (C₂H₄O₂) both reduce to the empirical formula CH₂O.
In line-angle drawings, students sometimes forget to count the hydrogens on carbon. Each carbon needs four bonds total; if only two lines meet at a vertex, there are two implied H atoms, not zero.
Students occasionally write heteroatoms (O, N) as implied in a line-angle formula. Only carbon and its hydrogens are implied; every other atom must be written explicitly.
⚠️ Converting between structural, condensed, and line-angle formulas is tested heavily. You may be given one form and asked to produce another.
⚠️ Empirical formula problems appear on nearly every exam. Know how to find the GCF of subscripts and reduce.
⚠️ Identifying isomers from a molecular formula is a common multiple-choice trap. Two structures can look quite different and still have the same molecular formula.
⚠️ Counting hydrogens from a line-angle formula (carbon has four bonds, subtract the lines you see) is a fast way to lose marks if you rush.
True or False: Two compounds with the same molecular formula must have the same physical properties.
False. Isomers share a molecular formula but can have very different properties.
Fill in the blank: The empirical formula of C₆H₁₂O₆ is ______.
CH₂O.
True or False: In a line-angle formula, oxygen atoms are implied and do not need to be written.
False. Only carbon and hydrogen on carbon are implied; all heteroatoms are written explicitly.
Fill in the blank: A polymer is formed when many ______ join together by covalent bonds.
Monomers.
True or False: The condensed structural formula for ethanol is CH₃CH₂OH.
True.
Q: What is the empirical formula of C₁₂H₁₀O₆?
A: Divide each subscript by 2. The empirical formula is C₆H₅O₃.
Q: Write the condensed structural formula for propane (a three-carbon alkane with only single bonds).
A: CH₃CH₂CH₃.
Q: A line-angle drawing shows a zigzag with three segments and an OH at the end. How many carbon atoms are in this molecule?
A: Four. Each vertex and each endpoint of the zigzag represents a carbon atom. Three segments produce four carbons.
Q: Explain why the molecular formula C₂H₆O is not sufficient to identify a single compound.
A: C₂H₆O corresponds to at least two different compounds: ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃). These are isomers with different structures and properties.
Q: What information does a structural formula provide that a molecular formula does not?
A: A structural formula shows how the atoms are connected and what types of bonds (single, double, triple) exist between them. A molecular formula only states the count of each element.
This material connects directly to nomenclature (Chapter 2 in most texts), because naming organic compounds requires you to read line-angle and condensed formulas fluently. Stereochemistry (Chapter 5 typically) builds on structural formulas by adding three-dimensional spatial information. Functional-group chemistry, covered in the second part of this same chapter and in later chapters, depends on your ability to spot groups like OH and COOH inside any formula representation.
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