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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

S3.2 - Functional groups - Classification of organic compounds

3.2.1 Representing Organic Compounds 3.2.2 Functional Group 3.2.3 Homologous Series 3.2.4 Trends and Properties of Homologous Series 3.2.5 Nomenclature 3.2.6 Structural Isomerism 3.2.7 Stereoisomerism and Chirality (AHL) 3.2.8 Mass Spectrometry (MS) of Organic Compounds (AHL) 3.2.9 Infrared (IR) Spectroscopy and Greenhouse Gases (AHL) 3.2.10 Proton NMR Spectroscopy (AHL) 3.2.11 Splitting Patterns in Proton NMR Spectroscopy (AHL) 3.2.12 Interpreting Spectra (AHL)

Representing Organic Compounds

Specification Reference 3.2.1

Quick Notes

  • Empirical formula – Simplest whole-number ratio of atoms in a compound. (e.g. CH2 for C2H4)
  • Molecular formula – Actual number of atoms of each element. (e.g. C2H4)
  • Full structural formula – Shows all atoms and all bonds.
  • Condensed structural formula – Groups atoms to simplify notation. (e.g. CH3CH3 for ethane)
  • Skeletal formula – Omits C and H atoms bonded to C; only bonds and functional groups are shown.
  • Stereochemical formula – Shows 3D arrangement (e.g. wedges and dashes); only drawn when required.

Full Notes

Overview of Formula Types

Organic molecules can be written using different formula types and you need to be comfortable using and changing between the following:

Formula Type Description Example (Butane)
Empirical Simplest ratio of atoms C2H5
Molecular Actual number of atoms C4H10
Full Structural Shows all atoms and all covalent bonds H3C–CH2–CH2–CH3
Condensed Structural Atoms grouped for compactness CH3CH2CH2CH3
Skeletal Carbon atoms implied at ends and bends, H not shown Zigzag line (structure only)

Molecular Formula – Shows the actual number of atoms in a molecule.

ExampleHexane → C6H14

IB Chemistry molecular formula example of hexane C6H14

Empirical Formula – Shows the simplest whole number ratio of atoms in a compound.

Example Hexane (C6H14) → C3H7

IB Chemistry empirical formula example showing hexane simplified to C3H7

Structural Formula – Shows how atoms are arranged in a molecule without drawing bonds.

Example Hexane → CH3CH2CH2CH2CH2CH3

IB Chemistry structural formula example for hexane showing CH3CH2 repeats

Displayed Formula – Shows all bonds and atoms.

Example Hexane

IB Chemistry displayed formula diagram of hexane with all bonds shown

Skeletal Formula – Simplified representation where carbon atoms are at the ends and bends of lines, and hydrogens bonded to carbon atoms aren’t shown.

Example Hexane

IB Chemistry skeletal formula diagram of hexane as a zig-zag line
Photo of Matt
Matt’s exam tip

Matt’s exam tip - if you find drawing skeletal formulas hard, start by drawing the displayed formula with a diagonal pattern between C-C bonds, then remove all the C-H bonds.
CIE A-Level Chemistry tip image showing how to convert from displayed to skeletal by removing C–H bonds

Constructing 3D Models

Molecules are 3D and we can represent this in several ways - such as using ball-and-stick kits (physical models) and virtual modelling software. We can also draw molecules using wedge and dash notation to show bond angles.

Example Methane (CH4) and Ethene (CH2CH2)

IB Chemistry stereochemical wedge-and-dash diagram showing methane tetrahedral 3D arrangement. IB Chemistry stereochemical diagram showing trigonal planar bonding of ethene with double bond.

Linked Course Questions

Structure 2.2 – Linked Course Question

What is unique about carbon that enables it to form more compounds than the sum of all the other elements’ compounds?

Carbon is unique because it can form four strong covalent bonds, allowing it to bond with many other atoms, including itself. This leads to long chains, rings, and branched structures. Carbon also forms single, double, and triple bonds, and can bond with a wide variety of elements. This versatility makes it the backbone of millions of compounds, especially in organic chemistry.


Nature of Science, Structure 2.2 – Linked Course Question

What are the advantages and disadvantages of different depictions of an organic compound?

Different representations of organic compounds each highlight different aspects:

  • Structural formulas show all atoms and bonds, making connectivity clear, but can become cluttered for large molecules.
  • Skeletal formulas simplify complex structures by omitting hydrogen and carbon labels, making them quick and efficient but harder for beginners to interpret.
  • Stereochemical formulas show 3D arrangement (e.g. wedges and dashes), important for understanding isomerism and reactivity, but can be more abstract.
  • 3D models (physical or digital) give the most realistic view of molecular shape and bond angles, but are less practical for quick drawing or notation.

Each depiction is useful in different contexts — scientists choose based on what information they need to communicate.

Summary