Types of Formulae of Organic Compounds Explained Simply | Easy Summary + Examples for MDCAT/ECAT

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Organic chemistry often feels like a puzzle, but its formulae act like the key that unlocks every shape and structure. Each formula style gives a different view of the same molecule: some show bonding, some show electrons, and some reveal the 3D form.

This guide by Inam Jazbi gathers all important types of formulae of organic compounds in one colourful, exam-friendly post, crafted especially for MDCAT, FSC and O-level learners.

🔍 🌈 Types of Formulae of Organic Compounds Explained Simply | Easy Summary + Examples✨ 🎯

Structures of organic compounds are represented in several ways. Structures of organic compounds are represented by dot or Lewis, dash, condensed, bond line and three dimensional structural formulas. Out of these condensed and bond line formulas are commonly used in nomenclature and chemical reactions.

Each organic compound has specific formula. There are five types of formulae of organic compounds:

1. Molecular formula

2.Structural Formula/Expanded structural formula/Lewis formula/Displayed formula

3. Condensed structural formula

4. Electronic formula/Cross-dot formula

5.Bond-line/Skeletal structural formula/Skeletal structural formula

🌟📘1. Molecular Formula

The formula which represents the actual number of atoms in one molecule of the organic compound is called the molecular formula, 

e.g., molecular formula of butane is C₄H₁₀ . It shows:

(i) Butane is made up of carbon and hydrogen atoms.

(ii) Each molecule of butane consists of 4 carbon atoms and 10 hydrogen atoms.

🌟📘2. Electronic or Dot and Cross Formula or Electron dot structure

The formula which shows the sharing of electrons between various atoms in one molecule of the organic compound is called dot and cross formula or electronic formula. Here dots or dot-cross are used to represent all of the valence electrons of all the bonded atoms in the molecule.

Writing dot structure is tedious and time-consuming. The other representation are more convenient and, are therefore, more often used.

🌟📘3. Lewis structures/Expanded structural formula/Dashed or complete structural formulas

Structural formula or Lewis structure of a compound represents the exact arrangement of the different atoms of various elements present in a molecule of a substance using a “stick or dash (−)” between the bonded atoms to represent a shared electron pair. Such a structural formula focuses on the electrons involved in bond formation.

In a structural formula, single bond is represented by a single line (−), a double bond by two lines (=) and a triple bond by three lines (≡) between the bonded atoms.

The unshared valence electrons that are not involved in covalent bonds are called non-bonding electrons or lone pair. The lone pairs are assigned to specific atoms using dots drawn next to the symbols for these atoms. Lone pairs of electrons on heteroatoms (e.g., oxygen, nitrogen, phosphorus, sulphur, halogens etc.) may or may not be shown. Organic compounds may have same molecular formulae but different structural formulae and these compounds are called isomers, e.g., butane C₄H₁₀  has two structural formulae

🌟📘4. Condensed Formula/Semi-structural formula/ condensed structural formula

The complete structural formulae can be shortened by leaving out some or all of the covalent bonds and by denoting the number of identical groups attached to an atom by a subscript and the resulting description of the molecule is called a condensed structural formula.

For example

The condensed formula for iso-propyl alcohol can be written in four different ways:

It is the formula in which the structure of a compound is written in a line of text omitting all the vertical and horizontal bonds. The formula that indicates the group of atoms joined together to each carbon atom in a straight chain or a branched chain is called the condensed formula. It is the formula in which bonds to each carbon are omitted and each distinct structural unit is written with subscript number for multiple substituents including hydrogen.

More Examples

(i) Pentanoic acid is expressed as CH₃CH₂CH₂CH₂CO₂H or CH₃ (CH₂)₃CO₂H

(ii) Similarly, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃ can be further condensed to CH₃(CH₂)₆CH₃.

(iii) n-butane and iso-butane are expressed as CH₃CH₂CH₂CH₃ and CH₃CH(CH₃)CH₃ respectively.

(iv) Ethane, ethene, ethyne and methanol have expressed as following condensed structure:

CH₃CH₃, H₂C=CH₂, HC≡CH, CH₃OH

🌟📘5.Bond-Line structures/skeletal Formula/ line-angle formulas/Bond Line Structural formula

Bond-line structures or skeletal structures is an abbreviated drawing convention or structural representation of organic molecules in short hand manner by replacing the letters C and H with series of lines in a zig-zag fashion in which each angle between the lines and each line end represent a carbon atom. (Hydrogen atoms are not shown but are assumed to be attached with to satisfy the tetravalencey of carbon atoms. Hydrogens bonded to nitrogen, oxygen, sulfur, or anything other than carbon are shown, but are usually drawn without showing the bond). The terminals denote methyl (–CH₃) groups (unless indicated otherwise by a functional group). All other heteroatoms like N, O, S, F, Cl, Br and I are represented by their symbols in skeleton. lone pairs are also not shown, it is assumed the person viewing the structure knows where they are.

The bond-line formula is the quickest of all to write because it shows only the carbon skeleton.

Line angle formulas help show structure and order of the atoms in a compound making the advantages and disadvantages similar to structural formulas.

Examples

The following examples illustrate the convention.

Multiple bonds are also indicated in bond-line formulae. For example:

Bond-Line Formulae for Ring or Cyclic Compounds

Cyclic compounds containing one or more rings is represented by drawing the suitable ring (polygon) without indicating the carbon and hydrogen atoms. The corner of the polygon denotes a carbon atom and its sides represent a carbon-carbon bond. The heteroatoms are always shown in the structure.

Various ways of representing 2-bromobutane are:

🌟📘6. Three-Dimensional Representation of Organic Molecules /Method of Draw Three-dimensional structural formulae

Examples

3-D representation using the wedge-dash representation for methane molecule on paper has been shown as below:

For example, you might want to show the 3-D arrangement of the groups around the carbon which has the -OH group in butan-2-ol. The normal structural formula (condensed formula), skeletal formula and the bond-line notation for Butan-2-ol are given below:

🌟 Quick Summary of Types of Formulae of Organic Compounds for MDCAT/ECAT

🧪 1. Molecular Formula
Shows the actual number of atoms in a molecule.
Example: C₂H₆O

📉 2. Empirical Formula
Shows the simplest whole-number ratio of atoms.
Example: CH₃ (for C₂H₆)

🧩 3. Structural Formula
Reveals how atoms are connected.
Three types:
💡Complete (all bonds shown)
💡Condensed (grouped)
💡Expanded (single bonds clearly drawn)

🎨 4. Displayed Formula
Shows every bond using lines; ideal for beginners.
Example: Full bond-by-bond drawing of ethanol.

📐 5. Skeletal / Line-Angle Formula
Carbon atoms shown as corners and line ends; hydrogens on carbon not shown.
Perfect for long chains and aromatic rings.

💡 6. Lewis (Electron-Dot) Formula
Shows valence electrons around atoms using dots.
Useful for predicting reactivity and bond formation.

🌀 7. Resonance Structures
Multiple valid structures showing electron delocalization.
Used for benzene, carbonate ion, phenoxide etc.

🔮 8. 3D Structural (Wedge) Formula
Uses solid wedges, dashed wedges, and lines to show spatial arrangement.
Crucial for stereochemistry (R/S).

🔥 9. General Formula
Represents a homologous series.
Examples:
• Alkanes: CₙH₂ₙ₊₂
• Alkenes: CₙH₂ₙ
• Alkynes: CₙH₂ₙ₋₂

🔎 10. Bond-Line / Zig-Zag Formula
A simplified version of skeletal formula used widely for organic mechanisms.

🧪🌈 MCQ Quiz (MDCAT Level) on Types of Formulae of Organic Compounds

1) Which formula gives only the simplest whole-number ratio of atoms?

🟦 A) Empirical Formula

🟩 B) Molecular Formula

🟧 C) Structural Formula

🟪 D) General Formula

2) Which formula can distinguish isomers best?

🟦 A) Molecular Formula

🟩 B) Empirical Formula

🟧 C) Structural Formula

🟪 D) General Formula

3) Which formula hides all hydrogens attached to carbon?

🟦 A) Displayed Formula

🟩 B) Wedge Formula

🟧 C) Skeletal Formula

🟪 D) Expanded Formula

4) Which representation is essential for showing stereochemistry (R/S)?

🟦 A) Skeleton formula

🟩 B) Wedge–dash formula

🟧 C) Condensed formula

🟪 D) Lewis structure

5) C₂H₆O and CH₃OH show the same molecular formula but differ in:

🟦 A) Empirical formula

🟩 B) Structural formula

🟧 C) General formula

🟪 D) Resonance structures

6) The formula CH₂O represents which type of formula for glucose C₆H₁₂O₆?

🟦 A) Structural

🟩 B) Molecular

🟧 C) Empirical

🟪 D) 3D Formula

7) A benzene ring drawn as a hexagon with alternating double bonds represents:

🟦 A) Wedge structure

🟩 B) Resonance structure

🟧 C) Lewis structure

🟪 D) Displayed structure

8) Which formula shows lone pairs most clearly?

🟦 A) Molecular

🟩 B) Condensed

🟧 C) Lewis dot

🟪 D) General

9) The general formula CₙH₂ₙ₊₂ belongs to:

🟦 A) Alkanes

🟩 B) Alkenes

🟧 C) Alkynes

🟪 D) Alcohols

10) In wedge–dash notation, a solid wedge represents:

🟦 A) Bond going behind the plane

🟩 B) Bond in the plane

🟧 C) Bond coming out of the plane

🟪 D) Lone pair

11) Ethanol written as CH₃CH₂OH is which type of formula?

🟦 A) Condensed formula

🟩 B) Molecular formula

🟧 C) Wedge formula

🟪 D) Bond-line formula

12) The zig-zag line structure of hexane is which formula?

🟦 A) Displayed

🟩 B) Structural

🟧 C) Bond-line formula

🟪 D) Lewis

13) Which formula shows all atoms and all bonds clearly?

🟦 A) Condensed

🟩 B) Expanded/Displayed

🟧 C) Empirical

🟪 D) Line-angle

14) Which formula is most useful for predicting reactivity?

🟦 A) Molecular

🟩 B) Lewis

🟧 C) Empirical

🟪 D) General

15) Resonance structures differ only in:

🟦 A) Atom positions

🟩 B) Electron positions

🟧 C) Number of atoms

🟪 D) Hybridization

16) Space-filling models fall under:

🟦 A) 3D structural formulae

🟩 B) Empirical formulae

🟧 C) General formulae

🟪 D) Linear formulae

🎯 ANSWERS 

🟩 1. A

🟩 2. C

🟩 3. C

🟩 4. B

🟩 5. B

🟩 6. C

🟩 7. B

🟩 8. C

🟩 9. A

🟩 10. C

🟩 11. A

🟩 12. C

🟩 13. B

🟩 14. B

🟩 15. B

🟩 16. A

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