Olevel/IGCSE | Molecular, Displayed, Structural Formulas | (lecture 3/29)

Molecular Formulas and Definitions
📌 A molecular formula shows the total number of atoms of all elements in a molecule.
📌 A displayed formula illustrates all the bonds between the atoms present in a molecule, providing more detail than the molecular formula.
📌 A structural formula (or condensed formula) is a shorthand way to show connectivity, condensing groups like $\text{CH}_3$ or $\text{CH}_2$ together, which is less time-consuming to draw than the displayed formula.

Alkanes: Formulas and Structures (C₁ to C₅)
📌 The general formula for alkanes is $\text{C}_n\text{H}_{2n+2}$.
📌 Methane ($n=1$) has the molecular formula $\text{CH}_4$; its displayed and structural formulas appear similar due to the single carbon atom.
📌 Ethane ($n=2$) has the molecular formula $\text{C}_2\text{H}_6$ and is characterized by a single $\text{C}-\text{C}$ bond ($\text{CH}_3\text{CH}_3$).
📌 For Propane ($n=3$, $\text{C}_3\text{H}_8$), the central carbon atom is bonded to only two hydrogen atoms ($\text{CH}_2$) because it forms two single bonds with adjacent carbons.
📌 Butane ($n=4$, $\text{C}_4\text{H}_{10}$) shows that terminal carbons are $\text{CH}_3$, while interior carbons are $\text{CH}_2$.

Structural Formula Shorthand
📌 The structural formula can be further condensed by grouping repeating units in parentheses, like $\text{CH}_3(\text{CH}_2)_3\text{CH}_3$ for Pentane ($n=5$).
📌 When writing structural formulas, $\text{CH}_3$ or $\text{CH}_2$ groups are written sequentially, and omitting the explicit line bonds between carbons implies a single bond, provided the valency of carbon is satisfied by surrounding H atoms.

States of Alkanes at Room Temperature
📌 Methane ($\text{CH}_4$), Ethane ($\text{C}_2\text{H}_6$), Propane ($\text{C}_3\text{H}_8$), and Butane ($\text{C}_4\text{H}_{10}$) are all gases at room temperature (e.g., cooking gas).
📌 Pentane ($\text{C}_5\text{H}_{12}$) is the first alkane in the series that exists as a liquid at room temperature.
📌 As the number of carbon atoms increases, the alkane fraction becomes heavier, and they transition from gas to liquid, eventually becoming solids (used as fuels derived from petroleum).

Key Points & Insights
➡️ Displayed formulas provide the most complete information (atoms and all bonds) but are time-consuming to draw.
➡️ Structural formulas ($\text{CH}_3\text{CH}_2\text{CH}_3$) are the preferred method for balancing detail with drawing efficiency.
➡️ Memorize that Pentane is the first alkane that is liquid at room temperature, differentiating it from the lighter gaseous alkanes ($\text{C}_1$ to $\text{C}_4$).
➡️ Organic chemistry is considered relatively easy and enjoyable compared to concepts like acid-base chemistry or mole calculations.

📸 Video summarized with SummaryTube.com on Nov 22, 2025, 13:54 UTC