6.7 Molecular and Structural Formula

This topic explains the difference between a molecular formula and a structural formula, and shows how to write a molecular formula from a given structural formula.

Definition

Molecular Formula

A molecular formula shows the number of atoms of each element in a molecule.

Simple explanation: The molecular formula tells you how many of each type of atom are in one molecule — but not how they are arranged.

Structural Formula

A structural formula of a compound shows the arrangement of atoms present in it.

Simple explanation: The structural formula shows how the atoms are connected to each other — which atom is bonded to which.

Main Idea

• Molecular formula → tells you how many atoms of each element.
• Structural formula → tells you how the atoms are arranged (which atoms are bonded together).

Example from the book:

• Structural formula of n-Butane: CH₃—CH₂—CH₂—CH₃
• Molecular formula of n-Butane: C₄H₁₀

Both represent the same compound — but the structural formula shows the arrangement, while the molecular formula just gives the count.

Simple explanation: The molecular formula is like a shopping list (4 C, 10 H); the structural formula is like a map (how they are connected).

Core Concept

Detailed Explanation

Molecular Formula

• Shows the actual number of atoms of each element in one molecule.
• Does not show how the atoms are arranged.

• C₄H₁₀ — butane (4 carbon atoms, 10 hydrogen atoms)
• C₂H₆O — ethanol (2 carbon atoms, 6 hydrogen atoms, 1 oxygen atom)

Structural Formula

• Shows the arrangement of atoms in the molecule.
• Uses dashes (–) or lines to represent bonds between atoms.

• CH₃—CH₂—CH₂—CH₃ — n-butane (a straight chain of 4 carbons)
• CH₃—CH₂—OH — ethanol (a 2-carbon chain with an OH group)

How to Write the Molecular Formula from the Structural Formula

Follow these steps (from the book):

Step 1: Identify different types of elements present in the structural formula.

Step 2: Write symbols of these elements side by side in a line.

Step 3: Count the number of atoms of each element from the structural formula.

Step 4: Show this number of atoms as subscripts of the symbol of the corresponding element.

Simple explanation: Look at the structural formula, count how many of each atom there are, then write the molecular formula.

Example 6.13 (from the book)

Problem: Write the molecular formula of the following compound: CH₃—CH₂—OH

Solution:

Step 1: Elements present: C, H, O

Step 2: Write symbols: C H O

• C: 2 (CH₃—CH₂—)
• H: 6 (3 in CH₃ + 2 in CH₂ + 1 in OH)
• O: 1 (in OH)

Step 4: Write as subscripts: C₂H₆O

Final Answer: C₂H₆O

Simple explanation: Count each type of atom in the structural formula and put the numbers as subscripts.

Practice Problems (from the book)

Write the molecular formulae of the following compounds:

1. CH₃—CH₂—OH

Solution: C₂H₆O

2. CH₃—CH₂—NH₂

Solution: C₂H₇N

3. CH₃—CO—CH₃

Solution: C₃H₆O

Simple explanation: Just count the atoms of each element in the structural formula.

Intermediate Level

Why We Need Both Formulas

Simple explanation: Molecular formula tells you "what's in it"; structural formula tells you "how it's put together."

Steps in Detail

Step 1: Identify elements. Look at the structural formula and list all different elements present.

Step 2: Write symbols. Write the symbols of these elements in a line.

Step 3: Count atoms. Count how many atoms of each element appear in the structural formula.

Step 4: Write subscripts. Write the count as a subscript after each symbol.

Example: CH₃—CH₂—CH₂—CH₃

• Elements: C, H
• Count: C = 4, H = 10
• Molecular formula: C₄H₁₀

Example: CH₃—CH₂—NH₂

• Elements: C, H, N
• Count: C = 2, H = 7, N = 1
• Molecular formula: C₂H₇N

Example: CH₃—CO—CH₃

• Elements: C, H, O
• Count: C = 3, H = 6, O = 1
• Molecular formula: C₃H₆O

Common Structural Formula Notations

Simple explanation: Structural formulas use dashes for bonds and group symbols like CH₃, OH, NH₂ to keep things compact.

Important Points

• - Molecular formula: shows the number of atoms of each element in a molecule.
• - Structural formula: shows the arrangement of atoms in a molecule.
• - Example: n-butane — structural: CH₃—CH₂—CH₂—CH₃; molecular: C₄H₁₀.
• - To write molecular formula from structural formula:
• 1. Identify elements.
• 2. Write symbols.
• 3. Count atoms.
• 4. Write subscripts.
• - Examples:
• - CH₃—CH₂—OH → C₂H₆O
• - CH₃—CH₂—NH₂ → C₂H₇N
• - CH₃—CO—CH₃ → C₃H₆O

Remember

• - Molecular formula = number of atoms.
• - Structural formula = arrangement of atoms.
• - n-Butane: CH₃—CH₂—CH₂—CH₃ → C₄H₁₀
• - Ethanol: CH₃—CH₂—OH → C₂H₆O
• - Ethylamine: CH₃—CH₂—NH₂ → C₂H₇N
• - Acetone: CH₃—CO—CH₃ → C₃H₆O
• - To get molecular formula: count atoms in structural formula.

Common Mistakes

• 1. Confusing molecular and structural formulas. Molecular = count; structural = arrangement.
• 2. Forgetting to count all atoms. In CH₃—CH₂—OH, H = 3 + 2 + 1 = 6, not 3 + 2 = 5.
• 3. Writing the wrong subscript. Always double-check the count.
• 4. Thinking the structural formula shows the shape of the molecule. It shows connectivity, not 3D shape.
• 5. Forgetting that group symbols represent multiple atoms. CH₃ = 1 C + 3 H; OH = 1 O + 1 H.
• 6. Thinking molecular formula gives arrangement. It only gives the number of atoms.

Quick Revision

• Molecular formula: shows the number of atoms of each element in one molecule.
• Structural formula: shows the arrangement of atoms present in the molecule.
• Example: n-Butane
• Structural formula: CH₃—CH₂—CH₂—CH₃
• Molecular formula: C₄H₁₀
• Steps to write molecular formula from structural formula:
• Examples:
• CH₃—CH₂—OH → C₂H₆O
• CH₃—CH₂—NH₂ → C₂H₇N
• CH₃—CO—CH₃ → C₃H₆O

60-Second Revision

• Molecular formula = number of atoms.
• Structural formula = arrangement of atoms.
• n-Butane: CH₃—CH₂—CH₂—CH₃ → C₄H₁₀.
• Ethanol: CH₃—CH₂—OH → C₂H₆O.
• Warning: don't confuse molecular formula with structural formula — one gives count, the other gives arrangement.