Draw and interpret the displayed formula of a molecule to show all the atoms and all the bonds

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IGCSE Chemistry - Organic Chemistry: Formulas, Functional Groups & Terminology - Drawing Molecular Formulas

IGCSE Chemistry 0620

Organic Chemistry - Drawing Molecular Formulas

This section focuses on understanding and drawing the displayed formula of a molecule to represent all atoms and bonds. It's a fundamental skill for interpreting chemical structures and understanding organic chemistry concepts.

Understanding Displayed Formulas

Displayed formulas provide a visual representation of a molecule's structure. They show the connectivity of atoms and the types of bonds between them. Let's break down how to interpret them.

  • Atoms: Each atom is represented by its symbol (e.g., C, H, O, N).
  • Bonds: Single bonds are indicated by a single line (e.g., -O-), double bonds by two lines (e.g., =O), and triple bonds by three lines (e.g., ≡N). A missing line implies a single bond.
  • Connectivity: The lines show which atoms are directly bonded to each other.
  • Hydrogen Atoms: Hydrogen atoms are often not explicitly shown in displayed formulas unless they are part of a functional group or have a specific role in the molecule. They are assumed to be present to satisfy the valency requirements of the atoms.

Drawing Molecular Formulas from Displayed Formulas

To draw a molecular formula from a displayed formula, systematically count the number of each type of atom and represent them with the appropriate symbols, connecting them with the correct number of bonds.

Example 1: Ethanol (CH3CH2OH)

Suggested diagram: Displayed formula of ethanol showing C-H, C-C, C-O, and O-H bonds.

The displayed formula shows:

  • Two Carbon atoms (C)
  • Six Hydrogen atoms (H)
  • One Oxygen atom (O)
  • One Oxygen-Hydrogen bond (O-H)
  • One Carbon-Oxygen bond (C-O)
  • One Carbon-Carbon bond (C-C)
Therefore, the molecular formula is C2H6O.

Example 2: Ethene (C2H4)

Suggested diagram: Displayed formula of ethene showing C=C bonds.

The displayed formula shows:

  • Two Carbon atoms (C)
  • Four Hydrogen atoms (H)
  • One Carbon-Carbon double bond (=C=C)
Therefore, the molecular formula is C2H4.

Functional Groups and Terminology

Functional groups are specific groups of atoms within a molecule that give the molecule characteristic chemical properties. Understanding functional groups is crucial in organic chemistry.

Functional Group Formula Example Molecule Properties
Hydrocarbon CnH2n+2 Methane (CH4) Relatively unreactive; primarily involved in combustion.
Alcohol -OH Ethanol (CH3CH2OH) Soluble in water; can undergo oxidation.
Ether R-O-R' Diethyl ether (CH3CH2OCH2CH3) Used as solvents; relatively unreactive.
Aldehyde -CHO Formaldehyde (HCHO) Reactive; can be oxidized to carboxylic acids.
Ketone -CO- Acetone (CH3COCH3) Reactive; can be reduced to alcohols.
Carboxylic Acid -COOH Acetic acid (CH3COOH) Acidic; reacts with bases to form salts.

Note: The number of hydrogen atoms in a hydrocarbon is determined by the number of carbon atoms. For example, a straight-chain alkane with 3 carbon atoms will have the formula C3H8.

Practice

Question 1: Draw the molecular formula for Benzene.

Question 2: Draw the molecular formula for Propane.

Question 3: Identify the functional group present in Acetone.