2.4 Chemical Formulas - Chemistry 2e | OpenStax

Learning Objectives

• Symbolize the composition of molecules using molecular and empirical formulas.
• Represent bonding arrangements of atoms within molecules using structural formulas.

Key Concepts

Molecular and Empirical Formulas

• Molecular Formula: Indicates the types and numbers of atoms in a molecule using chemical symbols and subscripts.
  • Example: Methane (CH₄) shows one C atom and four H atoms.
• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound.
  • Example: Titanium dioxide's empirical formula is TiO₂, indicating a 1:2 ratio of titanium to oxygen.

Structural Formulas

• Depict how atoms are connected in a molecule.
• Use lines to represent bonds between atoms.
• Examples include ball-and-stick and space-filling models to show geometric arrangements.

Diatomic Molecules

• Some elements form diatomic molecules, containing two atoms of the same element (e.g., H₂, O₂, N₂).
• Sulfur commonly forms S₈ molecules consisting of eight sulfur atoms.

Differences in Notation

• H₂: A diatomic molecule of hydrogen.
• 2H: Two separate hydrogen atoms not bonded as a unit.
• 2H₂: Two molecules of diatomic hydrogen.

Compounds

• Formed when atoms from different elements chemically combine.
• Example: Water (H₂O) and table salt (NaCl).

Deriving Formulas

• Molecular to Empirical: Divide by the greatest common factor.
  • Acetic acid, C₂H₄O₂, has an empirical formula CH₂O (1:2:1 ratio).
• Empirical to Molecular: Requires empirical formula and molecular mass.
  • Example: Benzene has an empirical formula of CH and molecular formula C₆H₆.

Isomers

• Compounds with the same molecular formula but different structures (e.g., acetic acid and methyl formate both have C₂H₄O₂).
• Structural isomers differ in atom-to-atom connections.
• Spatial isomers differ in the relative orientations of atoms (e.g., carvone isomers smell differently).

Example 2.6: Empirical and Molecular Formulas

• Glucose has a molecular formula C₆H₁₂O₆ and an empirical formula CH₂O.
• Metaldehyde has a molecular formula C₈H₁₆O₄ and an empirical formula C₂H₄O.

Portrait of a Chemist: Paula Hammond

• Work involves creating new molecular structures for cancer treatment, wound healing, etc.
• Leads research at MIT focusing on nanoparticle drug delivery systems.
• Contributions support advancements in treatment precision and reduced patient harm.

Important Notes

• Isomers and their varying structures affect chemical properties significantly.
• Molecular formulas provide detailed chemical composition, while empirical formulas give a simplified ratio of atoms.

Use these notes to review the key concepts of chemical formulas, structures, and their implications in chemistry.