Writing Chemical  Formulae

Key Notes:

Definition of Chemical Formula:

  • A chemical formula represents the elements in a compound and their relative proportions.
  • It consists of symbols for elements and subscripts indicating the number of atoms of each element in the molecule.

Symbols of Elements:

  • Every element has a unique symbol (e.g., H for hydrogen, O for oxygen, Na for sodium).
  • These symbols are used in writing chemical formulas.

Types of Chemical Formulas:

  • Empirical Formula: Represents the simplest whole-number ratio of elements in a compound (e.g., CH for benzene).
  • Molecular Formula: Represents the actual number of atoms of each element in a molecule (e.g., C6H6 for benzene).
  • Structural Formula: Shows the arrangement of atoms in a molecule.

Writing Formulae for Ionic Compounds:

  • Ionic compounds are formed by the transfer of electrons between metals and non-metals.
  • To write the formula:
    1. Write the symbol of the metal (cation) first.
    2. Write the symbol of the non-metal (anion) next.
    3. Balance the charges to ensure the compound is neutral (e.g., NaCl, MgO).
    4. Use subscripts to indicate the number of ions needed to balance the charges (e.g., Al₂O₃ for aluminum oxide).

Writing Formulae for Covalent Compounds:

  • Covalent compounds are formed by the sharing of electrons between non-metals.
  • Use prefixes (mono-, di-, tri-, etc.) to indicate the number of atoms of each element (e.g., CO₂ for carbon dioxide, N₂O₄ for dinitrogen tetroxide).
  • Do not use prefixes for the first element if it has only one atom (e.g., CO for carbon monoxide).

Polyatomic Ions:

  • Some compounds contain polyatomic ions (groups of atoms that carry a charge), such as sulfate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺).
  • In these cases, the polyatomic ion is treated as a single unit and placed in parentheses if more than one is needed (e.g., Ca(NO₃)₂ for calcium nitrate).

Balancing the Charges:

  • The total positive charge must equal the total negative charge to ensure the compound is neutral.
  • The number of cations and anions can be adjusted by using appropriate subscripts.

Common Examples:

  • Sodium chloride: NaCl (1:1 ratio of Na+ and Cl-).
  • Calcium carbonate: CaCO₃ (Ca²⁺ and CO₃²⁻ in a 1:1 ratio).
  • Water: H₂O (two hydrogen atoms for every oxygen atom).

Rules for Writing Chemical Formulae:

  • For Ionic Compounds: Balance charges, write the cation first, and use the lowest ratio of ions.
  • For Covalent Compounds: Use prefixes to indicate the number of atoms of each element.

Let’s practice!