Week 5 Ionic compounds, naming, formulas , polyatomic

Chemistry: An Introduction to General, Organic, and Biological Chemistry

Learning Goals Week 5

  1. Using charge balance to write the correct formula of an ionic compound

  2. Determine the formulas and names of ionic compounds formed between two given elements

  3. Describe characteristics (properties) of ionic compounds

  4. Name and write chemical formulas of ionic compounds with variable charge metal ions

  5. Name and write chemical formulas of ionic compounds with polyatomic ions

Writing Chemical Formulas for Ionic Compounds

Ionic Compounds

  • Definition: Ionic compounds consist of positive and negative charges that are held together by strong electrical attractions between oppositely charged ions.

  • Crystal Lattice Structure: In their solid state, ionic compounds exist in a crystal lattice that contains many ions, each of the cation (positive ion) and anion (negative ion).

  • Chemical Formula: A chemical formula, such as NaCl, indicates that sodium chloride is comprised of an equal number of sodium ions (Na⁺) and chloride ions (Cl⁻).

  • Example: Sodium sulfide is represented by the formula Na₂S, which indicates that this compound contains twice as many sodium ions as sulfide ions.

Determining Ionic Formulas

Key Concepts

  • The chemical formula represents the symbols and subscripts in the lowest whole-number ratio of the atoms or ions.

  • The charges of the ions must balance such that the sum of the ion charges equals zero:
    extTotalpositivecharge=extTotalnegativechargeext{Total positive charge} = ext{Total negative charge}

Criss-Cross Method for Writing Ionic Formulas

  1. Procedure:

    • Write the symbol and charge of the cation first and the anion second.

    • For instance, for sodium combined with sulfur:

      • Cation: Na⁺

      • Anion: S²⁻

    • Criss-Cross (transpose) only the number of the positive charge (1) to become the subscript of the anion and the number of the negative charge (2) to become the subscript of the cation.

    • Result: Na₂S (leave out all subscripts of 1).

Example

Writing the Formula from Element Symbols

  • Convert Pb⁴⁺ and O²⁻ into a formula:

    • Cation: Pb⁴⁺

    • Anion: O²⁻

    • Apply criss-cross:

      • Result: Pb₂O₄, but reduce to PbO₂.

Naming Ionic Compounds

General Rules

  • When naming an ionic compound:

    • The name of the metal (cation) is written first and is the same as the name of the element.

    • The name of the nonmetal (anion) is written second, with the first syllable of its name followed by the suffix '-ide'.

    • Example: KI is named potassium iodide.

Example Names of Some Ionic Compounds

Compound

Metal Ion

Nonmetal Ion

Name

KI

K⁺

I⁻

Potassium iodide

MgBr₂

Mg²⁺

Br⁻

Magnesium bromide

Al₂O₃

Al³⁺

O²⁻

Aluminum oxide

Properties of Ionic Compounds

  • Ionic compounds typically exhibit the following properties:

    • Solid at Room Temperature: They have a crystalline structure.

    • Brittle: They tend to be brittle and shatter under pressure.

    • High Boiling and Melting Points: Due to strong ionic bonds, they have high thermodynamic stability.

    • Electrical Conductivity: They can conduct electricity when dissolved in water or melted.

Metals with Variable Charge

Key Points

  • Transition metals (except Zn²⁺, Cd²⁺, and Ag⁺) can form two or more positive ions (cations).

  • Roman numerals are used in naming to indicate the charge of the metal ion.

Metal Ion

Charge

Roman Numeral

Cu²⁺

Copper(II)

Cu⁺: Copper(I)

Pb²⁺

Lead(II)

Pb⁴⁺: Lead(IV)

Fe²⁺

Iron(II)

Fe³⁺: Iron(III)

Polyatomic Ions

Definition and Properties

  • Polyatomic ions are groups of ions that have an overall ionic charge.

  • Typically have a charge of -1, -2, or -3, except for ammonium (NH₄⁺), which carries a positive charge.

  • Examples of common polyatomic ions include:

    • SO₄²⁻: Sulfate

    • PO₄³⁻: Phosphate

    • NO₃⁻: Nitrate

    • OH⁻: Hydroxide

    • CO₃²⁻: Carbonate

    • CH₃COO⁻: Acetate

Writing Formulas for Compounds Containing Polyatomic Ions

  • When writing formulas, maintain charge balance as with simple ionic compounds.

  • Example: For magnesium nitrate (Mg(NO₃)₂):

    • Magnesium ion: Mg²⁺

    • Nitrate ion: NO₃⁻

    • Apply charge balance: Mg²⁺ + 2 NO₃⁻ = 0.

Naming Compounds with Polyatomic Ions

  • The naming convention includes:

    • First, write the positive ion (usually a metal).

    • Then write the name of the polyatomic ion.

  • No prefixes are used in these compound names.

Practice and Study Check

  • Review exercises involving formula construction and naming for ionic compounds with variable charges and polyatomic ions.

  • Example questions include writing formulas for compounds based on given ions, identifying ionic compound names, and deducing properties or behaviors based on ionic characteristics.

Additional Exercises and Questions

  • Refer to relevant textbook questions on pages as specified in the document for further practice;

    • Q6.6, Q6.8, Q6.11, Q6.14, Q6.16, Q6.18, Q6.20 (p. 173)

    • Q6.27, Q6.34 (p. 180)

    • Q6.42, Q6.43, Q6.45, Q6.46 (p. 185)