Module 5 Lesson 3: Writing and Naming Ionic Compounds

Fundamentals of Chemical Formulas and IUPAC Nomenclature

• Chemical Formula Defined: This indicates the relative number of each kind of atom in a chemical compound. While these have been observed throughout previous modules, this serves as the formal definition.

• Formula Unit: This represents the simplest ratio of ions in an ionic compound. Unlike molecular compounds, ionic compounds exist as large crystalline solids within a crystal lattice structure containing numerous positive and negative ions. A formula unit reduces this lattice down to its most basic, simplest ratio.

• The International Union of Pure and Applied Chemistry (IUPAC):

  • IUPAC is the global organization responsible for establishing naming conventions in chemistry.
  • They manage chemical nomenclature (naming systems) and terminology.
  • They are responsible for naming new elements discovered for the periodic table.
  • They standardize measurements and data analysis methodologies to ensure consistency across the global chemistry community.
  • They provide recommendations that serve as the standard for chemists worldwide.

Naming Binary Ionic Compounds

• Binary Compound Definition: A compound composed of only two different kinds of elements.

• IUPAC Naming System Rules:

  1. Identify the Cation: Write the name of the cation (usually the metal) first. The name remains exactly as it appears on the periodic table.
  2. Identify the Anion: Write the name of the anion (the non-metal) second. Modify the ending of the element's name to $-ide$.
  3. Disregard Subscripts: When naming established binary ionic compounds from a formula (e.g., $CaCO_2$), subscripts do not influence the written name.

• Examples of Naming:

  • Magnesium and Oxygen: Magnesium is the metal. Oxygen's ending is dropped and replaced with $-ide$. The result is magnesium oxide.
  • Potassium and Iodine: Potassium is the metal. Iodine's ending is dropped and replaced with $-ide$. The result is potassium iodide.

Writing Binary Ionic Compound Formulas

• The Neutrality Rule: The total number of positive charges from the cations must be exactly equal to the total number of negative charges from the anions.

• Operational Steps for Formula Writing:

  1. Write the chemical symbols side-by-side, placing the cation first.
  2. Determine the oxidation numbers for each element. These are standardly derived from the group numbers on the periodic table.
  3. Cross Over Charges: Take the absolute value of the charge of one atom and make it the subscript of the other atom.
  4. Reduce to Simplest Ratio: If the subscripts are divisible by a common factor, they must be reduced (similar to reducing fractions) to reflect the formula unit.

• Periodic Table Oxidation Number Reference:

  • Group 1: $1+$
  • Group 2: $2+$
  • Transition Metals: Variable (requires specific notation, discussed later)
  • Group 13: $3+$
  • Group 14: $4+$ or $4-$
  • Group 15: $3-$
  • Group 16: $2-$
  • Group 17: $1-$
  • Group 18: $0$

• Procedural Examples:

  • Calcium Bromide: Calcium ($Ca$) is in Group 2 ($2+$ charge). Bromine ($Br$) is in Group 17 ($1-$ charge). Crossing the absolute values results in $Ca_1Br_2$. The final formula is $CaBr_2$ (the subscript 1 is implied and not written).
  • Barium Oxide: Barium ($Ba$) is in Group 2 ($2+$ charge). Oxygen ($O$) is in Group 16 ($2-$ charge). Crossing values results in $Ba_2O_2$. These are divisible by 2, so it reduces to the formula unit $BaO$.
  • Sodium Oxide: Sodium ($Na$) is $1+$. Oxygen ($O$) is $2-$. Crossing values results in $Na_2O$.
  • Sodium Chloride: Sodium ($Na$) is $1+$. Chlorine ($Cl$) is $1-$. Crossing values results in $NaCl$.

The Stock System for Multiple Oxidation States

• Context for Transition Metals: Certain metals, particularly transition metals and specific outliers like Tin ($Sn$) and Lead ($Pb$), can form multiple different oxidation numbers depending on the environment. One cannot determine the charge of these metals solely by looking at their position on the periodic table.

• Roman Numeral Notation: To specify which oxidation state is present in a compound, a Roman numeral is placed in parentheses immediately following the name of the metal. This Roman numeral indicates the positive charge of that specific metal ion.

• Writing Formulas from Stock Names:

  • Iron (II) Oxide: The name indicates Iron ($Fe$) has a $2+$ charge. Oxygen ($O$) has a $2-$ charge. Crisscrossing and reducing results in $FeO$.
  • Copper (II) Chloride: The name indicates Copper ($Cu$) has a $2+$ charge. Chlorine ($Cl$) from Group 17 has a $1-$ charge. Crisscrossing results in $CuCl_2$.

• Naming Compounds via Reverse Determination:

  • Formula $PbO_2$: To find the name, use the known charge of the anion to find the unknown cation. Oxygen is $2-$. Since there are two oxygen atoms, the total negative charge is $4-$. To balance this, the single Lead ($Pb$) atom must carry a $4+$ charge. The name is Lead (IV) oxide.
  • Formula $AgCl$: Chlorine is $1-$. To balance, Silver ($Ag$) must be $1+$. The name is Silver (I) chloride.

Ternary Ionic Compounds and Polyatomic Ions

• Definition: Ternary ionic compounds involve more than two elements, typically because they contain at least one polyatomic ion.

• Polyatomic Ions: These are groups of atoms covalently bonded together that act as a single unit with a net charge. These are found in specific chemistry reference tables (e.g., page 7 of the standard reference table).

• Writing Formulas with Polyatomic Ions:

  • Magnesium Hydroxide: Magnesium ($Mg$) is $2+$. Hydroxide ($OH$) is a polyatomic ion with a $1-$ charge. Because the $2+$ charge from magnesium requires two hydroxide units, you must use parentheses: $Mg(OH)_2$. Parentheses indicate the subscript applies to the entire ion unit.
  • Ammonium Chloride: Ammonium ($NH_4$) is a polyatomic ion with a $1+$ charge. Chlorine ($Cl$) is $1-$. Crisscrossing gives a 1:1 ratio: $NH_4Cl$. Parentheses are unnecessary when the subscript for the polyatomic ion is 1.
  • Sodium Sulfate: Sodium ($Na$) is $1+$. Sulfate ($SO_4$) is a polyatomic ion with a $2-$ charge. Crisscrossing results in $Na_2SO_4$.
  • Barium Phosphate: Barium ($Ba$) is $2+$. Phosphate ($PO_4$) is a polyatomic ion with a $3-$ charge. Crisscrossing gives $Ba_3(PO_4)_2$.

• Naming Ternary Compounds from Formulas:

  • $LiNO_3$: Identify the cation, Lithium ($Li$), and the polyatomic anion, nitrate ($NO_3$). The name is lithium nitrate.
  • $Na_2CO_3$: Identify the cation, Sodium ($Na$), and the polyatomic anion, carbonate ($CO_3$). The name is sodium carbonate.