Naming and Writing Ionic Formulas: Simple, Variable Charge, and Polyatomic Ions

6.3 Naming and Writing Ionic Formulas

Introduction to Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between positively charged cations (usually metals) and negatively charged anions (usually nonmetals or polyatomic ions). Understanding how to name and write formulas for these compounds is essential in general chemistry.

• Iodized salt contains potassium iodide (KI), which is used to prevent iodine deficiency.

• Learning Goal: Given the formula of an ionic compound, write the correct name; given the name, write the correct formula.

Naming Ionic Compounds

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

• The name of the nonmetal (anion) is the first syllable of the nonmetal name plus the suffix -ide, written second.

• A space is placed between the names of the metal and nonmetal ions.

Table: Names of Some Ionic Compounds

Compound	Metal Ion	Nonmetal Ion	Name
KI	K+ (Potassium)	I- (Iodide)	Potassium iodide
MgBr2	Mg2+ (Magnesium)	Br- (Bromide)	Magnesium bromide
Al2O3	Al3+ (Aluminum)	O2- (Oxide)	Aluminum oxide

Steps for Naming Ionic Compounds

1. Identify the cation and anion. Example: In K2O, K+ is the cation (Group 1A), O2- is the anion (Group 6A).

2. Name the cation by its element name: Potassium.

3. Name the anion by using the first syllable of its element name followed by -ide: Oxide.

4. Write the name for the cation first and the anion second: Potassium oxide.

Practice: Learning Check 1

• CaO: Calcium oxide

• Al2O3: Aluminum oxide

• MgCl2: Magnesium chloride

Metals with Variable Charge

Transition Metals and Roman Numerals

Many transition metals (except Zn2+, Cd2+, and Ag+) can form more than one type of positive ion (cation). Their charge is indicated by a Roman numeral in parentheses after the metal name.

• Examples: Cu2+ is copper(II), Fe3+ is iron(III), Pb4+ is lead(IV).

Determining the Charge of Variable Charge Metals

• Use the charge on the anion and charge balance to calculate the charge on the metal ion.

• Example: For MnF2:

$\text{Mn charge} + 2 \times (\text{F charge}) = 0$ $? + 2(-1) = 0$ $2 + (-2) = 0$ So, the compound is manganese(II) fluoride.

Naming Ionic Compounds with Variable Charge Metal Ions

1. Determine the charge of the cation from the anion.

2. Name the cation by its element name and use a Roman numeral in parentheses for the charge.

3. Name the anion by using the first syllable of its element name followed by -ide.

4. Write the name for the cation first and the anion second.

Example: FeCl2 is iron(II) chloride.

6.4 Polyatomic Ions

Introduction to Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms that carry an overall ionic charge. They often consist of a nonmetal (such as phosphorus, sulfur, carbon, or nitrogen) covalently bonded to oxygen atoms. Most polyatomic ions have a negative charge, except for ammonium (NH4+), which is positive.

Common Polyatomic Ions and Their Naming

• Most common polyatomic ions end in -ate (e.g., sulfate SO42-, phosphate PO43-, nitrate NO3-).

• If a related ion has one less oxygen, its name ends in -ite (e.g., sulfite SO32-, nitrite NO2-).

• Exceptions: CN- (cyanide), OH- (hydroxide).

• Adding an H+ to the polyatomic ion increases its charge by +1 and changes the name (e.g., CO32- + H+ = HCO3-, called bicarbonate or hydrogen carbonate).

• Halogens form four polyatomic ions with oxygen, each with a -1 charge: perchlorate (ClO4-), chlorate (ClO3-), chlorite (ClO2-), hypochlorite (ClO-).

Table: Common Polyatomic Ions

Ion	Name
SO42-	Sulfate
SO32-	Sulfite
PO43-	Phosphate
PO33-	Phosphite
NO3-	Nitrate
NO2-	Nitrite
CN-	Cyanide
OH-	Hydroxide
HCO3-	Bicarbonate (hydrogen carbonate)
HSO4-	Bisulfate (hydrogen sulfate)
ClO4-	Perchlorate
ClO3-	Chlorate
ClO2-	Chlorite
ClO-	Hypochlorite

Writing Formulas Containing Polyatomic Ions

General Rules

• Use the same rules of charge balance as for simple ionic compounds.

• When more than one polyatomic ion is needed, enclose the ion in parentheses and place the subscript outside the parentheses.

Example: Magnesium nitrate

$\text{Mg}^{2+} + 2 \times \text{NO}_3^- = 0$ $\text{Mg(NO}_3)_2$

Example: Aluminum Bicarbonate

1. Identify the cation and polyatomic ion: Al3+ and HCO3-.

2. Balance the charges: $1(3+) + 3(1-) = 0$

3. Write the formula: Al(HCO3)3

Practice: Learning Check 1

• Sodium phosphate: Na3PO4

Naming Ionic Compounds Containing Polyatomic Ions

• Write the positive ion (usually a metal) first.

• Write the name of the polyatomic ion second.

• No prefixes are used in naming ionic compounds.

Examples:

• Na2SO4: Sodium sulfate

• FePO4: Iron(III) phosphate

• Al2(CO3)3: Aluminum carbonate

Practice Problems

Learning Check 2: Select the Correct Formula

• Aluminum nitrate: Al(NO3)3

• Copper(II) nitrate: Cu(NO3)2

• Iron(III) hydroxide: Fe(OH)3

• Tin(IV) hydroxide: Sn(OH)4

Learning Check 4: Name Compounds with Polyatomic Ions

• MgSO3: Magnesium sulfite

• MgSO4: Magnesium sulfate

• Pb3(PO4)2: Lead(II) phosphate

Learning Check 3: Name Compound with Variable Charge Metal

• SnO2: Tin(IV) oxide

Writing Formulas from the Name of an Ionic Compound

• The formula is written from the first part of the name (metal ion) and the second part (nonmetal ion).

• Subscripts are added to balance the charge.

Table: Some Ionic Compounds of Metals That Form Two Compounds

Compound	Systematic Name
FeCl2	Iron(II) chloride
Fe2O3	Iron(III) oxide
Cu3P	Copper(I) phosphide
CrBr3	Chromium(III) bromide
SnCl4	Tin(IV) chloride
PbS2	Lead(IV) sulfide
BiF3	Bismuth(III) fluoride

Writing Formulas for Ionic Compounds: Example

1. Identify the cation and anion: Iron(III) chloride is Fe3+ and Cl-.

2. Balance the charges: $1(3+) + 3(1-) = 0$

3. Write the formula: FeCl3

Practice: Learning Check 4

• Nickel(II) sulfide: NiS

• Zinc chloride: ZnCl2

• Iron(III) oxide: Fe2O3

Additional info: The above notes cover the systematic approach to naming and writing formulas for ionic compounds, including those with variable charge metals and polyatomic ions, as required in General Chemistry. Practice problems and tables are included for reinforcement.