Ionic and Molecular Compounds: Naming, Formulas, and Polyatomic Ions

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Ch. 6: Ionic and Molecular Compounds

Introduction to Ionic and Molecular Compounds

Ionic and molecular compounds are two major classes of chemical compounds, each with distinct bonding and properties. Understanding how to name these compounds and write their chemical formulas is essential in general chemistry.

Naming and Writing Formulas for Ionic Compounds

Basic Rules for Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between cations (positively charged ions) and anions (negatively charged ions).
Most ionic compounds consist of a metal and a nonmetal.
The chemical formula is written with the cation first, followed by the anion.

Example: Calcium oxide (CaO) is formed from Ca2+ and O2−.

Polyatomic Ions

Polyatomic ions are groups of atoms covalently bonded together, carrying an overall charge. They can act as either cations or anions in ionic compounds.

Common polyatomic ions:

Ion	Name	Charge
NH4+	Ammonium	+1
NO3−	Nitrate	−1
SO42−	Sulfate	−2
OH−	Hydroxide	−1
CO32−	Carbonate	−2
PO43−	Phosphate	−3
CH3COO−	Acetate	−1

Writing Formulas for Ionic Compounds with Polyatomic Ions

Identify the cation and anion, including their charges.
Balance the total positive and negative charges to ensure the compound is neutral.
Use parentheses around polyatomic ions if more than one is needed.

Example 1: Magnesium nitrate Cation: Mg2+, Anion: NO3− Balance charges: Need two NO3− for each Mg2+ Formula: Mg(NO3)2

Example 2: Aluminum hydroxide Cation: Al3+, Anion: OH− Balance charges: Need three OH− for each Al3+ Formula: Al(OH)3

Example 3: Calcium carbonate Cation: Ca2+, Anion: CO32− Balance charges: 1:1 ratio Formula: CaCO3

Naming Polyatomic Ionic Compounds

Name the cation first (element name or polyatomic ion name).
Name the anion second (polyatomic ion name).

Example: (NH4)2CO3 is ammonium carbonate.

Example: Li2SO4 is lithium sulfate.

Ionic Compounds with Variable Charge Metals

Many transition metals can form more than one type of positive ion. The charge is specified using Roman numerals in parentheses.

Example: Copper (II) chloride is CuCl2 (Cu2+ and 2 Cl−).
Example: Iron (III) fluoride is FeF3 (Fe3+ and 3 F−).

Table of transition metals with common ionic charges

Naming and Writing Formulas for Molecular (Covalent) Compounds

Basic Rules for Molecular Compounds

Molecular (covalent) compounds are formed between nonmetals.
Prefixes are used to indicate the number of each atom present (mono-, di-, tri-, etc.).
The less electronegative element is named first.

Example: Dichlorine monoxide is Cl2O.

Example: Dihydrogen sulfide is H2S.

Example of writing the formula for dihydrogen sulfide as H2S

Applications of Ionic Compounds

Ionic compounds are widely used in everyday life, from medical applications to agriculture.

Plaster casts use calcium sulfate (CaSO4).
Fertilizers often contain ammonium nitrate (NH4NO3).

Examples of ionic compounds in real life: plaster cast (CaSO4) and fertilizer (NH4NO3)

Practice: Determining Formulas from Names

Sodium sulfate: Na2SO4
Ammonium carbonate: (NH4)2CO3
Magnesium phosphate: Mg3(PO4)2

Lecture Schedule Reference

The lecture schedule confirms that these topics are covered in Ch. 6 (Ionic and Molecular Compounds) and are relevant for exam preparation.

Lecture schedule showing coverage of ionic and molecular compounds

Summary Table: Common Polyatomic Ions

Formula	Name	Charge
NH4+	Ammonium	+1
NO3−	Nitrate	−1
SO42−	Sulfate	−2
CO32−	Carbonate	−2
PO43−	Phosphate	−3
OH−	Hydroxide	−1
CH3COO−	Acetate	−1

Key Equations

General formula for ionic compounds: (where charges balance: )

Additional info: The above notes integrate textbook-style explanations, examples, and applications, and are structured for effective exam preparation in a general chemistry course.