Molecules and Compounds: Structure, Types, and Nomenclature

Molecules & Compounds

Organizing Matter

Understanding matter at the particle level is essential in chemistry. Matter can be organized based on the types of particles present and how they are combined. The vocabulary of chemistry—such as atoms, molecules, and compounds—helps describe these combinations.

• Atoms: The basic units of chemical elements.

• Molecules: Groups of two or more atoms bonded together.

• Compounds: Substances composed of at least two different types of atoms in a fixed ratio.

Compounds can be represented by chemical formulas, which show the types and ratios of atoms present.

Compounds and Chemical Formulas

Compounds always have a consistent ratio of their constituent atoms, which is reflected in their chemical formulas. These formulas use subscripts to indicate the number of each type of atom and may use parentheses and coefficients to show multiples.

• Subscripts indicate the number of atoms of each element in a molecule (e.g., H2O).

• Coefficients multiply the entire formula (e.g., 4 H2O2 means 4 molecules of hydrogen peroxide).

• Parentheses are used when a group of atoms appears more than once (e.g., (NH4)2O).

Practice interpreting formulas by counting the number of each atom present in a given formula.

Types of Compounds

Ionic vs. Molecular Compounds

Compounds can be classified as ionic or molecular based on the types of elements involved and the nature of their bonding.

• Ionic Compounds: Formed from metals and nonmetals; consist of positive (cation) and negative (anion) ions held together by strong electrostatic forces in a crystal lattice. The smallest ratio of ions is called a "formula unit."

• Molecular Compounds: Formed from nonmetals; consist of discrete molecules held together by covalent bonds. Intermolecular forces between molecules are generally weaker than ionic bonds.

Binary Compounds

Binary compounds contain only two different elements. They can be either molecular or ionic.

• Molecular Binary Compounds: Usually consist of two nonmetals.

• Ionic Binary Compounds: Consist of a metal and a nonmetal.

Determining Compound Type by Formula

To determine if a compound is ionic or molecular, examine the first element in the formula:

• If it starts with a nonmetal, it is probably molecular (exceptions: ammonium ion, some metalloids).

• If it starts with a metal, it is ionic.

Naming Compounds

Naming Molecular Binary Compounds

Molecular binary compounds use prefixes to indicate the number of each atom. The general pattern is:

• prefix + name of first element + prefix + root of second element + "-ide"

• "Mono-" is never used for the first element.

Examples:

• PCl3: phosphorus trichloride

• N2O7: dinitrogen heptoxide

• BrF: bromine monofluoride

Naming Ionic Binary Compounds

Ionic binary compounds are named by stating the cation (metal) first, followed by the anion (nonmetal) with an "-ide" ending.

• Cation name + Anion name

• For transition metals with multiple possible charges, use Roman numerals to indicate the charge (IUPAC standard).

Examples:

• AlCl3: aluminum chloride

• FeCl3: iron (III) chloride

• CaI2: calcium iodide

Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms that carry a charge. Many common polyatomic ions contain oxygen and are called oxyanions.

• The most common form gets the "-ate" ending; one fewer oxygen gets "-ite." Prefixes like "per-" (more oxygen) and "hypo-" (less oxygen) are also used.

Ion	Formula	Name
SO42-	Sulfate
SO32-	Sulfite
ClO3-	Chlorate
ClO2-	Chlorite

Polyatomic Cations

• NH4+: ammonium

• Hg22+: mercury (I) ion (less common)

Writing Formulas

Writing Molecular Formulas

Prefixes in the name tell you the number of each atom. Write the formula accordingly.

• Dihydrogen monoxide: H2O

• Tricarbon octahydride: C3H8

• Phosphorus trichloride: PCl3

Writing Ionic Formulas

Ionic formulas must balance the total positive and negative charges. Use the criss-cross method or algebraic balancing.

• Calcium chloride: CaCl2

• Sodium sulfate: Na2SO4

• Ammonium phosphate: (NH4)3PO4

Naming Acids

Oxoacids

Oxoacids are acids that contain hydrogen, oxygen, and another element (the central atom). Their names are based on the oxyanion present:

• If the anion ends in "-ate," the acid name ends in "-ic acid" (e.g., H2SO4: sulfuric acid).

• If the anion ends in "-ite," the acid name ends in "-ous acid" (e.g., H2SO3: sulfurous acid).

• Prefixes "per-" and "hypo-" are retained (e.g., HClO4: perchloric acid; HClO: hypochlorous acid).

Binary Acids

Binary acids consist of hydrogen and one other nonmetal. Their names follow the pattern: "hydro-" + root of nonmetal + "-ic acid" (e.g., HCl: hydrochloric acid).

Summary Table: Naming Patterns

Type	Formula Example	Name Example
Molecular Binary	N2O5	dinitrogen pentoxide
Ionic Binary	NaCl	sodium chloride
Oxoacid	HNO3	nitric acid
Binary Acid	HBr	hydrobromic acid

Practice Problems

1. Sulfuric acid: H2SO4

2. HFO: hypofluorous acid

3. H3PO3: phosphorous acid

4. HBr: hydrobromic acid

5. Perbromic acid: HBrO4

6. HClO3: chloric acid