Ch 5: Molecules and Compounds: Structure, Classification, and Nomenclature

Chapter 5: Molecules and Compounds

Introduction

This chapter explores the structure, classification, and nomenclature of molecules and compounds. Understanding how atoms combine to form compounds, how to represent them, and how to name them is fundamental to the study of chemistry. The properties of compounds are often very different from those of the elements that compose them, and the diversity of compounds is essential for life and technology.

Law of Constant Composition

Definition and Application

• Law of Constant Composition: All samples of a given compound have the same proportions of their constituent elements by mass.

• Compounds vs. Mixtures: Compounds have fixed, definite proportions; mixtures can have variable proportions.

• Example: Water (H2O) always has a hydrogen to oxygen mass ratio of 2.0 g H : 16.0 g O = 1:8.

• Verification: Decomposition of different samples of compounds (e.g., ammonia, carbon dioxide, carbon monoxide) yields consistent mass ratios, confirming the law.

Formula for mass ratio:

$\text{Mass Ratio} = \frac{\text{mass of element 1}}{\text{mass of element 2}}$

Chemical Formulas: Representation of Compounds

Types of Chemical Formulas

• Empirical Formula: Simplest whole-number ratio of atoms in a compound (e.g., HO for hydrogen peroxide).

• Molecular Formula: Actual number of atoms of each element in a molecule (e.g., H2O2 for hydrogen peroxide).

• Structural Formula: Shows how atoms are connected using lines for bonds (e.g., H—O—O—H).

Subscripts and Order of Elements

• Subscripts indicate the number of atoms of each element; a subscript of 1 is omitted.

• The most metallic element is listed first; for nonmetals, the more metal-like (leftmost/lower in the periodic table) is listed first.

• Examples: NaCl (not ClNa), CO2 (not O2C), SO2 (not O2S).

Polyatomic Ions in Formulas

• Polyatomic ions are groups of atoms with a charge, acting as a unit in compounds.

• Parentheses are used when more than one polyatomic ion is present (e.g., Mg(NO3)2).

• To count atoms: multiply the subscript outside the parentheses by the subscript inside.

Example: In Mg(NO3)2: 1 Mg, 2 N, 6 O.

Classification of Elements and Compounds

Elements

• Atomic Elements: Exist as single atoms (e.g., Ne, He, Cu, Hg).

• Molecular Elements: Exist as molecules of two or more atoms of the same element (e.g., O2, N2, S8, P4).

• Diatomic Molecules: H2, N2, O2, F2, Cl2, Br2, I2.

Compounds

• Molecular Compounds: Composed of two or more nonmetals; basic units are molecules (e.g., H2O, CO2, C3H6O).

• Ionic Compounds: Composed of cations (usually metals) and anions (usually nonmetals); basic units are formula units (e.g., NaCl, KNO3).

Writing Formulas for Ionic Compounds

General Rules

• Ionic compounds must be electrically neutral; total positive charge equals total negative charge.

• Write the symbol and charge for each ion, then use the magnitude of each charge as the subscript for the other ion.

• Reduce subscripts to the smallest whole-number ratio.

Example: For Al3+ and O2−:

$\text{Al}_2\text{O}_3$

2 Al3+ (total +6) and 3 O2− (total −6) balance to zero.

Polyatomic Ions

• Common polyatomic ions include: acetate (C2H3O2−), carbonate (CO32−), nitrate (NO3−), sulfate (SO42−), ammonium (NH4+), etc.

• When writing formulas, use parentheses if more than one polyatomic ion is needed (e.g., Ca(NO3)2).

Nomenclature: Naming Compounds

Naming Ionic Compounds

• Type I (invariant charge): Metal forms only one type of cation. Name: metal + nonmetal base + -ide (e.g., sodium chloride).

• Type II (variable charge): Metal can form more than one cation. Name: metal + (charge in Roman numerals) + nonmetal base + -ide (e.g., iron(III) chloride).

• Polyatomic ions: Name the cation first, then the polyatomic ion (e.g., sodium nitrate).

Naming Molecular Compounds

• Use prefixes to indicate the number of atoms: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.

• First element: prefix + name (omit 'mono-' if only one atom).

• Second element: prefix + base name + -ide.

• Examples: CO2 is carbon dioxide; N2O4 is dinitrogen tetroxide.

Naming Acids

• Binary acids: Contain hydrogen and a nonmetal. Name: hydro- + base name + -ic + acid (e.g., hydrochloric acid for HCl(aq)).

• Oxyacids: Contain hydrogen, a nonmetal, and oxygen. If the oxyanion ends in -ate, use -ic acid; if -ite, use -ous acid (e.g., HNO3 is nitric acid, HNO2 is nitrous acid).

Summary Table: Classification and Naming of Compounds

Type	Composition	Naming Rule	Example
Ionic (Type I)	Metal (fixed charge) + Nonmetal	Metal + Nonmetal base + -ide	NaCl: sodium chloride
Ionic (Type II)	Metal (variable charge) + Nonmetal	Metal + (charge) + Nonmetal base + -ide	FeCl3: iron(III) chloride
Ionic (Polyatomic)	Metal + Polyatomic ion	Metal + Polyatomic ion	NaNO3: sodium nitrate
Molecular	Nonmetal + Nonmetal	Prefix + Name + Prefix + Base + -ide	CO2: carbon dioxide
Binary Acid	H + Nonmetal	Hydro- + Base + -ic acid	HCl(aq): hydrochloric acid
Oxyacid	H + Oxyanion	Base + -ic/-ous acid	HNO3: nitric acid

Formula Mass: Calculating the Mass of a Molecule or Formula Unit

• Formula Mass: The sum of the atomic masses of all atoms in a compound's chemical formula.

• Calculation: Multiply the number of each type of atom by its atomic mass and sum the results.

General formula:

$\text{Formula mass} = (n_1 \times \text{atomic mass}_1) + (n_2 \times \text{atomic mass}_2) + \ldots$

• Example 1: Water (H2O): $2 \times 1.01\,\text{amu} + 16.00\,\text{amu} = 18.02\,\text{amu}$

• Example 2: Sodium chloride (NaCl): $22.99\,\text{amu} + 35.45\,\text{amu} = 58.44\,\text{amu}$

• Example 3: Carbon tetrachloride (CCl4): $1 \times 12.01\,\text{amu} + 4 \times 35.45\,\text{amu} = 153.8\,\text{amu}$

Color Coding in Molecular Models

• Hydrogen: white

• Carbon: black

• Nitrogen: blue

• Oxygen: red

• Fluorine: light green

• Phosphorus: purple

• Sulfur: yellow

• Chlorine: green

Key Skills and Examples

• Counting Atoms: Multiply subscripts inside and outside parentheses (e.g., Pb(ClO3)2 has 1 Pb, 2 Cl, 6 O).

• Classifying Substances: Use composition to determine if a substance is an atomic element, molecular element, molecular compound, or ionic compound.

• Writing Formulas: Use ion charges to determine subscripts and ensure charge neutrality.

• Naming Compounds: Apply the correct naming convention based on the type of compound.

Summary

• Compounds have constant composition and are represented by chemical formulas.

• Elements and compounds are classified based on their basic units (atoms, molecules, ions).

• Nomenclature rules allow systematic naming of ionic, molecular, and acid compounds.

• Formula mass is calculated by summing the atomic masses of all atoms in a formula.