Chapter 5: Molecules and Compounds - Structured Study Notes

Molecules and Compounds

Introduction

This chapter explores the nature of molecules and compounds, their properties, and how they differ from the elements that compose them. It also covers the law of constant composition, chemical formulas, types of compounds, and nomenclature rules for naming compounds and acids.

The Properties of Compounds vs. Elements

Sucrose and Its Elements

Ordinary table sugar, known as sucrose, is a compound made of carbon, hydrogen, and oxygen atoms. The properties of sucrose are very different from those of its constituent elements: carbon (in the form of graphite), hydrogen, and oxygen.

Sodium Chloride and Its Elements

Elemental sodium is a highly reactive metal, while elemental chlorine is a poisonous yellow gas. When combined, they form sodium chloride (table salt), which is safe and commonly used in food. The properties of a compound are generally different from the properties of the elements that compose it.

Compounds and Mixtures

Constant Composition in Compounds

Most substances in everyday life are compounds, not elements. Free atoms are rare in nature. A compound is different from a mixture:

• In a compound, elements combine in fixed, definite proportions.

• In a mixture, elements combine in variable proportions.

Variable vs. Fixed Composition

Mixtures do not have constant composition, while compounds do. For example, a balloon filled with a mixture of hydrogen and oxygen has variable ratios, but a balloon filled with water has a fixed ratio of hydrogen to oxygen.

The Law of Constant Composition

Definition and Example

Formally stated by Joseph Proust, the law of constant composition says: All samples of a given compound have the same proportions of their constituent elements.

• For water: Decomposing 18.0 g of water yields 16.0 g of oxygen and 2.0 g of hydrogen. The mass ratio is always 8:1.

• For ammonia: Decomposing 17.0 g of ammonia yields 14.0 g of nitrogen and 3.0 g of hydrogen. The mass ratio is always about 4.67:1.

Chemical Formulas

Representing Compounds

Chemical formulas indicate the elements present in a compound and the relative number of atoms of each. Subscripts show the number of atoms; a subscript of 1 is omitted by convention.

• Examples: NaCl (table salt), CO2 (carbon dioxide), C12H22O11 (sucrose).

• Changing a subscript creates a different compound (e.g., CO vs. CO2).

Order of Elements in Formulas

Formulas list the most metallic element first. In compounds without metals, the more metal-like element is listed first. Among nonmetals, those to the left and bottom of the periodic table are more metal-like.

Polyatomic Ions

Some formulas contain groups of atoms that act as a unit, called polyatomic ions. Parentheses and subscripts indicate the number of each group. To find the total number of each atom, multiply the subscript outside the parentheses by the subscript inside.

• Example: Mg(NO3)2 contains one Mg atom and two NO3 groups.

Types of Chemical Formulas

Empirical, Molecular, and Structural Formulas

There are three main types of chemical formulas:

• Empirical formula: Simplest whole-number ratio of atoms.

• Molecular formula: Actual number of atoms in a molecule.

• Structural formula: Shows how atoms are connected.

Molecular models (ball-and-stick, space-filling) represent the 3D structure of molecules.

Connecting Macroscopic and Molecular Worlds

Structural formulas and models help connect what we see (macroscopic) with the atomic and molecular world.

A Molecular View of Elements and Compounds

Classification

Pure substances can be elements or compounds. Elements may be atomic (single atoms) or molecular (diatomic molecules). Compounds may be molecular (molecules) or ionic (formula units).

Atomic and Molecular Elements

• Atomic elements: Exist as single atoms (e.g., mercury).

• Molecular elements: Exist as diatomic molecules (e.g., chlorine).

Molecular and Ionic Compounds

• Molecular compounds: Formed from two or more nonmetals; basic units are molecules (e.g., CO2 in dry ice).

• Ionic compounds: Formed from metals and nonmetals; basic units are formula units (e.g., NaCl).

Writing Formulas for Ionic Compounds

Rules and Examples

Ionic compounds are formed when metals bond to nonmetals. The sum of the charges must be zero (charge-neutral).

• Write the symbol and charge for each ion.

• Use the magnitude of the charge as the subscript for the other ion.

• Reduce subscripts to smallest whole numbers.

Polyatomic ions are included in formulas using parentheses and subscripts.

Naming Ionic Compounds

Type I and Type II Compounds

Type I compounds contain metals with constant charges (main group elements). Type II compounds contain metals with variable charges (transition metals).

• Type I: Name the cation (element name), then the anion (base name + -ide).

• Type II: Specify the charge of the metal with a Roman numeral in parentheses.

Naming Compounds with Polyatomic Ions

Use the name of the polyatomic ion whenever it occurs. For compounds with both polyatomic cations and anions, use both names.

Naming Oxyanions

Oxyanions are anions containing oxygen. The one with more oxygen atoms gets the ending -ate; the one with fewer gets -ite. Prefixes hypo- and per- are used for series with more than two ions.

Polyatomic Ions in Everyday Chemistry

Examples

• Sodium hypochlorite (in bleach) decomposes color-causing molecules and kills bacteria.

• Sodium bicarbonate (in baking soda) releases CO2 gas in baking.

• Calcium carbonate (in antacids) neutralizes stomach acids.

• Sodium nitrite (in packaged meats) preserves food and inhibits bacterial growth.

Naming Molecular Compounds

Rules and Prefixes

Molecular compounds are formed from two or more nonmetals. The first element is the more metal-like one. Prefixes indicate the number of atoms:

• mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), etc.

• If there is only one atom of the first element, the prefix mono- is omitted.

Example: CO2 is carbon dioxide; N2O is dinitrogen monoxide.

Naming Acids

Binary Acids and Oxyacids

Acids are molecular compounds that form ions in water. Binary acids contain hydrogen and a nonmetal; oxyacids contain hydrogen, a nonmetal, and oxygen.

• Binary acids: hydro- + base name of nonmetal + -ic + acid (e.g., hydrochloric acid).

• Oxyacids: If oxyanion ends in -ate, use base name + -ic + acid; if ends in -ite, use base name + -ous + acid.

Formula Mass

Definition and Calculation

The formula mass of a compound is the sum of the atomic masses of all atoms in its chemical formula.

• Example: Calculate the formula mass of MgCl2 by adding the atomic masses of Mg and Cl atoms.

$\text{Formula mass} = \sum \text{atomic masses of all atoms in the formula}$

Summary and Learning Objectives

• Restate and apply the law of constant composition.

• Write chemical formulas and determine the number of each atom.

• Classify elements and compounds as atomic, molecular, or ionic.

• Name binary ionic compounds, compounds with polyatomic ions, molecular compounds, and acids.

• Calculate formula mass.