BackChapter 5: Molecules and Compounds – Structured Study Notes
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Molecules and Compounds
Introduction to Molecules and Compounds
Molecules and compounds are fundamental concepts in chemistry, describing how atoms combine to form substances with unique properties. The properties of compounds differ significantly from those of the elements that compose them.
Compound: A substance formed when two or more elements combine in fixed, definite proportions.
Molecule: The smallest unit of a compound that retains its chemical properties.
Example: Sucrose (table sugar) is a compound made of carbon, hydrogen, and oxygen, with properties distinct from its constituent elements.
Properties of Compounds vs. Elements
Compounds exhibit properties that are different from the elements they are made of. For example, sodium chloride (table salt) is formed from sodium (a reactive metal) and chlorine (a poisonous gas), yet salt is safe for consumption.
Example: Sodium chloride (NaCl) is a compound with properties distinct from sodium and chlorine.
Compounds vs. Mixtures
Most substances in nature are compounds, not elements. Free atoms are rare. Compounds differ from mixtures in that their constituent elements combine in fixed ratios, while mixtures can have variable proportions.
Compound: Elements combine in fixed, definite proportions.
Mixture: Elements can combine in any proportion.
Law of Constant Composition
The law of constant composition, stated by Joseph Proust, asserts that all samples of a given compound have the same proportions of their constituent elements.
Definition: The mass ratio of elements in a compound is constant for all samples.
Example: Water always has a mass ratio of 8.0:1 for oxygen to hydrogen.
Chemical Formulas
Chemical formulas represent compounds by indicating the elements present and the relative number of atoms of each. Subscripts denote the number of atoms; a subscript of 1 is omitted by convention.
Example: H2O for water, NaCl for table salt, CO2 for carbon dioxide.
Changing subscripts: Alters the compound entirely (e.g., CO vs. CO2).
Order of Elements in Chemical Formulas
Formulas list the most metallic element first. For compounds without metals, the more metal-like element is listed first. Table 5.1 provides the order for listing nonmetal elements.
Convention: NaCl, not ClNa; NO2, not O2N.
Polyatomic Ions in Chemical Formulas
Some compounds contain groups of atoms that act as a unit, called polyatomic ions. Parentheses and subscripts indicate the number of each group.
Example: Mg(NO3)2 contains one magnesium atom and two nitrate groups.
Calculation: Multiply the subscript outside the parentheses by the subscript inside.
Types of Chemical Formulas
There are three main types of chemical formulas: empirical, molecular, and structural. Molecular models visually represent these formulas.
Empirical formula: Shows the simplest ratio of atoms (e.g., HO for hydrogen peroxide).
Molecular formula: Shows the actual number of atoms (e.g., H2O2).
Structural formula: Shows how atoms are connected.
Molecular models: Ball-and-stick and space-filling models illustrate molecular geometry.
Representing Matter: Macroscopic, Molecular, and Symbolic Views
Chemists use macroscopic, molecular, and symbolic representations to connect the observable world with the atomic and molecular world.
Macroscopic: What we see (e.g., a glass of water).
Molecular: The arrangement of molecules.
Symbolic: Chemical formulas (e.g., H2O).
Classification of Elements and Compounds
Elements: Atomic vs. Molecular
Elements can exist as atomic or molecular units. Atomic elements are composed of single atoms, while molecular elements exist as diatomic molecules.
Atomic elements: Most metals (e.g., mercury).
Molecular elements: Diatomic molecules (e.g., Cl2, O2).
Compounds: Molecular vs. Ionic
Compounds are classified as molecular or ionic. Molecular compounds are formed from nonmetals, while ionic compounds consist of cations (usually metals) and anions (usually nonmetals).
Molecular compounds: Basic units are molecules (e.g., CO2).
Ionic compounds: Basic units are formula units (e.g., NaCl).
Writing Formulas for Ionic Compounds
Principles of Ionic Compound Formulas
Ionic compounds always contain positive and negative ions. The sum of the charges must be zero. The formula is written by balancing the charges of the cations and anions.
Steps: Write the symbols and charges, use the charge of one ion as the subscript for the other, reduce subscripts to smallest whole numbers.
Polyatomic Ions in Ionic Compounds
Polyatomic ions are groups of atoms with a charge, commonly found in ionic compounds. Familiarity with common polyatomic ions is essential for writing formulas.
Nomenclature: Naming Compounds
Naming Ionic Compounds
Ionic compounds are named systematically. The name depends on whether the metal forms only one type of ion (Type I) or more than one type (Type II).
Type I: Metal with invariant charge; name is cation followed by anion with -ide suffix.
Type II: Metal with variable charge; charge is specified with Roman numerals.
Naming Ionic Compounds with Polyatomic Ions
When polyatomic ions are present, their names are used directly in the compound name. Oxyanions are named based on the number of oxygen atoms.
-ate: More oxygen atoms.
-ite: Fewer oxygen atoms.
Prefixes: hypo- (less than), per- (more than).
Polyatomic Ions in Everyday Chemistry
Polyatomic ions are found in many household products, such as bleach (sodium hypochlorite), baking soda (sodium bicarbonate), and antacids (calcium carbonate).
Naming Molecular Compounds
Rules for Naming Molecular Compounds
Molecular compounds are named using prefixes to indicate the number of atoms. The first element is the more metal-like one, and the second element's name ends with -ide.
Prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.
Example: CO2 is carbon dioxide; N2O is dinitrogen monoxide.
Exception: The prefix mono- is omitted for the first element.
Naming Acids
Classification and Naming of Acids
Acids are molecular compounds that release ions in water. They are classified as binary acids (hydrogen and a nonmetal) or oxyacids (hydrogen, a nonmetal, and oxygen).
Binary acids: Named with "hydro-" + base name of nonmetal + "-ic" + "acid".
Oxyacids: Named based on the oxyanion present; "-ate" becomes "-ic acid", "-ite" becomes "-ous acid".
Nomenclature Flowchart and Examples
Flowcharts help systematically name compounds based on their formula and composition.
Chemistry in the Environment: Acid Rain
Acid Rain Formation and Effects
Acid rain is caused by the reaction of rainwater with air pollutants such as NO, NO2, and SO2, forming acids like HNO3 and H2SO4. Acid rain harms aquatic life, weakens trees, and damages buildings.
Formula Mass
Calculating Formula Mass
The formula mass of a compound is the sum of the atomic masses of all atoms in its chemical formula.
Formula:
Summary of Key Concepts
Compounds display constant composition; elements combine in fixed proportions.
Chemical formulas indicate the elements and their ratios in compounds.
Nomenclature rules allow systematic naming of ionic, molecular, and acid compounds.
Formula mass is calculated by summing atomic masses according to the chemical formula.
Learning Objectives
Restate and apply the law of constant composition.
Write and interpret chemical formulas.
Classify elements and compounds as atomic, molecular, or ionic.
Name compounds using systematic nomenclature rules.
Calculate formula mass for compounds.
