Study Notes: Molecules and Compounds (General Chemistry, Chapter 5)

Chapter 5: Molecules and Compounds

Introduction to Molecules and Compounds

Molecules and compounds are fundamental concepts in chemistry, describing how elements combine to form new substances with distinct properties. Understanding the differences between elements, compounds, and mixtures is essential for studying chemical reactions and the composition of matter.

• Compound Formation: When two or more elements combine, they form a compound, resulting in a new substance with properties different from the original elements.

• Properties: The properties of a compound are totally different from those of its constituent elements.

• Example: Sodium (Na) is a highly reactive metal, and chlorine (Cl) is a poisonous yellow gas. When combined, they form sodium chloride (NaCl), or table salt, which is safe to eat.

Elements, Compounds, and Mixtures

Most substances encountered in daily life are compounds, not elements. Free atoms are rare in nature, and compounds differ from mixtures in their composition.

• Elements: Pure substances consisting of only one type of atom.

• Compounds: Substances formed by elements combining in fixed, definite proportions.

• Mixtures: Combinations of elements or compounds in variable proportions.

• Example: A balloon filled with hydrogen and oxygen gases is a mixture; a balloon filled with water vapor is a compound with a fixed ratio of hydrogen to oxygen.

Law of Constant Composition (Definite Proportions)

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, regardless of the source.

• Definition: All samples of a given compound, regardless of their source, have the same proportions of their constituent elements.

• Example: Decomposing 18.0 g of water always yields 16.0 g of oxygen and 2.0 g of hydrogen.

• Formula: $\text{Mass ratio} = \frac{16.0\,g\,O}{2.0\,g\,H} = 8.0$ or $8.0:1$

• Practice Example: Decomposition of dinitrogen monoxide (N2O) yields consistent nitrogen-to-oxygen mass ratios in different samples:

  • Sample 1: $\frac{9.866\,g}{5.634\,g} = 1.751$

  • Sample 2: $\frac{16.23\,g}{9.269\,g} = 1.751$

  Both are consistent with the law of constant composition.

Chemical Formulas: Representation of Compounds

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

• Example: H2O indicates two hydrogen atoms and one oxygen atom.

• Changing Subscripts: Changing the subscript results in a different compound (e.g., CO is carbon monoxide, CO2 is carbon dioxide).

Order of Elements in Chemical Formulas

The order in which elements are listed in a chemical formula follows specific conventions.

• Most Metallic First: The most metallic element is listed first (e.g., NaCl, not ClNa).

• Periodic Table: Metals are on the left, nonmetals on the upper right.

• Nonmetal Compounds: The more metal-like nonmetal is listed first (e.g., NO2, not O2N).

• Exceptions: Some historical exceptions exist, such as OH- for hydroxide.

Polyatomic Ions in Chemical Formulas

Polyatomic ions are charged particles made up of multiple atoms. They can be positive (cations) or negative (anions) and act as a unit in chemical formulas.

• Examples: CaCO3 contains CO32-; Mg(ClO3)2 contains ClO3-.

• Common Polyatomic Ions: NH4+, OH-, Ca2+, SO42-, Na+, NO3-

Representing Compounds with Polyatomic Ions

When a compound contains multiple groups of the same polyatomic ion, parentheses and subscripts are used to indicate the number of each group.

• Example: Mg(ClO3)2 contains:

  • Mg: 1 atom

  • Cl: 2 atoms (1 × 2)

  • O: 6 atoms (3 × 2)

Types of Chemical Formulas

Chemical formulas can be categorized into three main types, each providing different levels of information about a compound.

• Empirical Formula: Shows the relative number of atoms of each element. Does not indicate the actual number, order, or shape. All ionic compounds are represented by empirical formulas.

  • Example: H2O, CHO2 (for C2H2O4), HO (for H2O2)

• Molecular Formula: Shows the actual number of atoms of each element in a molecule. Does not show order or shape.

  • Example: C2H2O4 (oxalic acid)

• Structural Formula: Uses lines to represent chemical bonds and shows how atoms are connected. Each line represents shared electrons (single, double, or triple bonds).

  • Example: H–H (single bond), H–C=C–H (double bond), H–C≡C–H (triple bond)

Molecular Models

Molecular models provide visual representations of molecules, helping to understand their structure and geometry.

• Ball-and-Stick Model: Atoms are balls, bonds are sticks; color-coded by element.

• Space-Filling Model: Atoms fill the space between each other, representing the molecule's actual size and shape.

• Example Table:

Name of Compound	Empirical Formula	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Benzene	CH	C6H6	Hexagonal ring with alternating double bonds	Ball-and-stick image	Space-filling image
Acetylene	CH	C2H2	H–C≡C–H	Ball-and-stick image	Space-filling image
Glucose	CH2O	C6H12O6	Ring structure with –OH groups	Ball-and-stick image	Space-filling image
Ammonia	NH3	NH3	Trigonal pyramidal	Ball-and-stick image	Space-filling image
Additional info:	Images referenced are typical textbook representations; actual images not included here.

Molecular View of Elements and Compounds

Elements and compounds can be classified based on their basic units and bonding.

• Elements:

  • Atomic Elements: Exist as single atoms (e.g., Ne, Al, K, Mg, Fe).

  • Molecular Elements: Exist as molecules—two or more atoms bonded together.

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

    • Polyatomic Molecules: P4, S8, Se8

• Compounds:

  • Molecular Compounds: Made of only nonmetals; basic units are molecules; formed via covalent bonds (e.g., H2O).

  • Ionic Compounds: Made of metals and nonmetals; basic units are formula units; formed via ionic bonds (e.g., NaCl).

Classification Practice

Classify substances as atomic element, molecular element, molecular compound, or ionic compound.

• Examples:

  • Krypton: atomic element

  • CoCl2: ionic compound (metal and nonmetal)

  • Nitrogen: molecular element (diatomic molecule)

  • SO2: molecular compound (nonmetal and nonmetal)

  • KNO3: ionic compound (metal and polyatomic ion)

• Practice:

  • Chlorine: molecular element (diatomic molecule)

  • NO: molecular compound (nonmetal and nonmetal)

  • Au: atomic element

  • Na2O: ionic compound (metal and nonmetal)

  • CrCl3: ionic compound (metal and nonmetal)

Summary Table: Classification of Elements and Compounds

Type	Basic Unit	Bond Type	Example
Atomic Element	Atom	None	Ne, Au
Molecular Element	Molecule	Covalent	O2, S8
Molecular Compound	Molecule	Covalent	H2O, CO2
Ionic Compound	Formula Unit	Ionic	NaCl, KNO3

Additional info: These notes cover the essential concepts from Chapter 5 of an introductory college chemistry course, including definitions, examples, formulas, and classification tables. The content is structured to provide a comprehensive yet concise study guide for exam preparation.