Chapter 3: Molecules, Compounds, and Chemical Equations – Structured Study Notes

Chapter 3: Molecules, Compounds, and Chemical Equations

Introduction to Molecules and Compounds

Molecules and compounds are fundamental units in chemistry, formed by atoms held together through chemical bonds. Understanding their properties, formation, and classification is essential for mastering general chemistry.

Selected Properties of Hydrogen, Oxygen, and Water

• Hydrogen: Gas at room temperature, explosive, boiling point −253°C.

• Oxygen: Gas at room temperature, necessary for combustion, boiling point −183°C.

• Water: Liquid at room temperature, used to extinguish flames, boiling point 100°C.

Mixtures vs. Compounds

• Mixtures: Can have any ratio of components (e.g., hydrogen and oxygen mixture).

• Compounds: Have a fixed ratio of elements (e.g., water always has two hydrogen atoms to one oxygen atom).

Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. They arise from attractions between charged particles (electrons and protons).

• Ionic Bonds: Occur between metals and nonmetals; involve transfer of electrons, forming cations and anions.

• Covalent Bonds: Occur between nonmetals; involve sharing of electrons, forming molecules.

The Formation of Ionic Compounds

• Metal atoms lose electrons to become cations.

• Nonmetal atoms gain electrons to become anions.

• Electrostatic attraction between oppositely charged ions forms ionic bonds.

Ionic vs. Molecular Compounds

• Ionic Compounds: Composed of cations (usually metals) and anions (usually nonmetals), bound by ionic bonds. Basic unit is the formula unit (e.g., NaCl).

• Molecular Compounds: Composed of molecules formed by covalent bonds between nonmetals.

Types of Chemical Formulas

Chemical formulas represent the composition of compounds in different ways:

• Empirical Formula: Shows the simplest whole-number ratio of atoms.

• Molecular Formula: Shows the actual number of atoms of each element in a molecule.

• Structural Formula: Shows how atoms are connected, including geometry.

Molecular Models

• Ball-and-Stick Model: Atoms as balls, bonds as sticks; color-coded for elements.

• Space-Filling Model: Atoms fill space between each other, representing molecular shape more realistically.

Comparison of Formula Types

Name of Compound	Empirical Formula	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Benzene	CH	C6H6	Ring structure	Ball-and-stick	Space-filling
Acetylene	CH	C2H2	Linear structure	Ball-and-stick	Space-filling
Glucose	CH2O	C6H12O6	Ring structure	Ball-and-stick	Space-filling
Ammonia	NH3	NH3	Trigonal pyramidal	Ball-and-stick	Space-filling

Classification of Elements and Compounds

Pure substances are classified as elements or compounds. Elements can be atomic or molecular, while compounds can be molecular or ionic.

• Atomic Elements: Exist as single atoms (e.g., Ne, Na).

• Molecular Elements: Exist as molecules (e.g., O2, N2, P4, S8).

• Molecular Compounds: Molecules formed by covalent bonds (e.g., H2O).

• Ionic Compounds: Formula units formed by ionic bonds (e.g., NaCl).

Molecular Elements in the Periodic Table

• Seven diatomic elements: H2, N2, O2, F2, Cl2, Br2, I2.

• Polyatomic elements: P4, S8.

Naming Ionic Compounds

Ionic compounds are named based on the type of metal present:

• Type I: Metal forms only one type of ion (charge is invariant).

• Type II: Metal forms more than one type of ion (charge varies).

Type I Ionic Compounds

• Metal ion always has the same charge.

• Binary compounds contain only two different elements.

• Name: cation name + base name of anion (nonmetal) + -ide.

Examples of Type I Binary Ionic Compounds

• KCl: Potassium chloride

• CaO: Calcium oxide

Type II Ionic Compounds

• Metal can form more than one kind of cation (often transition metals).

• Name: cation name + charge in Roman numerals (in parentheses) + base name of anion + -ide.

Polyatomic Ions

• Groups of covalently bonded atoms with an overall charge.

• Common polyatomic ions include nitrate (NO3−), carbonate (CO32−), sulfate (SO42−), etc.

• Name ionic compounds containing polyatomic ions using the ion's name.

Oxyanions

• Anions containing oxygen and another element.

• Named based on the number of oxygen atoms: -ate (more O), -ite (less O).

• Prefixes hypo- (less than) and per- (more than) are used for series with more than two ions.

Hydrated Ionic Compounds

• Contain a specific number of water molecules per formula unit.

• Named using prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-.

• Example: MgSO4·7H2O is magnesium sulfate heptahydrate.

Naming Molecular Compounds

Molecular compounds are named using prefixes to indicate the number of each element present.

• Prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.

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

• Name: prefix + name of first element + prefix + base name of second element + -ide.

Formula Mass and Molar Mass

The mass of a molecule or formula unit is calculated by summing the atomic masses of its constituent atoms.

• Formula Mass: Sum of atomic masses in a chemical formula.

• Molar Mass: Mass of one mole of a compound, expressed in grams per mole.

Acids

Acids are molecular compounds that release hydrogen ions (H+) when dissolved in water. They are characterized by sour taste and ability to dissolve many metals.

• Binary Acids: Contain H+ and a nonmetal anion.

• Oxyacids: Contain H+ and a polyatomic anion with oxygen.

Naming Binary Acids

• Prefix hydro- + base name of nonmetal + -ic + "acid".

• Example: HCl(aq) is hydrochloric acid.

Naming Oxyacids

• If polyatomic ion ends in -ate, change to -ic + "acid".

• If polyatomic ion ends in -ite, change to -ous + "acid".

Inorganic Nomenclature Flow Chart

Composition of Compounds

The percentage by mass of each element in a compound can be determined from its formula or experimental analysis.

• Mass Percent Formula:

$\text{Mass percent of element X} = \frac{\text{mass of element X in 1 mol of compound}}{\text{mass of 1 mol of compound}} \times 100\%$

Determining Empirical and Molecular Formulas

The empirical formula gives the simplest ratio of atoms, while the molecular formula gives the actual number of atoms. Experimental data and molar mass are used to determine these formulas.

• Empirical Formula: Simplest whole-number ratio.

• Molecular Formula: Whole-number multiple of empirical formula.

• Formula: $\text{Molecular formula} = (\text{empirical formula})^n$

• n calculation: $n = \frac{\text{molar mass}}{\text{empirical formula molar mass}}$

Combustion Analysis

• Used to determine empirical formulas by burning a compound in oxygen and measuring the products.

Organic Compounds and Hydrocarbons

Organic compounds are primarily composed of carbon and hydrogen, sometimes with other elements. Hydrocarbons are the simplest organic compounds, containing only carbon and hydrogen.

• Alkanes: Only single bonds (suffix -ane).

• Alkenes: At least one double bond (suffix -ene).

• Alkynes: At least one triple bond (suffix -yne).

• Base names: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-.

Functionalized Hydrocarbons

• Functional groups impart specific chemical properties to organic compounds.

• Families of organic compounds are defined by their functional groups.

Example: Alcohols (–OH group), carboxylic acids (–COOH group), amines (–NH2 group).