Chapter 3: Molecules and Compounds

Objectives and Overview

This chapter introduces the foundational concepts of molecules and compounds, including the types of chemical bonds, chemical formulas, atomic-level structure, and systematic naming (nomenclature) of compounds. It also covers the calculation of formula mass, the mole concept, and an introduction to organic compounds.

Chemical Bonds

Ionic and Covalent Bonds

Chemical bonds are the forces that hold atoms together in compounds. There are two primary types:

• Ionic Bonds: Involve the transfer of electrons from a metal to a nonmetal, resulting in the formation of oppositely charged ions (cations and anions) held together by electrostatic attraction.

• Covalent Bonds: Involve the sharing of electrons between two or more nonmetal atoms, resulting in the formation of molecules.

Example: Sodium (Na) transfers an electron to chlorine (Cl), forming Na+ and Cl- ions, which combine to form NaCl.

Chemical Formulas

Types of Chemical Formulas

Chemical formulas indicate the elements present in a compound and the relative number of atoms or ions of each. There are three main types:

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

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

• Structural Formula: Shows how atoms are bonded to each other within the molecule.

Example: Hydrogen peroxide has the empirical formula HO, molecular formula H2O2, and a structural formula showing the connectivity of atoms.

Comparison of Compounds

Elements and Compounds at the Atomic Level

Classification of Pure Substances

Pure substances can be classified as elements or compounds. Elements can exist as atomic (single atoms) or molecular (diatomic or polyatomic molecules). Compounds can be molecular (covalently bonded) or ionic (composed of ions).

Atomic and Molecular Elements

Some elements exist as single atoms (atomic elements), while others exist as molecules (molecular elements). Diatomic molecules consist of two atoms (e.g., O2, N2), and polyatomic molecules contain more than two atoms (e.g., P4, S8).

Types of Compounds

Ionic vs. Molecular Compounds

Compounds are classified based on the types of elements involved and the nature of their bonding:

• Ionic Compounds: Composed of metal cations and nonmetal anions, held together by ionic bonds. Example: NaCl.

• Molecular Compounds: Composed of nonmetals bonded covalently. Example: H2O.

Naming Compounds (Nomenclature)

Inorganic Nomenclature Flowchart

Naming inorganic compounds follows systematic rules based on the type of compound (ionic, molecular, or acid) and the elements involved.

Ionic Compounds

• Composed of cations (usually metals) and anions (usually nonmetals).

• The smallest electrically neutral collection of ions is called a formula unit.

• Metals may form only one type of ion (invariant charge) or more than one (variable charge).

Writing Formulas for Ionic Compounds

1. Write the symbol and charge for the metal cation and nonmetal anion.

2. Cross the charges (without sign) to become subscripts for the opposite ion.

3. Reduce subscripts to the smallest whole-number ratio.

4. Check that the total positive and negative charges balance.

Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms with an overall charge. Common examples include ammonium (NH4+), nitrate (NO3-), and sulfate (SO42-).

Oxyanions

Oxyanions are polyatomic ions containing oxygen and another element. Naming depends on the number of oxygen atoms:

• More oxygen: -ate (e.g., NO3-: nitrate)

• Fewer oxygen: -ite (e.g., NO2-: nitrite)

• Prefixes: per- (more than -ate), hypo- (less than -ite)

Hydrates

Hydrates are ionic compounds with a fixed number of water molecules associated with each formula unit. Example: CaCl2 · 6H2O is calcium chloride hexahydrate.

Naming Binary Molecular Compounds

Binary molecular compounds (two nonmetals) use prefixes to indicate the number of atoms. The prefix mono- is never used for the first element.

Acids

Acids are molecular compounds that release H+ ions when dissolved in water. They are named based on their composition:

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

• Oxyacids: Based on the polyatomic ion (e.g., H2SO4: sulfuric acid, HNO2: nitrous acid)

Formula Mass and the Mole Concept

Formula Mass

The formula mass (or molecular mass) is the sum of the atomic masses of all atoms in a molecule or formula unit, expressed in atomic mass units (amu).

Formula:

Mole Concept

The mole is a counting unit in chemistry, relating the mass of a substance to the number of particles (Avogadro's number: particles/mol).

Formula:

Composition of a Compound

Mass Percent Composition

The mass percent of an element in a compound is calculated as:

Empirical and Molecular Formulas from Composition

1. Convert mass percentages to grams (assume 100 g sample if not given).

2. Convert grams to moles for each element.

3. Divide by the smallest number of moles to get the simplest ratio.

4. If necessary, multiply to obtain whole numbers for subscripts.

To determine the molecular formula, divide the molar mass by the empirical formula mass and multiply the empirical subscripts by this factor.

Introduction to Organic Compounds

Carbon Bonding

Carbon forms four covalent bonds and can create chains, branches, and rings, leading to a vast diversity of organic compounds.

Classification of Organic Compounds

• Hydrocarbons: Contain only carbon and hydrogen. Types include alkanes (single bonds), alkenes (double bonds), alkynes (triple bonds), and aromatics (benzene rings).

• Functionalized Hydrocarbons: Contain additional functional groups (e.g., alcohols, amines) that determine chemical reactivity.

Naming Hydrocarbons

The base name is determined by the number of carbons (meth-, eth-, prop-, etc.), and the suffix indicates the type of bond (-ane, -ene, -yne).

Summary Table: Properties of Hydrogen, Oxygen, and Water

Key Visuals