Chapter 4: Molecules and Compounds – Study Notes

Chapter 4: Molecules and Compounds

Introduction

This chapter covers the fundamental concepts of molecules and compounds, focusing on the types of chemical bonds, the formation and properties of ions, lattice energy, empirical and molecular formulas, and nomenclature of inorganic compounds. Mastery of these topics is essential for understanding chemical structure and reactivity in general chemistry.

Types of Chemical Bonds

Covalent Bonds

Covalent bonds involve the sharing of two electrons between two atoms. These bonds are typical in molecules formed between nonmetals.

• Single, Double, and Triple Bonds:

  • Single bonds share one pair of electrons.

  • Double bonds share two pairs of electrons.

  • Triple bonds share three pairs of electrons.

• Bond Strength and Length:

  • Triple bonds are shorter and stronger than double bonds.

  • Double bonds are shorter and stronger than single bonds.

  • The internuclear distance at which potential energy is minimized is the bond length, and the minimum energy is the bond energy.

Ionic Bonds

Ionic bonds are formed by the transfer of one or more electrons from one atom to another, resulting in the formation of charged particles called ions.

• Cations: Positively charged ions formed when metals lose electrons.

• Anions: Negatively charged ions formed when nonmetals gain electrons.

• Roman Numerals: Used for transition metals to indicate the charge of the cation (e.g., Fe2+ is iron(II)).

• Example: In FeCl2, iron has a 2+ charge (Fe2+); in FeCl3, iron has a 3+ charge (Fe3+).

Ionic vs. Covalent Compounds

It is important to distinguish between ionic and molecular (covalent) compounds in molecular representations.

Ionic	Covalent
Composed of cations and anions (metal + nonmetal)	Composed of nonmetals only
Electrostatic attraction between ions	Atoms share electrons
Forms crystalline solids with high melting points	Can be gases, liquids, or solids; lower melting points

Lattice Energy of Ionic Compounds

Lattice energy is the energy released when one mole of an ionic compound forms from its free ions in the gas phase. It is a measure of the strength of the forces holding the ions together in the crystal lattice.

• Factors Affecting Lattice Energy:

  • Depends on the charge of the ions and the size (radius) of the ions.

  • The greater the charge on the ions, the higher the lattice energy.

  • The smaller the ionic radius, the higher the lattice energy.

• Relationship to Melting Point: Higher lattice energy generally means a higher melting point for the ionic solid.

Formula:

$\text{Lattice\ Energy} \propto \frac{q_1 q_2}{r}$

where $q_1$ and $q_2$ are the charges of the ions, and $r$ is the distance between their centers.

Empirical and Molecular Formulas

Empirical and molecular formulas are used to represent the composition of compounds.

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

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

Calculating Empirical Formula from Percent Composition

• Assume 100 g of the sample to convert percent to grams.

• Convert grams to moles using molar masses.

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

• Multiply to obtain whole numbers if necessary.

Example: If a compound contains 84.1 g of carbon and 15.9 g of hydrogen, convert to moles and find the simplest ratio.

Calculating Molecular Formula

• Determine the empirical formula mass.

• Divide the molar mass of the compound by the empirical formula mass to find the multiplier.

• Multiply the subscripts in the empirical formula by this multiplier.

Formula:

$\text{Molecular\ Formula} = (\text{Empirical\ Formula})_n$

where $n = \frac{\text{Molar\ Mass}}{\text{Empirical\ Formula\ Mass}}$

Percent Yield

Percent yield measures the efficiency of a chemical reaction.

Formula:

$\text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%$

Polyatomic Ions

Polyatomic ions are charged groups of covalently bonded atoms that act as a single ion. Memorizing common polyatomic ions is essential for naming and writing formulas of compounds.

Name	Formula & Charge	Name	Formula & Charge
Ammonium	NH4+	Nitrate	NO3-
Acetate	CH3COO-	Nitrite	NO2-
Sulfate	SO42-	Sulfite	SO32-
Cyanide	CN-	Hydroxide	OH-
Permanganate	MnO4-	Phosphate	PO43-
Perchlorate	ClO4-

Naming Ionic and Molecular Compounds

Correct nomenclature is crucial for clear communication in chemistry. The process depends on whether the compound is ionic or molecular.

Inorganic Nomenclature Flowchart

• Ionic Compounds:

  • Metal + nonmetal or polyatomic ion

  • Use the name of the cation (with Roman numeral if needed) and the anion

• Molecular Compounds:

  • Nonmetals only

  • Use prefixes to indicate the number of each atom (mono-, di-, tri-, etc.)

Key Definitions

• Molecule: A unit of matter formed when two or more atoms are joined by covalent bonds.

• Ionic Bond: A bond resulting from the electrostatic attraction between a cation and an anion.

• Polyatomic Ion: A charged group of more than one kind of atom joined together by covalent bonds.

• Electrostatic Potential: The energy a charged particle has due to its position relative to another charged particle. It is directly proportional to the product of the charges and inversely proportional to the distance between them.

• Lattice Energy: The energy released when one mole of an ionic compound forms from its free ions in the gas phase.

Summary Table: Key Concepts

Concept	Description
Covalent Bond	Atoms share electrons; forms molecules
Ionic Bond	Transfer of electrons; forms ions
Lattice Energy	Energy released upon formation of ionic solid from ions
Empirical Formula	Simplest whole-number ratio of atoms
Molecular Formula	Actual number of atoms in a molecule
Percent Yield	Efficiency of a chemical reaction

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard general chemistry curriculum.