Chapter 3: Chemical Bonding and Ionic Compounds

3.1 Introduction to Bonding

Chemical bonding is fundamental to understanding how atoms combine to form compounds. Atoms bond to achieve a more stable electron configuration, often resembling that of the nearest noble gas.

• Bonding is the joining of two atoms in a stable arrangement.

• Atoms will gain, lose, or share electrons to reach a stable electron configuration.

• There are two main types of chemical bonds:

  • Ionic bonds: Formed by the transfer of electrons from one atom (typically a metal) to another (typically a nonmetal).

  • Covalent bonds: Formed by the sharing of electrons between two atoms (usually nonmetals or a metalloid and a nonmetal).

Ionic bonds typically form between a metal (left side of the periodic table) and a nonmetal (right side of the periodic table). Covalent bonds form between two nonmetals or a metalloid and a nonmetal.

Molecule: A compound or element containing two or more atoms joined together with covalent bonds.

Example: The hydrogen molecule (H2) forms when two hydrogen atoms share two electrons in a covalent bond.

3.2 Ions

Ions are charged species formed when atoms gain or lose electrons, resulting in an unequal number of protons and electrons.

A. Cations and Anions

• Cations: Positively charged ions formed when an atom loses one or more electrons. Cations have fewer electrons than protons.

• Anions: Negatively charged ions formed when an atom gains one or more electrons. Anions have more electrons than protons.

• Octet Rule: Main group elements are especially stable when they possess eight valence electrons (an octet) in their outer shell.

Example: Sodium (Na) loses one electron to form Na+ (cation); chlorine (Cl) gains one electron to form Cl- (anion).

B. Relating Group Number to Ionic Charge

• For main group elements:

  • Groups 1A, 2A, 3A: The cation charge equals the group number.

  • Groups 6A, 7A: The anion charge equals 8 minus the group number.

C. Metals with Variable Charge

• Some transition metals can form more than one type of cation with different charges (e.g., Fe2+ and Fe3+).

D. Common Ions in the Human Body

• Important biological ions include Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, and PO43-.

3.3 Ionic Compounds

An ionic compound is formed when a metal transfers one or more electrons to a nonmetal, resulting in a compound composed of cations and anions held together by strong electrostatic forces.

• The sum of the charges in an ionic compound must be zero (electrical neutrality).

• Example: Na+ and Cl- combine to form NaCl (sodium chloride).

Writing Formulas for Ionic Compounds

1. Identify the cation (metal) and anion (nonmetal).

2. Determine the charge on each ion using the group number.

3. Balance the total positive and negative charges to achieve a net charge of zero.

4. Write the formula with the cation first, followed by the anion. Use subscripts to indicate the number of each ion if needed.

Examples:

• K+ + Cl- → KCl

• Ca2+ + 2Cl- → CaCl2

3.4 Naming Ionic Compounds

A. Naming Cations

• Main group cations are named after the element (e.g., Na+ is sodium, Ca2+ is calcium).

• Metals with variable charge use:

  • Systematic name: Element name followed by Roman numeral (e.g., iron(II), iron(III)).

  • Common name: Suffix “-ous” for lower charge, “-ic” for higher charge (e.g., ferrous, ferric).

B. Naming Anions

• Anions are named by replacing the ending of the element name with “-ide” (e.g., Cl- is chloride, O2- is oxide).

C. Compounds of Main Group Metals

• Name the cation first, then the anion (e.g., NaF is sodium fluoride).

• Do not specify the charge or the number of ions in the name.

D. Compounds of Metals with Variable Charge

1. Determine the charge on the cation based on the anion and the overall neutrality.

2. Name the cation (systematic or common name) and the anion.

3. Write the cation name first, then the anion (e.g., CuCl2 is copper(II) chloride or cupric chloride).

E. Writing a Formula from the Name

1. Identify the cation and anion and their charges.

2. Balance the charges to achieve neutrality.

3. Write the formula with the cation first, using subscripts as needed (e.g., tin(IV) oxide is SnO2).

Physical Properties of Ionic Compounds

• Ionic compounds are crystalline solids with high melting and boiling points.

• They conduct electricity when dissolved in water due to the movement of ions.

3.6 Polyatomic Ions

A polyatomic ion is a charged species (ion) composed of two or more atoms covalently bonded, acting as a single unit.

Writing Formulas for Ionic Compounds with Polyatomic Ions

• Follow the same rules as for simple ions: balance the total positive and negative charges.

• If more than one polyatomic ion is needed, use parentheses (e.g., Ba(NO3)2).

Examples:

• Na+ + NO3- → NaNO3

• Mg2+ + 2OH- → Mg(OH)2

Naming Ionic Compounds with Polyatomic Ions

• Name the cation first, then the polyatomic anion (e.g., NaHCO3 is sodium bicarbonate).

• Do not specify the charge or the number of ions in the name.

3.7 Identifying and Naming Compounds

• Ionic compounds are formed from metals and nonmetals or contain polyatomic ions.

• Molecular compounds are formed from nonmetals only.

• Use prefixes for naming molecular compounds (e.g., CO2 is carbon dioxide, N2O4 is dinitrogen tetroxide).

Common Prefixes for Molecular Compounds

Examples: SO2 is sulfur dioxide, N2O4 is dinitrogen tetroxide.

3.8 Naming Acids

• Acids are named based on their anion component.

• If the anion ends in “-ide,” the acid name begins with “hydro-” and ends with “-ic acid” (e.g., HCl(aq) is hydrochloric acid).

• If the anion ends in “-ate,” the acid name ends with “-ic acid” (e.g., HNO3 is nitric acid).

• If the anion ends in “-ite,” the acid name ends with “-ous acid” (e.g., HNO2 is nitrous acid).

3.9 Properties and Classification of Compounds

• Ionic compounds have high melting and boiling points, are solid at room temperature, and conduct electricity when dissolved in water.

• Molecular compounds generally have lower melting and boiling points and may exist as gases, liquids, or solids at room temperature.

3.10 Practice: Writing and Naming Compounds

• Write formulas by balancing charges between cations and anions.

• Name compounds by following the rules for ionic, molecular, or acid nomenclature.

Examples:

• Ca2+ and N3-: Ca3N2 (calcium nitride)

• Ba2+ and NO3-: Ba(NO3)2 (barium nitrate)

• SO2: sulfur dioxide (molecular compound)

• HNO2(aq): nitrous acid

Additional info: For more details on covalent bonding and molecular structure, see Chapter 4.