Chemical Bonding, Lattice Energy, and Chemical Formulas: A Study Guide

Chemical Bonds

Introduction to Chemical Bonds

Chemical bonds are the attractive forces that hold atoms together in compounds. These bonds form because they lower the potential energy of the charged particles that compose atoms, resulting in more stable arrangements.

• Chemical Bonds form by lowering the potential energy of the system.

• Polyatomic Species are results of chemical bonding involving more than two atoms.

• Valence Electrons are key in chemical bonding, as they are the outermost electrons involved in bond formation.

• There are three main types of chemical bonding:

  • Ionic Bonding

  • Covalent Bonding

  • Metallic Bonding

Ionic Bonding

Formation and Properties

Ionic bonds form between metals (which have low ionization energies) and nonmetals (which have strongly negative electron affinity values). This results in the transfer of electrons from metal atoms to nonmetal atoms, creating ions.

• Metal atoms become cations by losing electrons (positively charged).

• Nonmetal atoms become anions by gaining electrons (negatively charged).

• Ionic Compounds are formed from ionic bonds and are composed of a lattice of alternating cations and anions.

Ionic Crystal Lattice and Lattice Energy

• Ionic Crystal Lattice forms when ionic compounds become solid, resulting in a highly ordered structure with high melting and boiling points.

• Lattice Energy is the energy associated with the formation of a crystalline lattice of alternating cations and anions from the gaseous ions.

  • It is usually exothermic (energy is released).

  • Greater lattice energy means a more stable ionic compound.

Compound	Lattice Energy (kJ/mol)
NaF	-910
CaO	-3414

Additional info: Lattice energy increases with higher charge and smaller ionic radius.

Metallic Bonding

Properties and Structure

In metallic bonding, metal atoms lose some valence electrons, which become delocalized and move freely throughout the metal lattice. This results in a 'sea of electrons' that holds the metal cations together.

• Typical of group 1 and 2 metals, which have relatively low melting and boiling points.

• Accounts for properties such as electrical conductivity, malleability, and ductility.

Covalent Bonding

Formation and Types

Covalent bonds form between two or more nonmetals or metalloids by sharing pairs of valence electrons. This allows each atom to attain a stable electron configuration.

• Two atoms share some of their valence electrons.

• Molecular Substances/Molecules/Network Solids are formed from covalent bonds.

• Metals with relatively high ionization energy (most d-block and some p-block metals) can also form covalent bonds.

Chemical Formulas and Molecular Models

Types of Chemical Formulas

• Chemical Formulas represent a compound, indicating the elements present and the number of atoms or ions of each.

  • Empirical Formula: Shows the simplest whole-number ratio of atoms (e.g., H2O for water).

  • Molecular Formula: Shows the actual number of atoms of each element (e.g., H2O2 for hydrogen peroxide).

  • Formula Unit Formula: Used for ionic compounds, gives the lowest whole-number ratio of ions (e.g., Al2(SO4)3).

• Structural Formulas show the connectivity of atoms (e.g., H–O–O–H for H2O2).

• Models:

  • Ball-and-Stick Model: Atoms as balls, bonds as sticks.

  • Space-Filling Model: Atoms as spheres scaled to size, showing how molecules fill space.

Lewis Theory

Lewis Structures and Electron Dots

Lewis theory states that chemical bonds form when atoms share or transfer valence electrons to achieve a stable noble gas configuration.

• Lewis Electron-Dot Structures (Lewis structures) depict valence electrons as dots around the atomic symbol.

• Lewis Symbol: Represents valence electrons of main-group elements as dots.

• Example: Electron configuration of O is 1s22s22p4; 6 valence electrons.

Ionic Bonding and Electron Transfer

Electron Configurations and Octet Rule

• For s-block metals, stable electron configuration is that of the preceding noble gas (atoms lose all valence electrons to form cations).

• For nonmetals, stable electron configuration is that of the following noble gas (atoms gain electrons to form anions).

• Example: Lithium cation (Li+) and Fluoride anion (F-).

Formula Unit

• The formula unit is the smallest, electrically neutral collection of ions in an ionic compound (e.g., NaCl is composed of Na+ and Cl-).

• Example: Potassium and Chlorine:

  • K: 4s1 → K+ + e- (transfers valence electron to Cl)

  • Cl: 3s23p5 + e- → Cl- (gains electron to complete octet)

Ionic Compounds: Formulas and Names

Common Ions and Polyatomic Ions

It is essential to memorize the charges and formulas of common monatomic and polyatomic ions for naming and writing formulas of ionic compounds.

Monatomic Cations	Charge	Monatomic Anions	Charge
Na+	+1	Cl-	-1
Ca2+	+2	O2-	-2
Al3+	+3	N3-	-3
Additional info: See full tables in textbook for more ions.

Polyatomic Ion	Formula	Charge
Sulfate	SO42-	-2
Nitrate	NO3-	-1
Ammonium	NH4+	+1
Phosphate	PO43-	-3

Naming Ionic Compounds

• Ionic Compound Name: (name of cation) (name of anion)

  • For monatomic ions: Sodium chloride (NaCl)

  • For polyatomic ions: Calcium nitrate (Ca(NO3)2)

• In a chemical formula, the sum of the charges of the positive ions (cations) must equal the sum of the charges of the negative ions (anions).

• Empirical formula for ionic compounds reflects the smallest whole-number ratio of ions.

• Example: Formula for ionic compound between aluminum and oxygen is Al2O3.

Covalent Bonding (Expanded)

Electron Pairs and Lewis Structures

• Shared Electron Pairs are produced when atoms combine to achieve a stable configuration.

• The stable electron configuration is that of the following noble gas.

• Bonding Electron Pairs are the shared electron pairs, represented with a single line in Lewis structures.

• Lone Electron Pairs are electron pairs not shared by atoms and belong to a particular atom.

• Dative Covalent (Coordinate) Bonds occur when both electrons in a shared pair come from one atom (electron pair donor) and are accepted by another atom (electron pair acceptor).

Additional Key Concepts

• Mass Percent Composition: The mass percent of an element X in a compound is given by: $\text{mass percent of element X} = \frac{\text{mass of X in 1 mol of compound}}{\text{mass of 1 mol of compound}} \times 100\%$

• Drawing Lewis Structures: Shows how atoms are bonded and the arrangement of valence electrons.

  • Count total valence electrons.

  • Arrange atoms and connect with single bonds.

  • Distribute remaining electrons to complete octets.