4.1 Ionic Bonding | Chemistry: Atoms First 2e

Learning Objectives

Explain the formation of cations, anions, and ionic compounds
Predict the charge of common metallic and nonmetallic elements and write their electron configurations

Core Concepts & Principles

Ions form when neutral atoms gain or lose electrons to achieve stable electron configurations:

Cations: Positive ions formed when a neutral atom loses valence electrons (common in metals).
Anions: Negative ions formed when a neutral atom gains valence electrons (common in nonmetals).

Compounds composed of these ions are ionic compounds (salts). They are held together by ionic bonds, which are robust electrostatic forces of attraction between oppositely charged ions.

Properties of Ionic Compounds

Because ionic bonds are exceptionally strong, ionic solids form rigid, crystalline structures with high melting and boiling points.

Solid State Conductivity: Poor conductors of electricity because ions are locked rigidly in place within the crystal lattice and cannot move.
Molten or Dissolved State Conductivity: Excellent conductors of electricity and heat because melting or dissolving breaks the lattice, allowing ions to move freely.

The Nature of Ionic Lattices

Ionic compounds do not exist as discrete, individual molecules. Instead, electrostatic attractions are isotropic (equal in all directions), causing ions to organize into a 3D crystal lattice where each ion surrounds itself with multiple oppositely charged ions. Chemical formulas like $\text{NaCl}$ represent the simplest whole-number ratio of ions, not molecular units.

Electronic Structures of Ions

Main-Group Cations: Lose all valence electrons to adopt the electron configuration of the preceding noble gas. Groups 1 and 2 form ions with charges equal to their group number. Groups 13–17 form cations with charges equal to their group number minus 10.
Transition Metals: Lose their outermost $s$ electrons first, followed by one or more $d$ electrons from the next shell (e.g., $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$ ). Inner transition metals typically form $3^+$ ions.
Monatomic Anions: Nonmetal atoms gain enough electrons to completely fill their outer $s$ and $p$ orbitals, matching the electron configuration of the next noble gas.

Problem-Solving Routines & Methods

Writing Ion Electron Configurations

1
Write the full electron configuration for the neutral parent atom.
2
For main-group elements, remove all valence electrons (for cations) or add electrons to fill outer s and p orbitals (for anions).
3
For transition metal cations, always remove electrons from the highest-energy s orbital first, and then from the d orbitals.

Pro-Tip: Do not confuse Aufbau filling order with ionization order! Transition metals fill d-orbitals last, but they lose s-orbitals first when ionizing.

Lattice Dissociation Energy

NaCl(s) \longrightarrow Na^+(g) + Cl^-(g) \quad \Delta H = 769\text{ kJ}

The energy required to break apart one mole of a solid ionic compound into separate gaseous ions.

Variables & Constants

\Delta H

=enthalpy change (

769\text{ kJ}

for

\text{NaCl}

);

s

=solid phase;

g

=gaseous phase

Practice & Concept Checks

Concept Check

Why are ionic solids poor conductors of electricity, whereas molten or aqueous ionic compounds are excellent conductors?

Concept Check

What is the electron configuration of the zinc ion,

\text{Zn}^{2+}

Key Terms & Vocabulary

cationIons

A positively charged ion formed when a neutral atom loses one or more valence electrons.

Example:

\text{Ca}^{2+}

anionIons

A negatively charged ion formed when a neutral atom gains one or more valence electrons.

Example:

\text{Cl}^-

ionic compoundsCompounds

Chemical compounds composed of cations and anions held together by electrostatic forces.

Example:

\text{NaCl}

(table salt)

ionic bondsBonding

Electrostatic forces of attraction holding oppositely charged cations and anions together in an ionic substance.

binary ionic compoundCompounds

An ionic compound consisting of exactly two elements: a metallic cation and a nonmetallic anion.

Example:

\text{Al}_2\text{O}_3

inert pair effectPeriodic Trends

The tendency of heavy post-transition elements in groups 13–15 to retain their valence s-electron pair, resulting in lower-than-expected positive charges.

Example:

\text{Tl}^+, \text{Sn}^{2+}, \text{Pb}^{2+}

isoelectronicElectronic Structure

Having the exact same electron configuration as another chemical species, typically a noble gas.

Example:

\text{Ca}^{2+}

is isoelectronic with

\text{Ar}