Main Types of Chemical Bonds

Overview and Classification

Atoms can join together in different ways to form molecules, ions, and extended solids. The way in which atoms are held together is called a chemical bond. In this chapter, we focus on the main types of chemical bonds and how they are distinguished from one another. Detailed discussions of individual bond types will follow in their own chapters.

The main bond types traditionally distinguished in introductory chemistry are:

• Covalent bonding
• Ionic bonding
• Metallic bonding

In addition, there is a separate group of intermolecular interactions (such as van der Waals forces and hydrogen bonding) that are usually weaker and act between molecules or ions rather than within them; these are covered later under “Special Intermolecular Interactions”.

The classification into “covalent”, “ionic”, and “metallic” is a simplification: real bonds often show mixed character and can be placed on a continuum between idealized types.

Basic Criteria for Distinguishing Bond Types

Different types of bonds can be characterized and distinguished according to several criteria:

• How electrons are organized
• Are electrons locally shared between specific atoms?
• Are electrons largely transferred from one atom to another?
• Are electrons delocalized over many atoms?
• Where electrostatic attraction occurs
• Between nuclei and a common electron pair (covalent)
• Between oppositely charged ions (ionic)
• Between positive metal ion cores and a “sea” of electrons (metallic)
• Which particles are formed
• Discrete molecules (typical for many covalent bonds)
• Crystal lattices of ions (typical for ionic bonding)
• Metallic lattices (typical for metallic bonding)
• Observable macroscopic properties
• Electrical conductivity (in solid, molten, or dissolved states)
• Melting and boiling points
• Solubility in polar vs nonpolar solvents
• Mechanical properties (hardness, brittleness, malleability, ductility)

These criteria will reappear in the more detailed chapters on each bond type and help to assign a dominant bonding character to a substance.

Idealized Main Bond Types

Covalent Bonding (Qualitative Characterization)

In an ideal covalent bond, atoms are connected by shared electron pairs. The electrons are localized between specific nuclei and contribute to holding them together.

Characteristic ideas (without detailed mechanisms):

• Bonds connect specific pairs (or occasionally groups) of atoms.
• The resulting entities are often molecules with definite compositions and shapes.
• The bonding electrons are usually pictured as belonging to both atoms simultaneously.

Properties typically associated with predominantly covalent substances:

• Often form gases, liquids, or low-melting solids (molecular substances).
• Poor electrical conductors in solid and liquid states (if no ions or mobile electrons).
• Solubility can vary widely, but many covalent molecules dissolve better in nonpolar or slightly polar solvents.

Detailed aspects such as bond polarity, bond order, and directional characteristics of covalent bonds are treated in the dedicated chapter on covalent bonding.

Ionic Bonding (Qualitative Characterization)

In an ideal ionic bond, electrons are transferred from one atom (typically a metal) to another (typically a nonmetal), resulting in positively and negatively charged ions. The bonding is the electrostatic attraction between these oppositely charged ions.

Characteristic ideas:

• The basic particles are ions: cations ($\ce{M^{n+}}$) and anions ($\ce{X^{m-}}$).
• Ions arrange in ordered three-dimensional lattices (ionic crystals) to maximize attractive and minimize repulsive interactions.
• No discrete molecules; instead, an extended ionic network.

Properties typically associated with predominantly ionic substances:

• Usually crystalline solids with high melting and boiling points.
• Generally hard but brittle.
• Usually nonconducting as solids, but conduct electricity when molten or dissolved in water, due to mobile ions.
• Frequently soluble in water and other polar solvents, often poorly soluble in nonpolar solvents.

The construction of ionic lattices, the role of lattice energy, and the energetics of ion formation are developed in detail in the ionic bonding chapter.

Metallic Bonding (Qualitative Characterization)

In metallic bonding, atoms in a metal give up some of their valence electrons, which become delocalized throughout the entire crystal. One often speaks of positive metal ion cores in a “sea” of mobile electrons.

Characteristic ideas:

• Electrons are delocalized over many atoms rather than shared between just two.
• Atoms occupy regular positions in a metallic crystal lattice.
• The bonding is due to electrostatic attraction between the positive cores and the delocalized electrons.

Properties typically associated with metallic substances:

• High electrical and thermal conductivity in the solid state.
• Malleability (can be hammered into sheets) and ductility (can be drawn into wires).
• Often metallic luster.
• Usually moderately high melting points, though there are wide variations.

Details such as different metallic crystal structures, band models for metallic conduction, and alloys are discussed in the chapter on metallic bonding.

Continuum and Mixed Bond Character

The three main bond types are idealized extremes; real substances often lie in between:

• A bond between atoms with moderate electronegativity difference can show both:
• Covalent character (shared electrons),
• Ionic character (partial charge separation, bond polarity).
• Bonds in many solid substances (e.g., network solids, semiconductors, many oxides) are more accurately described as partly ionic, partly covalent.

It is therefore helpful to think in terms of a bonding continuum:

• Predominantly covalent ←→ mixed covalent/ionic ←→ predominantly ionic
• Localized electron pairs ←→ increasing delocalization ←→ metallic-like delocalization

Later chapters introduce tools (such as electronegativity differences and qualitative band models) to estimate where a particular bond or substance lies on this continuum.

Comparing Macroscopic Properties

The differences in how electrons are held and how particles are organized in the solid or liquid state are reflected in typical observable properties. The table below summarizes the main trends, without going into the explanations that belong to the individual bond-type chapters.

\*General trends; many exceptions exist and are treated in context.

Intra- vs Intermolecular Interactions

The main bond types discussed here (covalent, ionic, metallic) are usually intramolecular or intra-lattice interactions: they hold atoms together within a molecule or extended solid.

In contrast, intermolecular interactions act between molecules, ions, or parts of large molecules. These are generally weaker and include:

• van der Waals forces
• Hydrogen bonds
• Other specific weak interactions

They play crucial roles in determining:

• Boiling and melting points of molecular substances,
• Solubilities,
• Structures of biological macromolecules (proteins, DNA, etc.).

These interactions form the subject of the “Special Intermolecular Interactions” part of the course and are conceptually separated from the main, stronger bond types introduced here.

Summary of the Main Types

• Covalent bonding: localized sharing of electron pairs between specific atoms, often forming molecules.
• Ionic bonding: electrostatic attraction between oppositely charged ions in a lattice, following electron transfer.
• Metallic bonding: attraction between positive metal cores and delocalized electrons in a metallic lattice.

These three main bond types form the foundation for understanding the structures and properties of substances in further chapters. Subsequent sections on covalent, ionic, and metallic bonding will examine each type in more depth, including their microscopic models and quantitative aspects.