Metallic bonding

Lore & Background

As chemistry developed, it became clear that metals formed the majority of the periodic table, and with electrochemistry, metals were understood to go into solution as positively charged ions. A picture emerged of metals as positive ions held together by an ocean of negative electrons. With quantum mechanics, this was formalized into the free electron model and the nearly free electron model, where electrons are seen as a gas traveling through the solid with isotropic energy. The nearly-free model added Brillouin zones to k-space, breaking isotropy mildly. X-ray diffraction and thermal analysis allowed study of crystalline solids, but intermetallic compounds and alloys remained mysterious, often studied empirically by metallurgy rather than chemistry. The nearly-free electron model was used by researchers like Hume-Rothery to predict alloy compositions, but cyclotron resonance later showed the Fermi surface was not spherical except perhaps in caesium, revealing that a model can give correct predictions yet be wrong in basic assumptions.

Reader's Guide

Metallic bonding is fundamental to understanding the physical properties of metals and their alloys. The concept evolved from early electrochemical observations to quantum mechanical models, including the free electron model and band structure calculations. The nearly-free electron model, despite its initial success in predicting alloy compositions, was later found to be based on incorrect assumptions about the shape of the Fermi surface. This led to more sophisticated models such as density functional theory and band structure calculations based on molecular orbitals. The bonding is characterized by electron delocalization and electron deficiency, with far more available energy states than shared electrons, enabling electrical conductivity. The study of metallic bonding also involves understanding the transition from localized to itinerant electrons, particularly for d- and f-electrons, which retain spin and add magnetic properties. The concept remains dominant in introductory metallurgy courses, though it is recognized that metallic bonding is not a unique type of bond but describes bonding in condensed matter, with metallic vapors often containing molecules held by conventional covalent bonds.

Did You Know?

• Metallic bonding is not the only type of chemical bonding a metal can exhibit; elemental gallium consists of covalently-bound pairs of atoms in both liquid and solid-state, with metallic bonding between the pairs.
• The nearly-free electron model predicted a fairly large number of alloy compositions that were later observed, but cyclotron resonance revealed the Fermi surface was not spherical except perhaps in caesium.
• Graphene is an example of two-dimensional metallic bonding, similar to aromatic bonding in benzene, naphthalene, anthracene, and ovalene.
• For caesium, the picture of Cs+ ions held together by a negatively charged electron gas is very close to accurate, though not perfectly so.