Metallic bonding

Lore & Background

As chemistry developed into a science, it became clear that metals formed the majority of the periodic table. With the advent of electrochemistry, metals generally go into solution as positively charged ions, and a picture emerged of metals as positive ions held together by an ocean of negative electrons. With quantum mechanics, this picture was given a formal interpretation in the free electron model and the nearly free electron model, where electrons are seen as a gas traveling through the structure of the solid. The nearly-free electron model was used by researchers such as Hume-Rothery to explain why intermetallic alloys with certain compositions would form, though later cyclotron resonance revealed the Fermi surface was not spherical as believed, showing a model can give correct predictions yet be wrong in its basic assumptions. The nearly-free electron debacle compelled researchers to modify the assumption that ions flowed in a sea of free electrons. Quantum mechanical models were developed, such as band structure calculations based on molecular orbitals and density functional theory. The electronic band structure model became a major focus for the study of metals and semiconductors. Researchers such as Mott and Hubbard realized that the one-electron treatment was appropriate for strongly delocalized s- and p-electrons, but for d- and f-electrons, interaction with nearby individual electrons may become stronger than the delocalized interaction, explaining the transition from localized unpaired electrons to itinerant ones partaking in metallic bonding.

Reader's Guide

Metallic bonding is significant because it explains the characteristic properties of metals, including strength, ductility, electrical and thermal conductivity, opacity, and lustre. The concept evolved from a simple picture of positive ions in a sea of electrons to more sophisticated quantum mechanical models, including the free electron model, nearly free electron model, and band structure calculations. The nearly-free electron model, though initially successful in predicting alloy compositions, was later found to be based on incorrect assumptions about the shape of the Fermi surface. This led to the development of more accurate models, such as density functional theory and band structure calculations. The study of metallic bonding also revealed the importance of electron delocalization and electron deficiency, where there are far more available energy states than shared electrons, enabling conductivity. The work of researchers like Mott and Hubbard highlighted that for d- and f-electrons, localized interactions can dominate, leading to magnetic properties. Metallic bonding is not a unique type of bond but describes bonding in condensed matter, whether crystalline solid or liquid, and is an extremely delocalized communal form of covalent bonding.

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.
• Graphene is an example of two-dimensional metallic bonding, similar to aromatic bonding in benzene.
• For caesium, the picture of Cs+ ions held together by a negatively charged electron gas is very close to accurate.
• The nearly-free electron model predicted a fairly large number of alloy compositions that were later observed, yet the Fermi surface was found not to be spherical as believed.