Metallic Solids (College Board AP® Chemistry)

Metallic Solids

• Metallic solids are made up of metal atoms held together by metallic bonds

• These metallic bonds result from the fact that the valence electrons are delocalized

  • This means that the electrons are spread throughout the solid structure and not fixed to specific atoms

• Metallic solids can be visualized as an array of positive ions immersed in a “sea” of delocalized electrons

Bonding in metallic solids

Diagram showing the interaction between the “sea” of delocalized valence electrons and positive metal ions

Properties of Metallic Solids

• Unlike ionic and molecular solids, metallic solids are very good conductors of electricity

  • This is due to the presence of the delocalized electrons

• They also have high thermal conductivity which means they have high melting points

  • This is due to the strong electrostatic force of attraction between the positive metal ions and the sea of delocalized valence electrons

• However, most metallic solids are malleable and ductile

  • Malleable means that the metallic solid can be hammered into thin sheets

  • Ductile means that the metallic solid can be pulled or drawn into wires

  • This is due to the layers of positive ions being able to slide over each other

Alloys

• Alloys are materials made from a mixture of two elements and possess metallic properties

• The process of mixing metals to form alloys is one of the primary ways of modifying the properties of pure metallic elements

  • For example, pure gold is too soft to be used in jewelry, but alloys of gold are much harder

• Alloys can be divided into two main categories:

  • Substitutional alloys

  • Interstitial alloys

• Substitutional and interstitial alloys are both homogeneous mixtures in which components are uniformly and randomly dispersed

  • The homogenous mixtures formed are called solid solutions

Substitutional alloys

• Substitutional alloys are formed when the solute and solvent atoms have similar atomic radii and chemical-bonding characteristics

  • For example, in brass, one-third of the copper atoms (solvent atoms) are substituted with zinc atoms

  • Other examples of substitutional alloys include:

    • Sterling silver - 93% silver and 7% copper

    • Pewter - 85% tin, 7% copper, 6% bismuth and 2% antimony

    • Plumber’s solder - 95% tin and 5% antimony

Substitutional Alloy

Diagram of brass showing zinc atoms occupying some of the positions in a lattice of carbon atoms

Interstitial alloys

• Interstitial alloys are formed when the solute atoms and solvent atoms have different bonding atomic radii and chemical-bonding characteristics

• In these alloys, the solute atoms occupy interstitial positions in the “holes” between solvent atoms

• Typically, the interstitial atom is a nonmetal that makes covalent bonds to the neighboring metal atoms

• The presence of the interstitial atoms changes the properties of the solvent metal atoms

• The presence of the extra bonds provided by the interstitial atom causes the metal lattice to become harder, stronger, less malleable and ductile

• For example, steel, which is much harder and stronger than pure iron, is an alloy of iron that contains up to 3% carbon

• However, the ability of the metal to conduct electricity is not affected because the delocalized electrons are retained in the alloys

Interstitial Alloy

Diagram of steel showing carbon atoms occupying the “holes” in a lattice of iron atoms