Metallic Solids (College Board AP® Chemistry): Revision Note

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 are also good conductors of heat, because the free valence electrons transfer energy through the structure

• Many metals have high melting points, due to the strong electrostatic attraction between the positive metal ions and the sea of delocalized valence electrons

• Most metallic solids are also 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

• An alloy is a mixture of a metal with one or more other elements that keeps its 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

  • These homogeneous mixtures 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

Diagram of a substitutional alloy (brass)

Brass: zinc atoms occupy some of the positions in a lattice of copper atoms

Interstitial alloys

• Interstitial alloys are formed when the solute and solvent atoms have significantly different atomic radii, so the smaller atoms fit into the holes (interstices) between the larger ones

• 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

• Alloys still conduct electricity, because they keep a sea of mobile delocalized electrons

Diagram of an interstitial alloy (steel)

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