2.4 Metallic Bonding

Metallic Bonding & Electron Sea

Positive metal cores in a live sea of delocalized valence electrons. Apply a voltage, heat one edge, shear the lattice, or alloy it, and see why metals conduct, bend, and stiffen with carbon.

Electron SeaConductionMalleabilityBrass & Steel
Topic 2.4

Structure of Metals and Alloys

Represent a metallic solid and/or alloy using a model to show essential characteristics of the structure and interactions present in the substance.

Metallic bonding is modeled as an array of positive metal ions surrounded by delocalized valence electrons — the "sea of electrons". The electrons belong to the whole solid, not to individual atoms.

That model explains every classic metallic property:

  • Electrical and thermal conductivity — the delocalized electrons move freely, carrying charge and energy.
  • Malleability and ductility — sliding one layer of cations past another does not create charge repulsion, because the electron sea simply flows around the rearranged cores. Contrast this directly with ionic brittleness (2.3).
  • Luster — mobile electrons absorb and re-emit light across the visible spectrum.

Alloys are solid mixtures of a metal with one or more other elements, and the CED names exactly two structural types:

  • Interstitial alloys form between atoms of significantly different radii. The smaller atoms fill the interstitial spaces between the larger ones. The canonical example is steel: carbon occupying interstices in iron. Wedging atoms into the gaps makes the lattice more rigid and decreases malleability and ductility — which is precisely why steel is harder than pure iron.
  • Substitutional alloys form between atoms of comparable radius, where one atom substitutes for the other in the lattice. The CED's example is brass, where zinc substitutes for copper.

Either way, alloys retain a sea of mobile electrons, so they remain conducting.

Key points

  • Delocalized valence electrons are the single cause of conductivity, malleability, and luster.
  • Interstitial alloy = very different radii, smaller atoms in the gaps, harder and less malleable (steel).
  • Substitutional alloy = comparable radii, one atom swapped for another (brass).
  • Alloys still conduct, because the electron sea survives.

Common mistakes

  • Malleable vs. brittle is the ionic/metallic dividing line. Be ready to explain both in the same answer using layer displacement.
  • Interstitial alloys are harder, not softer. Students often assume "adding impurities weakens it" — the opposite is true here.
  • Do not describe metallic bonding as electron transfer. Nothing is transferred; the electrons are shared over the entire lattice.
  • An alloy is a mixture, so its composition (and its properties) can be varied continuously.

Worked example

Bronze is made from copper (r = 128 pm) and tin (r = 141 pm). Steel is made from iron (r = 126 pm) and carbon (r = 70 pm). Identify the alloy type in each case and predict how each differs from the pure host metal.

Bronze — substitutional. Tin and copper have comparable radii (141 vs 128 pm, about a 10% difference), so tin atoms can take the place of copper atoms in the lattice. The lattice is distorted somewhat, which impedes layer sliding and makes bronze harder than pure copper, but the effect is milder than in an interstitial alloy.

Steel — interstitial. Carbon (70 pm) is roughly half the radius of iron (126 pm), far too small to substitute, so carbon atoms occupy the interstitial spaces between iron atoms. These wedged-in atoms lock the layers, making the lattice much more rigid: steel is significantly harder and less malleable and ductile than pure iron.

Both remain conductive, because in each case the delocalized electron sea is preserved.

Full notes for topic 2.4 →