AP Chemistry · Unit 2 · Topic 2.4
Structure of Metals and Alloys
A metal wire can bend and still conduct. That combination is difficult to explain with a model of rigid, isolated molecules. Metallic bonding instead allows the arrangement of positive cores to change while mobile electrons continue to provide cohesion and transport charge.
The useful introductory model
Represent a metal as an array of positive atomic cores surrounded by delocalized valence electrons. These electrons are not permanently assigned to one neighboring pair of atoms. An applied electric field can produce a net electron drift, giving electrical conductivity. Thermal energy can also be transferred efficiently through the material, although a simple electron-sea picture does not capture every quantitative detail.
Why deformation need not destroy cohesion
Metallic attraction is less tied to a particular directional pair than a localized covalent bond. Layers of metal cores can move while remaining attracted to the redistributed electron population. This helps explain malleability and ductility. The ease of movement depends on crystal defects and microstructure; “all metals are equally soft” does not follow from the bonding model.
Two ways to make an alloy
| Alloy model | Where the added atoms sit | Likely structural effect |
|---|---|---|
| Substitutional | Replace some host atoms at lattice positions | Different atom sizes distort local spacing |
| Interstitial | Occupy gaps between host atoms | Small atoms obstruct relative layer movement |
| Pure metal reference | One predominant atomic species | Provides a comparison for the added component |
Alloys are materials with composition-dependent properties, not necessarily compounds with one fixed chemical formula. Some real alloys contain multiple phases or ordered structures beyond these simplified pictures.
Worked example: mass composition versus atom count
An illustrative copper–zinc alloy contains 12.0 g Cu and 8.00 g Zn. It is 60.0% copper by mass. Using 63.55 and 65.38 g mol−1, the amounts are 0.1888 mol Cu and 0.1224 mol Zn. The copper atom fraction is 0.1888/(0.1888 + 0.1224) = 0.607. Similar atomic masses make the two percentages close here, but they are conceptually distinct.
Link hardness to microscopic movement
Added atoms can make it harder for defects and atomic layers to move through a metal, increasing hardness or strength. Interstitial carbon in an iron-based material is a useful conceptual example. Composition alone does not uniquely determine mechanical behavior: heat treatment and the resulting phases also matter. Avoid claiming that adding any impurity always improves every property.
Read a materials experiment cautiously
Suppose wires of equal length and cross-sectional area show greater resistance after alloying. The comparison is consistent with added atoms disrupting electron transport. If the alloy wire were thinner, geometry would provide an alternative explanation. Likewise, a hardness comparison should control loading method and specimen preparation before attributing the change entirely to bonding.
Where the model can mislead
The positive cores are not freely drifting ions in the solid, and the mobile electrons do not mean the sample has a net negative charge. The metal remains electrically neutral overall. Alloying can trade higher strength for lower ductility or conductivity, so selecting a material requires specifying the intended property rather than calling one sample simply “better.”
Practice, then check your reasoning
Which model fits very small atoms placed in the gaps of a host-metal lattice?
Answer and reasoning: An interstitial alloy.
Why should wire dimensions be controlled in a conductivity comparison?
Answer and reasoning: Resistance also depends on length and cross-sectional area; changing them confounds the comparison of materials.
Scientific references: OpenStax: metallic solids and alloys; OpenStax: lattice structures. OpenStax Chemistry 2e authors: Paul Flowers, Klaus Theopold, Richard Langley, and William R. Robinson. Consult source pages for their license terms. Accessed September 13, 2026. Explanations, comparison tables, and practice tasks were independently written for this site; numerical teaching datasets are illustrative unless explicitly identified otherwise.
Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.