2.4 Structure of Metals and Alloys

Metallic Bonding

Metal atoms pack closely together and pool their valence electrons into a shared, mobile "sea of electrons". Metal cations are held in a lattice, surrounded by these delocalized electrons. This bonding explains why metals conduct electricity and heat well (electrons move freely) and are malleable/ductile (electrons don't lock atoms in fixed positions, so the lattice can deform without breaking).

Alloys: Interstitial vs. Substitutional

An alloy is a mixture of a metal with one or more other elements. In an interstitial alloy, small atoms (like carbon) fit into gaps between larger metal atoms, distorting the lattice and making it harder/less malleable (e.g., steel). In a substitutional alloy, atoms of similar size replace host metal atoms in the lattice (e.g., brass, sterling silver).

Properties Relate to Structure

Alloying changes a metal's properties by disrupting the uniform lattice, which affects electron mobility and atom movement. This typically increases hardness and strength but can decrease conductivity and ductility compared to the pure metal, since irregularities in the lattice impede electron flow and atomic slipping.

Key terms

Metallic bonding

Bonding in which metal cations are held together by a shared, delocalized "sea" of valence electrons.

Alloy

A mixture of a metal with one or more other elements, altering its properties.

Interstitial alloy

An alloy where small atoms fit into the gaps (interstices) between larger metal atoms (e.g., steel).

Substitutional alloy

An alloy where atoms of similar size replace host metal atoms in the lattice (e.g., brass, sterling silver).

Why are metals malleable/ductile?

Delocalized electrons allow atoms to slide and reposition without breaking the metallic bond, so the lattice deforms rather than fractures.

Try an AP-style question

Why are metals malleable?

1. Their electrons are localized between neighboring pairs
2. Their ions are fixed in place, so the shape stays rigid
3. Layers of cations can slide within the electron sea without breaking bonds
4. They have strong, directional covalent bonds between atoms