Structure of Metal and Alloys

Metallic bond

• A bond formed between metal atoms where valence electrons are delocalized in a "sea" that moves freely throughout the solid

• Explains conductivity, malleability, ductility, luster

Electron sea model

• A model that describes metallic bonding as positive metal ions surrounded by a "sea" of mobile valence electrons

• Electrons are not tied to any one atom → can move → conduct electricity

Why metals are good conductors

• Because their delocalized electrons can move freely through the lattice when a voltage is applied

• Works in solid and liquid states (unlike ionic compounds)

Why metals are malleable and ductile

• Because layers of metal ions can slide past each other without breaking bonds, the electron sea adjusts and holds the structure together

• Unlike ionic solids, which shatter when layers shift

Lattice structure of metals

Metal atoms pack together in highly ordered, repeating 3D patterns called crystal lattices to maximize attraction and minimize energy

Unit cell

The smallest repeating unit in a crystal lattice; stacking many unit cells forms the entire metal structure

Alloy

• A mixture of two or more elements, where at least one is a metal. Alloys often have improved properties over pure metals

• Example: Brass (Cu + Zn), Steel (Fe + C)

Substitutional alloy

• An alloy where atoms of one metal replace atoms of another in the lattice

• Atoms must be similar in size

• Example: Brass (Zn replaces some Cu atoms)

Interstitial alloy

• An alloy where smaller atoms (like C or B) fit into the spaces (holes) between larger metal atoms

• Strengthens the metal by preventing layers from sliding

• Example: Steel (carbon fits between iron atoms)

Why are alloys are stronger than pure metals

• Because the added atoms disrupt the regular lattice, making it harder for layers to slide past each other

• Substitutional: different-sized atoms distort the lattice

• Interstitial: small atoms "pin" the structure

Properties of metals

• ✅ Good conductors of heat and electricity

• ✅ Shiny (lustrous)

• ✅ Malleable and ductile

• ✅ High melting points (generally)

• ✅ Form cations easily (low ionization energy)

Why metals don’t form molecules like covalent compounds

Because metallic bonding is non-directional — every atom is bonded equally to many neighbors → forms extended networks, not discrete molecules

Metal point trends in metals

• Higher melting point = stronger metallic bonding

• Increases with:

• More valence electrons (e.g., W has 6 → very high mp)

• Smaller atomic radius

• Greater charge on ions

• Example: Na (1 valence e⁻) → low mp

• Example: W (6 valence e⁻) → very high m

Pure metal vs. alloy

Property                           Pure Metal         Alloy                     

Strength                           Lower                Higher

Conductivity                     Higher               Slightly lower

Corrosion Resistance      Often lower       Often higher