Metals and Alloys - Comprehensive Notes

Analyze data comparing metals and nonmetals and construct explanations for their differences.
Summarize defining properties of metals.
Describe how delocalized electrons give rise to metallic properties.

Metals have delocalized electrons, which are not associated with a single atom or bond.

Two very important properties of metals are ductility and malleability.
Ductility: The ability to be drawn into wires.
Malleability: The ability to be hammered or pressed into shapes.
Both ductility and malleability can be explained by the mobility of delocalized valence electrons in a metal.
Metals vs. Ionic Compounds
- A sea of valence electrons allows metals to be squeezed into wire or hammered without breaking.
- Ionic compounds do not behave the same way.
Drifting electrons insulate metal cations from each other.
When metal is forced through a die, the cations easily slide past one another, allowing the metal to be formed into wire. This is an example of ductility.

When an external force is applied to a metal, it deforms due to the mobility of the delocalized electrons.
When an external force is applied to an ionic compound, it leads to repulsive forces, causing the crystal to crack.

Metals are good conductors of both heat and electricity.
Thermal conductivity: A material's ability to conduct heat.
Electrical conductivity: A material's ability to conduct electricity.
- These properties make metals highly useful for cookware (pots and pans) and electronics.
Luster: The way light interacts with a material's surface.
- Light causes the free electrons in metals to move, and then the excited electrons re-emit the light, which is why metals aren't transparent and look shiny.
- This property makes metals highly valued in jewelry making.
Much of the light that hits the surface of a metal such as gold is reflected. This makes metals appear shinier than nonmetals.

A warm air molecule transfers energy to the metal.
A battery "pushes" the electrons in one direction, creating an electric current in the metal.

A rumor spreads faster in a gym where all of the students can move around freely, or in a classroom where students are confined to desks?
Analogy: Freedom of movement increases the ability of a rumor to travel around a room, just as the freedom of electrons to move increases the conductivity of a material like metal.

The three packing arrangements for atoms in a metal crystal are:
- Face-centered cubic (FCC)
- Body-centered cubic (BCC)
- Hexagonal close-packed (HCP)

FCC structure: Every atom has 12 neighbors arranged as shown.
- This tightly-packed structure makes FCC metals very ductile.
- FCC metals include gold, copper, and aluminum.
BCC structure: Every atom has eight neighbors.
- Because BCC is less closely packed than FCC, BCC metals are less ductile.
- BCC metals include lithium, potassium, and sodium.
HCP structure: Each atom has 12 neighbors, like FCC.
- However, HCP metals tend to be brittle since its non-cubic geometry means it has fewer planes across which atoms can slip past each other.
- HCP metals include zinc, cobalt, and cadmium.
The atoms in FCC materials are more closely packed than in BCC materials, so pulling atoms out of the "dip" as the planes slip over each other takes less energy. That's why FCC materials are more ductile than BCC materials.

Young's modulus is a measure of a solid material's stiffness.
A highly ductile material will typically have a very low modulus.
The three metals with BCC structures have much higher moduli and therefore they are much less ductile.
The FCC metals, on the other hand, have low moduli and are more ductile.
- Metal
  - Structure
    - Young's Modulus (GPa)
      - Chromium
        BCC
        $279$
      - Iron
        BCC
        $210$
      - Tungsten
        BCC
        $411$
      - Copper
        FCC
        $110$
      - Silver
        FCC
        $83$
      - Aluminum
        FCC
        $70$
      - Lead
        FCC
        $16$

All of these gemstones contain defects of aluminum oxide, which is clear and colorless in its pure form.
- Blue sapphires result from substitutions of titanium and iron atoms.
- Green sapphires result from iron impurities.
- Chromium atom substitutions result in the formation of red rubies.

Defects can create dislocations when propagated through the structure.
A dislocation happens when entire planes of atoms get inserted or removed.
Dislocations affect the properties of metals.
For example, the more dislocations in a metal structure, the lower its malleability.
The intentional introduction of dislocations in metal is called work hardening.
When a metal is bent or shaped repeatedly, the number of dislocations increases.
These dislocations get tangled, preventing further movement and making it harder to deform the metal.
Work hardening also results in the movement of dislocations, which leads to hardening through a process called pinning.
Work-hardened metals are stronger than unworked metals, but they are also more brittle.

Folding or hammering the metal creates shear forces along the slip plane.
These forces can cause dislocations to move along the slip plane.
A dislocation can be moved through the metal without having to break all of the bonds across the plane.
Eventually, the dislocation will run into another dislocation or a boundary defect.
This is called pinning. Pinning a dislocation hardens the metal.

Most of the metals you encounter every day are alloys.
Alloys are mixtures of two or more elements, at least one of which is a metal.
Alloys are important because their properties are often superior to those of their constituent elements.
Steel is a combination of a metal element (iron) and up to 2% by weight of a nonmetal element (carbon).
The carbon atoms are a type of point defect that makes steel less malleable than iron. That's why support structures in buildings are made from steel and not iron.

Carbon occupies interstitial sites in iron's structure.
These carbon atoms get trapped at dislocation sites, preventing the movement of the dislocation (pinning).
This makes steel stronger than pure iron.