MYP Chemistry 4.4 Alloys Notes

What Happens to the Structure When We Make an Alloy?

Pure Metals: Neat Layers that Slide

1. In a pure metal:
   1. All atoms are the same size.
   2. They are arranged in regular, closely packed layers (a metallic lattice).
   3. Positive metal ions are surrounded by a “sea” of delocalised electrons that hold the lattice together.
2. Because the atoms are all identical and neatly arranged, the layers can slide easily over each other when a force is applied.
3. That’s why many pure metals (like pure copper or pure iron) are relatively soft and malleable.

Alloys: Distorted Layers

1. An alloy is a mixture of a metal with:
   1. another metal, or
   2. a non-metal (often a small atom like carbon).
2. When we add these new atoms into the metallic lattice:
   1. The added atoms are usually a different size from the main metal atoms (either larger or smaller).
   2. This distorts the regular layers.
   3. The layers can no longer slide past each other as easily → the metal becomes harder and stronger.

Types of Alloys

Substitutional Alloys

1. The added atoms are similar in size to the host metal atoms.
2. They replace some of the original metal atoms in the lattice.

Interstitial Alloys

1. The added atoms are small and fit into the spaces (interstices) between metal atoms.
2. This significantly blocks movement of the layers.

How Do These Structural Changes Affect Properties?

Changing the atomic arrangement changes the properties.

Hardness and Strength

1. In pure metals, layers of identical atoms slide easily → softer, more malleable.
2. In alloys, distorted layers can’t move as easily → harder and stronger.

So:

1. Brass (Cu + Zn) is harder and stronger than pure copper.
2. Steel (Fe + C) is harder and stronger than pure iron.

Resistance to Corrosion

Some alloys are designed to resist corrosion.

Density and Conductivity

1. Density: Because alloy atoms have different masses and sizes, the density of an alloy can be different from its pure metal.
   1. Brass has a slightly different density from pure copper.
   2. Solder (tin + lead) has a different density from either pure tin or pure lead.
2. Electrical conductivity:
3. Pure metals: very good conductors (regular lattice, electrons move easily).
4. Alloys: usually lower conductivity than the pure metal.
5. Distortions in the lattice scatter electrons, so they don’t flow as freely.

Why Are Alloys So Important in Modern Society?

Alloys let us tune the properties of a metal to match a job. We almost never want a metal “as it comes” – we want:

1. stronger,
2. lighter,
3. more corrosion-resistant,
4. sometimes cheaper or easier to shape.

Why Alloys Matter

Alloys are crucial because they:

1. Improve safety – stronger steels in bridges, cars, and buildings reduce failures.
2. Increase durability – corrosion-resistant alloys last longer, reducing waste and cost.
3. Enable technology – specialized alloys in electronics, turbines, batteries, and medical implants.
4. Support sustainability – longer-lasting materials mean fewer replacements and less resource use.