Metal structures: strong metallic bonding gives most metals high melting and boiling points
Definition
Giant metallic structure
A very large, regular arrangement of metal atoms held together by strong metallic bonding.
Definition
Metallic bonding
The strong electrostatic attraction between positive metal ions and delocalised electrons.
- Most metals exist as a giant metallic structure, a regular arrangement of a very large number of atoms.
- The outer electrons of each metal atom become delocalised, so they are free to move throughout the whole structure.
- This leaves a lattice of positive metal ions surrounded by a sea of delocalised electrons.
- The strong electrostatic attraction between the positive ions and the shared electrons is the metallic bonding.
- This metallic bonding acts in every direction throughout the structure, so it is very strong.
- A large amount of energy is needed to overcome this strong bonding and separate the particles.
- As a result, most metals have high melting points and high boiling points.
- This is why most metals are solid at room temperature.
Key Idea
- Always link the property back to the bonding when you explain it.
- High melting and boiling points arise because a large amount of energy is needed to overcome the strong metallic bonding in the giant structure.
Pure metals: regular layers can slide, so the metal can be bent and shaped
Definition
Pure metal
A metal that contains only one metallic element rather than a mixture of elements.
- A pure metal contains atoms of only one metallic element.
- These identical atoms pack together in regular layers.
- When a force is applied, one layer of atoms can slide over the layer next to it.
- The metallic bonding is not broken as the layers move, because the delocalised electrons keep holding the ions together.
- This is why a metal can be bent, hammered and drawn into shape rather than shattering.
- Because the regular layers slide so easily, pure metals are often too soft for many everyday uses.
Common Mistake
- Do not say that the individual metal atoms bend or stretch when the metal changes shape.
- The atoms keep their identity while whole layers of atoms slide past one another.
Alloys: distorted layers cannot slide, so alloys are harder than pure metals
Definition
Alloy
A mixture of two or more elements in which at least one element is a metal.
- An alloy is made by mixing a metal with one or more other elements.
- The added atoms are usually a different size from the atoms of the original metal.
- These different-sized atoms distort the regular layers of the structure.
- The distorted layers can no longer slide over one another easily when a force is applied.
- More force is therefore needed to change its shape, so the alloy is harder than the pure metal.
- For example, brass is copper Cu\text{Cu}Cu mixed with zinc Zn\text{Zn}Zn, and is harder than pure copper.
- Similarly, steel is an alloy of iron Fe\text{Fe}Fe with a small amount of carbon C\text{C}C, and is harder than pure iron.
Self review
- Why do most metals have high melting and boiling points?
- Why can a pure metal be bent and shaped without breaking apart?
- Why are pure metals too soft for many uses?
- Why does distorting the layers of atoms make an alloy harder than a pure metal?
- Name an alloy and state the metal it is harder than.
