Giant metallic structures and alloys (AQA GCSE Chemistry): Revision Notes
Giant metallic structures and alloys
What are giant metallic structures?
Metals form giant structures made up of metal atoms arranged in layers. These structures are held together by strong metallic bonding. The metal atoms lose electrons to become positive ions, and these loose electrons can move freely around the structure.
These freely moving electrons are called delocalised electrons. They act like a "sea of electrons" that holds all the metal ions together.
The concept of delocalised electrons is fundamental to understanding metallic properties. Unlike in ionic or covalent compounds where electrons are fixed in specific locations, metallic electrons are free to move throughout the entire structure.
Properties of metals
High melting and boiling points
Most metals have very high melting and boiling points. This happens because:
- Metallic bonds are very strong
- You need lots of energy to break these bonds
- The metal structure only breaks apart at very high temperatures
Exception: Mercury is liquid at room temperature and melts easily compared to other metals. This is because mercury has weaker metallic bonding due to its electronic structure.
Metals conduct electricity
Metals are excellent conductors of electricity because:
- The delocalised electrons are free to move through the metal
- When you connect a metal to a battery, these electrons flow as an electric current
- The electrons can move from one end of the metal to the other very easily
This is why electrical wires are made from metals like copper - the delocalised electrons can carry electrical current efficiently over long distances with minimal resistance.
Metals conduct heat
Metals are also good heat conductors because:
- When you heat one part of a metal, the delocalised electrons start moving faster
- These fast-moving electrons carry heat energy through the metal
- This is why metal spoons get hot when you put them in hot soup!
Practical Example: Heat Conduction
When you place a metal spoon in hot soup:
- Heat energy is absorbed by the metal atoms at the bottom of the spoon
- The delocalised electrons gain kinetic energy and move faster
- These energetic electrons transfer heat energy up through the spoon
- Within seconds, the handle becomes too hot to touch!
Pure metals vs alloys
Pure metals
Pure metals contain only one type of metal atom. For example:
- Pure copper contains only copper atoms
- These atoms are arranged in neat, regular layers
Pure metals can be bent and shaped easily because the layers of atoms can slide over each other when you apply force.
The regular arrangement of identical atoms in pure metals creates weak points where layers can slide. This is why pure gold is too soft for jewellery and must be mixed with other metals to create harder alloys.
What are alloys?
An alloy is a mixture of two or more metals. For example:
- Brass is made by mixing copper with zinc
- The zinc atoms are different sizes from copper atoms
Why are alloys harder than pure metals?
Alloys are much harder and stronger than pure metals because:
- The different-sized atoms disrupt the regular layers
- When you try to bend the alloy, the layers cannot slide past each other easily
- The larger atoms get in the way and stop the layers from moving
- This makes the alloy much harder to bend or break
Worked Example: Brass Formation
When copper and zinc are mixed to form brass:
- Copper atoms arrange in regular layers
- Zinc atoms (which are larger) are inserted between copper layers
- The larger zinc atoms distort the regular copper structure
- This prevents layers from sliding easily
- Result: Brass is much harder than pure copper
Key Concept: The hardness of alloys comes from disruption of the regular atomic structure, not from stronger bonding. The different-sized atoms act like "roadblocks" that prevent layers from sliding smoothly.
Key Points to Remember:
- Metals have giant structures held together by strong metallic bonding
- Delocalised electrons make metals good conductors of electricity and heat
- Pure metals are soft because their layers can slide easily
- Alloys are harder than pure metals because different-sized atoms disrupt the layers
- Most metals have high melting points due to strong metallic bonds