Bonding & Substance Properties (AQA GCSE Chemistry Combined Science): Revision Notes
Giant metallic structures and alloys
What are giant metallic structures?
Metals form huge structures made of metal ions arranged in regular patterns. These structures are held together by strong metallic bonds.
In metallic bonding, metal atoms lose electrons to become positive ions. The lost electrons become delocalised electrons - they are free to move around the whole structure like a "sea of electrons."
The concept of delocalised electrons is crucial to understanding metal properties. Unlike in covalent compounds where electrons are fixed between specific atoms, metal electrons can move freely throughout the entire structure.
Key properties of metals
High melting and boiling points
Metals have high melting and boiling points because metallic bonds are very strong. You need lots of energy to break these bonds, which means most metals have high melting and boiling points.
Exception: Mercury is liquid at room temperature because it has relatively weak metallic bonds compared to other metals.
Metals conduct electricity
Metals are excellent electrical conductors because the delocalised electrons can move freely through the metal. When you apply electricity, these electrons flow and create an electric current. The electrons can move from one end of the metal to the other without obstruction.
Metals conduct heat
Metals are also good heat conductors because when you heat a metal, the delocalised electrons gain energy. These energetic electrons move around and carry the heat energy with them, spreading the heat quickly through the metal.
What are alloys?
An alloy is a mixture of metals, or a metal mixed with small amounts of other elements.
Why make alloys?
Pure metals can be quite soft and easy to bend. Alloys are usually:
- Harder than pure metals
- Stronger than pure metals
- More useful for specific jobs
How alloys work
In pure metals, all the ions are the same size, so the layers of ions can slide over each other easily. In alloys, you have different sized ions mixed together. The different sizes stop the layers sliding as easily, which makes the alloy harder and stronger.
Example: Copper and brass
Worked Example: Comparing Pure Copper and Brass
Pure copper:
- Contains only copper ions (all the same size)
- Layers can slide easily over each other
- Result: Can be bent and shaped easily
Brass (copper + zinc alloy):
- Contains copper ions and larger zinc ions
- The larger zinc ions disrupt the regular arrangement
- Layers cannot slide as easily
- Result: Brass is harder than pure copper
Common alloys you should know
Steel is iron mixed with carbon. Pure iron is quite soft, but adding carbon makes it much stronger. Different amounts of carbon give different strengths, and steel is used to make cars, buildings, and tools.
Steel is one of the most important alloys in modern construction and manufacturing. The carbon content can be adjusted from 0.3% to 2% to create steels with different properties.
Key Points to Remember:
- Metals have giant structures held together by strong metallic bonding
- Delocalised electrons make metals good conductors of electricity and heat
- Most metals have high melting points because metallic bonds are strong
- Alloys are mixtures of metals that are usually harder and stronger than pure metals
- Different sized ions in alloys stop layers sliding, making them stronger than pure metals