Ionic, Covalent & Metallic Bond (AQA GCSE Chemistry): Revision Notes
Metallic bonding
What is metallic bonding?
Metallic bonding happens in metal elements and alloys. The outer electrons from metal atoms become delocalised. This means they are no longer attached to individual atoms.
When electrons become delocalised, they leave behind positive metal ions. These metal ions form a regular structure called a lattice.
The delocalised electrons are free to move around the whole structure. They form what we call a "sea of electrons".
The key to understanding metallic bonding is the concept of delocalisation. Unlike ionic or covalent bonding where electrons are either transferred or shared between specific atoms, in metallic bonding the electrons become mobile and belong to the entire metal structure.
How metallic bonding works
Worked Example: Sodium Metal Bonding
Let's trace through what happens when sodium atoms form metallic bonds:
Step 1: Sodium atoms lose their outer electrons Step 2: This creates positive sodium ions (Na+) Step 3: The lost electrons become delocalised Step 4: These delocalised electrons can move freely around all the sodium ions Step 5: The sodium ions arrange themselves in a regular pattern
The electrostatic attraction between the positive metal ions and the negative delocalised electrons creates the strong metallic bond.
Why metals have their properties
The "sea of electrons" explains why metals have special properties:
- Electrical conductivity: The delocalised electrons can move freely, carrying electric current
- Malleability: The structure can bend without breaking because electrons can move around
- Metallic lustre: The free electrons reflect light, making metals shiny
All the characteristic properties of metals can be traced back to the same fundamental feature: the mobile sea of delocalised electrons that can respond to different external forces and conditions.
Metals and alloys
Pure metals are often too soft for many everyday uses. For example:
- Pure copper is soft
- Pure gold is soft
- Pure iron is soft
To make metals more useful, we mix different metals together to create alloys.
Alloys are mixtures of metals that are much harder than pure metals.
This happens because different sized atoms disrupt the regular lattice structure, making it harder for layers of atoms to slide over each other.
Examples of alloys
Example: Solder Alloy
Solder is a useful alloy made from tin and lead with these properties:
- It has a low melting point (183°C)
- This makes it perfect for joining electrical components
- It melts easily but doesn't damage delicate parts
- It conducts electricity well
Steel is an alloy of iron and carbon that is much stronger than pure iron.
Why we choose specific metals
Different metals have different properties. We choose metals based on what we need them to do:
- Copper is used for electrical wires because it conducts electricity very well
- Copper is used for cooking pans because it has a high melting point (1085°C) and conducts heat well
- We don't use sodium for cooking pans because it has a very low melting point (98°C) and would melt!
Material selection in engineering involves matching the properties of metals to the specific requirements of each application. The same metal might be perfect for one use but completely unsuitable for another.
Remember!
Key Points to Remember:
- Metallic bonding occurs when outer electrons become delocalised, creating positive metal ions surrounded by a "sea of electrons"
- The electrostatic attraction between positive ions and delocalised electrons creates strong metallic bonds
- Alloys are mixtures of metals that are harder and more useful than pure metals
- Delocalised electrons can move freely, which explains why metals conduct electricity
- We choose different metals for different jobs based on their melting points and other properties