Metallic bonding and alloysIB MYP Chemistry: Revision notes
Section 1
Metallic bonding
Metals are made of a regular lattice of positive ions arranged in layers. The outer-shell electrons of each metal atom are not held to one atom. They are delocalised: they move freely through the whole structure, forming a sea of delocalised electrons.
Metallic bonding is the strong electrostatic attraction between the positive metal ions and the sea of delocalised electrons. This attraction holds the metal together.
Metals do not have shared pairs of electrons between pairs of atoms (covalent) and do not transfer electrons to non-metals (ionic).
Section 2
Conductivity
Metals conduct electricity because the delocalised electrons are free to move through the metal and carry charge when a voltage is applied.
The positive ions stay in fixed positions in the lattice. Only the electrons move.
Say delocalised electrons carry the charge in metals. In molten or dissolved ionic compounds it is the ions.
Section 3
Malleability
Metals are malleable: they can be hammered or bent into shape without breaking.
The layers of positive ions can slide over each other. As they slide, the delocalised electrons still attract the ions, so the metallic bonding is not broken and the metal stays in one piece.
Section 4
Alloys
A pure metal is often too soft for use. An alloy is a mixture of a metal with one or more other elements. Examples: brass (copper and zinc), steel (iron and carbon), bronze (copper and tin).
Alloys are harder than pure metals because the atoms of the other element are a different size from the metal atoms. They disrupt the regular layers, so the layers cannot slide over each other easily.
Steel is used for bridges and buildings because it is stronger than pure iron.
The extra atoms in an alloy do not form new bonds. They are simply a different size and stop the layers sliding.
Section 5
Worked example: comparing hardness
A student drops a ball bearing onto pure copper and onto brass. The mean dent in copper is 3.4 mm and in brass is 2.1 mm.
- The smaller dent shows brass is harder.
- Explanation: zinc atoms are a different size from copper atoms, so they disrupt the layers in brass.
- The layers cannot slide easily, so brass resists denting better.
To make the test fair, use the same ball bearing, the same drop height and the same thickness of strip.
Must know
- Metals: a lattice of positive ions in a sea of delocalised electrons.
- Conduct because delocalised electrons are free to move.
- Malleable because layers of ions slide over each other.
- Alloys are mixtures of a metal with other elements.
- Alloys are harder because different-sized atoms stop the layers sliding.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Metallic bonding and alloys
- An electrician in Johannesburg chooses copper for the electrical cables in a new school building because copper conducts electricity very well. Copper is a metal.Describe the structure of a metal such as copper.2 marks
- A craftsperson in Marrakech hammers a flat sheet of copper into a bowl. The copper changes shape without cracking or breaking.Explain why the copper changes shape without breaking.2 marks
- A student compares the hardness of pure copper with brass, an alloy of copper and zinc. She drops the same steel ball bearing from the same height onto a strip of each metal and measures the diameter of the dent. She does three trials on each strip. The dents in copper measured 3.5 mm, 3.4 mm and 3.3 mm. The dents in brass measured 2.2 mm, 2.0 mm and 2.1 mm.Calculate the mean dent diameter for each metal and state which metal is harder, giving a reason.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).