Edexcel IGCSE Chemistry · Spec 1.52C-1.54C
Metallic Bonding
How metallic bonding works and how it explains the properties of metals.
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Metallic Bonding
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Explained
Metallic bonding, and the properties that follow from it
In a metal, the atoms pack into a regular lattice and each one loses its outer electrons. What is left is a lattice of positive metal ions, with those electrons free to move throughout the whole structure.
Those free electrons are described as delocalised, meaning they no longer belong to any particular atom. The metallic bond is the electrostatic attraction between the positive ions and the sea of delocalised electrons around them.
Every property of a metal comes out of that one picture, so it is worth being able to draw it before you try to explain anything.
Why metals conduct electricity
The delocalised electrons are free to move through the lattice. Apply a potential difference and they drift, and a flow of charge is a current.
They conduct heat for the same reason. The electrons gain kinetic energy at the hot end, move through the metal, and transfer that energy elsewhere, which is much faster than passing it along by vibration alone.
Why metals bend rather than shatter
The ions are arranged in layers. When a force is applied, the layers slide over one another into a new position.
What matters is that the bonding survives the move. The delocalised electrons are everywhere, so wherever an ion ends up it is still attracted to them, and the metal simply takes a new shape. That is why metals are malleable and can be hammered into sheets, and ductile and can be drawn into wires.
Compare that with an ionic solid, where sliding the layers brings ions of the same charge next to each other. They repel, and the crystal cracks. The contrast is the point of the question when it is asked.
Why melting points are high
The attraction between the positive ions and the delocalised electrons is strong and acts throughout the structure. A large amount of energy is needed to overcome it, so most metals melt and boil at high temperatures.
Metals with ions of higher charge release more electrons each, so the attraction is stronger and the melting point is higher. Magnesium melts at a higher temperature than sodium for exactly this reason.
What the mark schemes accept and reject
Edexcel International GCSE Chemistry mark schemes police bonding vocabulary more strictly here than almost anywhere else in the course.
On a question about a giant covalent structure, the note says that any mention of intermolecular forces, forces between molecules or ions, ionic bonding or metallic bonding scores zero out of three. On a question about ionic bonding, the note says that mentioning covalent bonding, intermolecular forces, metallic bonding or molecules loses the first four marks.
In both cases a correct explanation is thrown away by naming the wrong type of bonding anywhere in it. There is no reason to expect a metallic bonding question to be marked any differently, so keep ionic and covalent out of your answer entirely.
One further note transfers directly. On the graphite question the mark scheme ignores free electron, spare electron and unbonded, crediting only delocalised. That is the word to use for the electrons in a metal too.
Alloys
An alloy is a mixture of a metal with another element, and it is harder than the pure metal.
The reason follows from the layers. The added atoms are a different size, so they disrupt the regular arrangement, and the layers can no longer slide over one another easily. Less sliding means a harder, less malleable material, which is why steel is used for structures and pure iron is not.
Spec 1.52C-1.54C
What you need to know
- Describe metallic bonding
- Explain why metals conduct electricity and can be bent into shape
- Explain why metals have high melting points
Active recall
Quick check
Answer each question before opening the answer.
Describe metallic bonding.
A lattice of positive metal ions surrounded by a 'sea' of delocalised electrons, held together by electrostatic attraction.
Why are metals good conductors and malleable?
Delocalised electrons are free to move and carry charge; the layers of ions can slide over each other.
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