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Principles of Chemistry

Edexcel IGCSE Chemistry 4CH0 — 1.8 Metallic Bonding — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 16 of 25 Unit quiz

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Lesson summary

1.8 Metallic Bonding — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe metallic bonding and use it to explain the properties of metals and alloys.Metals have a giant structure of positive metal ions arranged in regular layers.

  • delocalised
  • High melting and boiling points
  • Good electrical conductivity
  • Good thermal conductivity
  • Malleable and ductile
  • Worked example 1

Part 1 of 6What is metallic bonding?

Metals have a giant structure of positive metal ions arranged in regular layers. The outer-shell electrons of each atom are released and delocalised: they form a "sea" of electrons free to move throughout the structure.

Metallic bonding is the strong electrostatic attraction between the positive metal ions and the delocalised electrons.

What is metallic bonding?
Its strength increases with the number of delocalised electrons per atom, the charge on the ion and the smaller size of the ion.
This is why magnesium (two delocalised electrons per atom, Mg2+) has a higher melting point than sodium (one electron, Na+).

Part 2 of 6Properties explained by the structure

Properties explained by the structure
High melting and boiling points
  • High melting and boiling points a lot of energy is needed to overcome the strong attraction between ions and electrons.
Good electrical conductivity
  • Good electrical conductivity delocalised electrons carry charge through the metal in solid and liquid states.
Good thermal conductivity
  • Good thermal conductivity free electrons pass kinetic energy quickly along the metal, and vibrating ions also transfer energy.
Malleable and ductile
  • Malleable and ductile layers of ions can slide over each other without breaking the bonding,
  • because the electrons keep holding the ions together;
    • metals can be hammered into sheets and drawn into wires.
Lustrous
  • Lustrous the surface reflects light.

Part 3 of 6Comparing metallic, ionic and covalent substances

PropertyMetallicGiant ionicSimple molecular
Melting pointHighHighLow
Conducts when solidYesNoNo
Conducts when moltenYesYesNo
Behaviour when hitBends (malleable)Shatters (brittle)Soft/weak

Ionic crystals shatter because shifting layers brings like charges together and they repel.

Part 4 of 6Alloys

An alloy is a mixture of a metal with one or more other elements. Pure metals are often too soft because the layers slide easily. In an alloy, atoms of different sizes disrupt the regular layers and make it harder for them to slide, so alloys are stronger and harder.

1
Brass (copper and zinc) — hard, used for taps and instruments.
2
Steel (iron with a small amount of carbon) — strong and used in construction.
3
Stainless steel (iron, chromium, nickel) — resists rusting.
4
Solder (tin and lead, or modern lead-free) — low melting point, used for electrical joints.

Part 5 of 6Explaining properties in the exam

Explaining properties in the exam
Exam answers on bonding need the right particles and forces.
  • A reliable structure isname the structure, name the particles, name the force, then link to the property.
Example for melting point: "Metals have a giant lattice of positive ions in a sea of delocalised electrons.
  • There are strong electrostatic forces between the ions and the electrons.
  • A lot of energy is needed to overcome these forces,
  • so the melting point is high." Do not say that bonds between atoms are broken,
  • and avoid using the word "molecules".
For the trend across a period: sodium, magnesium and aluminium have 1, 2 and 3 delocalised electrons per atom, with ionic charges of 1+, 2+ and 3+ and decreasing ionic radius.
Explaining properties in the exam
The attraction therefore strengthens from Na to Al, and melting points rise (98 °C, 650 °C and 660 °C).
Conductivity also rises because there are more charge carriers.
The same explanation applies to the group trend: down Group I, the larger ions and weaker attraction give lower melting points and softer metals.

Alloys appear in many uses, including aircraft (aluminium alloys, light and strong), coins (nickel and copper) and dental fillings, where the choice of alloy gives the correct hardness.

Part 6 of 6Worked examples

  1. Explain why magnesium has a higher melting point than sodium. [3]3 marks

  2. Explain why metals conduct electricity. [2]2 marks

  3. Explain why a metal can be bent without breaking. [2]2 marks

  4. Why is an alloy harder than the pure metal? [2]2 marks

Key terms

TermMeaning
Metallic bondAttraction between positive metal ions and delocalised electrons.
Delocalised electronsOuter electrons free to move through the metal structure.
MalleableCan be hammered or bent into shape.
DuctileCan be drawn into wires.
AlloyA mixture of a metal with other elements.
LatticeA regular arrangement of particles.

Avoid these mistakes

1
Saying metals conduct because of free ions — it is delocalised electrons.
2
Saying metallic bonding is attraction between atoms — it is between positive ions and delocalised electrons.
3
Saying metals are brittle — it is ionic solids that shatter.
4
Thinking an alloy is a compound — it is a mixture.
5
Comparing strengths without mentioning charge and number of delocalised electrons.

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