# Metallic bonding and alloy structure

> IB Chemistry HL · IB Chemistry HL (2025 syllabus)
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u2-metallic-bonding-and-alloy-structure/

This subtopic covers the electron sea model of metallic bonding, explains the characteristic physical properties of metals, and classifies the two main types of alloy structure, relating structure to altered properties of metallic materials.

**Prerequisites:** [Basic ionic and covalent bonding concepts](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ionic-and-covalent-bonding/)

## Learning objectives

- Describe the electron sea model of metallic bonding
- Explain key physical properties of metals using the bonding model
- Distinguish between substitutional and interstitial alloy structures
- Relate alloy structure to changes in physical properties compared to pure metals

## The Electron Sea Model of Metallic Bonding

**Metallic Bonding** — The strong non-directional electrostatic attraction between a regular lattice of positively charged metal cations and a delocalized 'sea' of free-moving valence electrons that are not bound to any individual atom

*Example:* In sodium metal, each atom donates one valence electron to the sea, leaving Na⁺ cations in the lattice.

All metal atoms have low ionization energies, so they readily lose their valence electrons to form the delocalized electron sea. The entire solid structure is held together by the electrostatic attraction between the positive cations and negative delocalized electrons.

**Worked example:** Use the metallic bonding model to explain why magnesium has a higher melting point (650°C) than sodium (98°C)

1. Identify the charge of the metal cations and number of delocalized electrons per atom
2. Magnesium has 2 valence electrons per atom, forming $Mg^{2+}$ cations. Sodium has 1 valence electron per atom, forming $Na^+$ cations.
3. Compare the strength of electrostatic attraction between cations and the electron sea
4. A higher cation charge and greater number of delocalized electrons creates stronger electrostatic attraction between the cations and electron sea.
5. Relate attraction strength to melting point
6. More thermal energy is required to overcome stronger metallic bonding, so magnesium has a higher melting point than sodium.

> **tip**
>
> Metallic bonding is non-directional — this is the key property that explains why metals are malleable, not brittle.

## Physical Properties of Metals (Structure → Property Links)

All core physical properties of metals follow directly from the electron sea model, and IB exam questions very frequently ask you to connect structure to property. The key relationships are summarized below:

- **Electrical conductivity**: Delocalized electrons can move through the lattice when a potential difference is applied, making metals good conductors.
- **Thermal conductivity**: Free-moving electrons transfer kinetic energy rapidly through the structure.
- **Malleability/ductility**: Non-directional bonding allows layers of cations to slide past each other without breaking the bonding interaction.
- **High melting/boiling points**: Strong electrostatic attraction requires large amounts of energy to overcome.

**Worked example:** Explain why metals are malleable but ionic solids are brittle using their respective bonding models

1. Describe the response of metallic structure to applied force
2. When force is applied to a metal, layers of cations slide past each other. Non-directional metallic bonding re-forms between the new layer positions and the delocalized sea, so the metal bends instead of breaking.
3. Describe the response of an ionic solid to applied force
4. In an ionic lattice, force shifts the position of ions so that like-charged ions are adjacent. Strong electrostatic repulsion between like charges splits the lattice, making ionic solids brittle.

## Alloy Structure and Properties

**Alloy** — A homogeneous mixture of a metal with one or more other elements (usually another metal or carbon), designed to have modified physical properties compared to the pure metal.

Alloys are classified based on the relative size of the added element atoms, which determines their position in the host metal lattice. The two main classes are summarized below:

| Alloy Type | Relative size of added atoms | Position in lattice | Common Example |
| --- | --- | --- | --- |
| Substitutional | Similar to host atoms | Replaces host atoms in the lattice | Brass (copper + zinc) |
| Interstitial | Much smaller than host atoms | Fits in gaps between host atoms | Carbon steel (iron + carbon) |

Alloying almost always increases hardness and strength, and reduces electrical conductivity compared to the pure metal. The added atoms disrupt the regular repeating lattice, making it harder for layers of cations to slide past each other, which increases hardness. Disruption of the continuous electron sea reduces electron mobility, lowering conductivity.

**Worked example:** Explain why pure copper is much softer than brass, a substitutional alloy of copper and zinc

1. Identify the structure of brass
2. Brass is a substitutional alloy, so zinc atoms (similar size to copper) replace some copper atoms in the regular pure copper lattice.
3. Explain the effect of zinc atoms on the lattice
4. The different-sized zinc atoms disrupt the uniform regular structure of the pure copper lattice.
5. Relate lattice disruption to hardness
6. Disruption makes it far harder for layers of copper cations to slide past each other when force is applied, so brass is harder and stronger than pure copper.

## Common pitfalls

- **Wrong:** Describing metallic bonding as attraction between neutral metal atoms
  - Why it fails: Valence electrons are delocalized, so lattice sites are positively charged cations, not neutral atoms
  - Correct: Always describe metallic bonding as attraction between positive metal cations and delocalized valence electrons
- **Wrong:** Mixing up substitutional and interstitial alloy size rules
  - Why it fails: Many students incorrectly assign small atoms to substitutional alloys
  - Correct: Remember: interstitial = interstices = gaps = small atoms fit in gaps; substitutional = similar size to replace host atoms
- **Wrong:** Claiming alloys are stronger than pure metals because they have stronger metallic bonding
  - Why it fails: Increased strength comes from lattice disruption, not stronger bonding between particles
  - Correct: Explain increased strength as a result of disrupted lattice that prevents layers of cations from sliding easily
- **Wrong:** Saying metals conduct electricity because electrons vibrate in place
  - Why it fails: Conductivity requires mobile charge carriers; vibration is a property of all solid particles
  - Correct: State that conductivity arises from delocalized electrons that move freely through the lattice under a potential difference

## Cheatsheet

| Concept | Key Exam Point |
| --- | --- |
| Metallic bonding | Electrostatic attraction between metal cations + delocalized electron sea |
| Non-directional bonding | Explains why metals are malleable (layers slide, bonds re-form) |
| Melting point trend | Higher cation charge = stronger bonding = higher melting point |
| Substitutional alloy | Similar size added atoms replace host (example: brass) |
| Interstitial alloy | Small added atoms fit in lattice gaps (example: steel) |
| Alloy property change | Hardness/strength ↑, electrical conductivity ↓ vs pure metal |

## What's next

Understanding metallic bonding and alloys completes your overview of the four main types of solid structure for IB Chemistry, and reinforces the core skill of connecting microscopic structure to macroscopic observable properties. This subtopic is a common source of both multiple choice and short answer questions in Paper 1 and Paper 2, so mastering the structure-property links here will earn you easy marks on exam day. The bonding concepts you learned here build directly into more advanced topics in solid state chemistry and materials science, which are common themes in exam questions and extended essay projects.

- [Crystal lattice structures](https://www.owlsprep.com/study/ib-chemistry-hl-u2-crystal-lattice-structures/)
- [AHL: Covalent bond order and electron delocalization](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ahl-covalent-bond-order-and/)
- [AHL: Advanced hybridization and delocalized pi systems](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ahl-advanced-hybridization-and-delocalized/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-hl-u2-metallic-bonding-and-alloy-structure/