# Metallic bonding

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u3-metallic-bonding/

This module covers the electron sea model of metallic bonding, links between bonding structure and physical properties, and common exam questions on melting point trends for CIE A-Level Chemistry 9701.

**Prerequisites:** [Basic ionic and covalent bonding](https://www.owlsprep.com/study/cie-9701-u3-ionic-covalent-bonding/)

## Learning objectives

- Describe the electron sea model of metallic bonding
- Relate metallic bonding structure to key physical properties of metals
- Explain trends in melting points of metals across periods and down groups

## The Electron Sea Model of Metallic Bonding

Metallic bonding occurs in pure metal elements and alloys. It is fundamentally different from ionic and covalent bonding because it is non-directional, meaning attraction acts equally in all directions throughout the giant lattice structure.

**Metallic Bonding** — The strong electrostatic force of attraction between closely packed positive metal cations and a surrounding sea of delocalised outer electrons that are not bound to any single atom.

*Example:* In solid sodium, each Na atom loses its single outer electron to form a Na+ cation, which is fixed in a regular lattice surrounded by a sea of mobile delocalised electrons.

**Worked example:** Describe the structure of pure metallic copper.

1. Pure copper has a giant metallic lattice structure, made up of positively charged Cu²⁺ ions arranged in a regular, repeating 3D pattern.
2. The outer electrons from each copper atom are delocalised, forming a 'sea' of mobile negative charge that fills the spaces between the positive cations.
3. The metallic bond is the strong electrostatic attraction between the positive Cu²⁺ ions and the negative delocalised electron sea.

> **Exam tip:** Always explicitly mention electrostatic attraction in your definition of metallic bonding — you will lose a mark if you only state attraction between ions and electrons.

## Physical Properties of Metals Explained by Bonding

**Worked example:** Explain why copper is ductile can be drawn into wires, while sodium chloride (an ionic solid) is brittle and cannot.

1. Copper has a giant metallic structure with non-directional metallic bonding between Cu²⁺ cations and delocalised electrons.
2. When a force is applied to draw copper into a wire, layers of Cu²⁺ cations slide past each other, but the electrostatic attraction between cations and the electron sea is maintained. The structure does not break.

> **Exam tip:** Always mention non-directional bonding when explaining malleability/ductility — this is a common standalone marking point in CIE exams.

The strength of metallic bonding determines the melting point of a metal. Three key factors affect bond strength:

- Higher charge on the metal cation = stronger electrostatic attraction
- Smaller ionic radius = closer proximity between cations and electrons = stronger attraction
- More delocalised electrons per atom = stronger attraction

**Worked example:** Explain why aluminium has a higher melting point (660°C) than sodium (98°C).

1. Sodium forms Na⁺ ions with a +1 charge, while aluminium forms Al³⁺ ions with a +3 charge.
2. Aluminium has 3 delocalised electrons per atom, compared to only 1 delocalised electron per atom for sodium.
3. Al³⁺ also has a smaller ionic radius (0.054 nm) than Na⁺ (0.102 nm), so attraction between ions and electrons is stronger.
4. The combination of these factors creates much stronger metallic bonding in aluminium, so more energy is required to break the bonds, leading to a higher melting point.

**Check your understanding**

Test your understanding of trends:

1. Which of the following group 1 metals has the highest melting point?

   - Lithium (Li)
   - Sodium (Na)
   - Potassium (K)
   - Rubidium (Rb)

   *Why:* All group 1 metals form +1 ions with 1 delocalised electron per atom. Ionic radius increases down the group, so Li⁺ has the smallest radius, strongest attraction, highest melting point.

### Common pitfalls

- **Wrong:** Describing metallic bonding as attraction between neutral metal atoms, not positive cations.
  - Why it fails: CIE marking schemes require explicit mention that outer electrons are delocalised, leaving behind positive ions.
  - Correct: Always describe metallic bonding as attraction between positive metal cations and delocalised electrons.
- **Wrong:** Explaining electrical conductivity by saying ions can move through the solid metal.
  - Why it fails: In solid metals, positive cations are fixed in the lattice. Only electrons are mobile.
  - Correct: State that delocalised free electrons move through the lattice to carry charge.
- **Wrong:** Claiming melting point always increases down a group of metals.
  - Why it fails: Down a group, ionic radius increases while cation charge stays the same, so attraction weakens and melting point decreases.
  - Correct: Down a metal group, melting point generally decreases due to increasing ionic radius and weaker metallic bonding.
- **Wrong:** Explaining malleability without mentioning non-directional bonding.
  - Why it fails: CIE regularly awards a separate mark for stating that metallic bonding is non-directional.
  - Correct: Add that non-directional bonding means attraction is maintained even when layers slide past each other.

### Cheatsheet

| Factor | Effect on Metallic Bond Strength | Effect on Melting Point |
| --- | --- | --- |
| Higher cation charge | Increases strength | Increases |
| More delocalised electrons per atom | Increases strength | Increases |
| Smaller ionic radius | Increases strength | Increases |
| Larger ionic radius (down a group) | Decreases strength | Decreases |

### What's next

Metallic bonding is a foundational concept for understanding periodicity, where you will compare melting point trends across period 3 and explain why metals have different properties to non-metals. You will also build on this knowledge when studying alloys, which are mixtures of metals with different cation sizes that change the malleability and strength of the material. Extended response questions in CIE exams often require you to compare metallic bonding with ionic and covalent bonding, so mastering this concept is key to scoring full marks on these questions.

- [Ionic bonding](https://www.owlsprep.com/study/cie-9701-u3-ionic-bonding/)
- [Covalent bonding](https://www.owlsprep.com/study/cie-9701-u3-covalent-bonding/)
- [Periodicity](https://www.owlsprep.com/study/cie-9701-u5-periodicity/)

---

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/cie-9701-u3-metallic-bonding/