Home Notes Papers

Metallic bonding

Paper 2Paper 4

This section is examined in Paper 2 and Paper 4.

The Structure of Metals
To understand metallic bonding, we must first look at the structure. Metals do not exist as isolated molecules (like O_2 or H_2O). Instead, they form a giant metallic lattice.

In this structure:

  1. Metal atoms lose their outer shell electrons to become positive ions (cations).
  2. These positive ions are arranged in a regular, repeating pattern (a lattice).
  3. The lost electrons do not belong to any specific ion; they move freely throughout the entire structure.

This creates a 'sea' of delocalised electrons surrounding the fixed positive ions.

Metallic Bonding

Metallic bonding is defined as the strong electrostatic attraction between the positive ions (in a giant metallic lattice) and the sea of delocalised electrons.

Key points to remember:

  • The force is electrostatic (between opposite charges: positive ions and negative electrons).
  • It is non-directional, meaning the attraction pulls equally in all directions, holding the lattice together.
Describing Metallic Bonding (e.g., in Iron or Copper)

When asked to describe the bonding in a metal, you must mention three specific components. Missing any one will cost marks.

Model Answer Structure:

  1. Mention positive ions (or cations).
  2. Mention a sea of delocalised electrons (or mobile/free electrons).
  3. State that there is electrostatic attraction between the positive ions and the electrons.
ComponentRequired Phrase
  1. The Particles
Positive ions (or cations)
  1. The Electrons
Sea of delocalised electrons (or mobile/free electrons)
  1. The Force
Electrostatic attraction between the positive ions and the electrons
⚠︎ Confusing Bonding Types
Mistake: Saying 'metals share electrons' or 'metals have covalent bonds.'

Correction: Metals do not share electrons between specific pairs of atoms (which is covalent bonding). Instead, the electrons are delocalised (free to move) among all ions. Do not say 'electrons are transferred' (which implies ionic bonding where electrons stay with one ion); in metals, they are shared collectively by the whole lattice.

Describing Bonding for Marks
When to use: When asked to 'describe' or 'name and describe' metallic bonding.

Why examiners accept this: The markscheme is strict about identifying the charges involved. You must explicitly state that the ions are positive and the force is electrostatic. Simply saying 'attraction between atoms' is incorrect because it ignores the charge separation.

Correct Usage Example:
'In metallic bonding, there is a strong electrostatic attraction between the lattice of positive metal ions and the sea of delocalised electrons.'

Tip: Use the word 'delocalised'. It shows you understand that the electrons are not fixed to one atom.

Explaining Properties from Structure
Q:
Explain why solid metals are good conductors of electricity. [1]
A:
Because there are delocalised electrons (or mobile/free electrons) that can move (or flow/transfer charge) through the structure.
Q:
Explain why metals are malleable (can be hammered into sheets). [2]
A:
  1. The layers of positive ions can slide (or move/glides) over each other.
  2. The bonding is non-directional (or the electrostatic attraction holds the structure together even when ions move), so the metal does not shatter.
Q:
Explain why metals are ductile (can be drawn into wires). [1]
A:
Because the layers of positive ions can slide over each other without breaking the metallic bonding.
Why Metals Conduct Electricity
Conductivity requires charged particles that are free to move.

In metals:

  • The delocalised electrons are negatively charged.
  • They are not bound to any specific ion and can move freely throughout the lattice.
  • When a voltage is applied, these electrons drift towards the positive terminal, carrying charge.

Note: The positive ions are fixed in place and do not move, so they do not contribute to electrical conduction.

Explaining Conductivity for Marks
When to use: When asked 'Explain why metals conduct electricity'.

Why examiners accept this: The key concept is charge carrier mobility. You must identify what moves (electrons) and that they are free to move. Saying 'metals have electrons' is insufficient; you must say they are delocalised or mobile.

Correct Usage Example:
'Metals conduct electricity because there are delocalised electrons that are free to move (or flow) through the structure and carry charge.'

Tip: Do not mention 'ions moving'. Ions only move in molten or aqueous solutions, not in solid metals.

Why Metals are Malleable and Ductile
Malleability (hammering into sheets) and Ductility (drawing into wires) depend on the arrangement of ions.

  1. In a metal lattice, positive ions are arranged in layers.
  2. Because the bonding is non-directional (the sea of electrons surrounds all ions equally), the layers can slide over each other when force is applied.
  3. The metallic bond is not broken during this sliding; the electrons simply adjust to the new positions of the ions.

This contrasts with ionic compounds, where shifting layers causes like charges to align (positive next to positive), causing repulsion and shattering.

Explaining Malleability for Marks
When to use: When asked 'Explain why metals are malleable'.

Why examiners accept this: You must explain the mechanism of movement. The key is that layers move/slides. You do not need to overcomplicate it with 'non-directional bonding' unless asked for a deeper explanation, but mentioning that the structure remains intact is helpful.

Correct Usage Example:
'The layers of positive ions can slide (or move) over each other without breaking the metallic bonds.'

Tip: Avoid saying 'atoms slide'. Use the term ions because the atoms have lost electrons to become charged.

Comparing Structure and Bonding
Q:
Which row describes the structure and bonding in solid cobalt? [1]
A:
Giant metallic lattice with positive ions and delocalised electrons.
Q:
Describe the bonding in a Group II element like Magnesium. [3]
A:
  1. Positive ions (or cations).
  2. Sea of delocalised electrons.
  3. Electrostatic attraction between the positive ions and the electrons.
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