Metallic Bonding

This lesson covers: 

1. The structure of metals
2. How metallic bonds are formed
3. Factors affecting the strength of metallic bonds
4. The properties of metals

Metals have metallic lattice structures

Metals exist as giant lattice structures made up of:

• Positively charged metal cations (e.g. Na⁺, Mg²⁺)
• Delocalised electrons that move freely between the cations

The outer electrons of metal atoms leave to become delocalised, resulting in positively charged metal cations.

A 2D model of sodium’s giant metallic lattice structure is shown above.

Metallic bonding occurs between metal cations and delocalised electrons

• Metallic bonding refers to the electrostatic attractions between positively charged metal ions (e.g. Na⁺, Mg²⁺) and negatively charged delocalised electrons.
• These metallic bonds are very strong and hold the metals in their lattice structure.

Strength of metallic bonds

The strength of metallic bonds determines the melting point of a metal. The stronger the metallic bonds, the higher the melting point.

The strength of these bonds is influenced by three main factors:

1. Number of delocalised electrons per atom:

• Metals with a higher number of delocalised electrons per atom tend to form stronger metallic bonds.
• The increased number of delocalised electrons allows for stronger electrostatic attractions between the electrons and the metal cations.

1. Charge of the metal cation:

• A higher cation charge results in stronger electrostatic attractions between the cation and the delocalised electrons.

1. Radius of the metal cation:

• Smaller metal cations have a higher charge density, which allows them to hold the delocalised electrons closer to the nucleus.
• This proximity enhances the electrostatic attractions between the cation and the electrons, resulting in a stronger metallic bond. 

Magnesium has a higher melting point than sodium

To illustrate these concepts, let's compare the melting points of magnesium and sodium:

Magnesium has a melting point of 650°C, while sodium has a melting point of 98°C.

Magnesium has a higher melting point than sodium because:

1.  Each magnesium atom donates two electrons to the delocalised electron sea, whereas each sodium atom donates only one electron.

2.  Magnesium has a +2 charge, while sodium has a +1 charge.

3.  Magnesium cations (Mg²⁺) are smaller than sodium cations (Na⁺).

Properties explained by metallic bonding

The properties of metals result from their metallic bonding and lattice structure:

1. High melting and boiling points - The strong electrostatic forces of attraction between the positively charged metal cations and the sea of delocalised electrons must be overcome for the lattice to break apart; this requires large amounts of energy.
2. Good conductors of electricity and heat - The delocalised electrons can flow freely through the lattice to transfer charge and heat energy.
3. Malleable and ductile - Layers of the cation lattice can slide over one another when the metal is hammered or pulled because there are no bonds locking individual cations together.
4. Insoluble - The strength of the metallic bonding prevents water or other solvent molecules from pulling the cations away from the lattice structure and dissolving the metal.