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.