Question

Both solid \(\mathrm{Kr}\) and solid Cu consist of individual atoms. Why do their physical properties differ so much?

Short answer

Krypton is held together by weak Van der Waals forces, while copper is held together by strong metallic bonds, resulting in drastically different physical properties.

Step-by-step solution

Identify the Types of Bonding

Solid krypton (Kr) is held together by weak Van der Waals forces (London dispersion forces), whereas solid copper (Cu) is held together by metallic bonds.

Understand Van der Waals Forces

In krypton, the atoms are neutral and only weakly attract each other through temporary dipoles. These forces are relatively weak, leading to low melting and boiling points, as well as low hardness.

Understand Metallic Bonding

In copper, the atoms release some of their electrons to form a 'sea of electrons' that freely move throughout the lattice. This strong attraction between the positively charged ions and the sea of electrons results in high melting and boiling points, good electrical conductivity, and considerable hardness.

Compare Physical Properties

Due to the weak Van der Waals forces in krypton, it is soft and has low melting and boiling points. In contrast, the strong metallic bonds in copper make it hard, with high melting and boiling points, and excellent electrical and thermal conductivity.

Key concepts

Van der Waals Forces

Van der Waals forces are a type of weak intermolecular force present in many solid materials. These forces arise from temporary dipoles that form when the distribution of electrons around an atom or molecule becomes momentarily asymmetrical. In the case of solid krypton (Kr), these temporary dipoles cause weak attractions between its atoms.
Because of their weakness, Van der Waals forces result in materials with low melting and boiling points. These materials are often soft and easily broken.

• For instance: Solid krypton is a gas at room temperature and only becomes a solid at very low temperatures due to these weak forces.

Metallic Bonds

Metallic bonds are a type of chemical bond found in metals, including solid copper (Cu). In these types of bonds, atoms release some of their electrons to create a 'sea of electrons' that freely move throughout the material. The positively charged metal ions are held together by the attraction to these free-moving electrons.
This results in strong, cohesive forces that give metals their characteristic properties:

• Metals usually have high melting and boiling points because a lot of energy is required to break the metallic bonds.
• They are excellent conductors of electricity and heat due to the free movement of electrons.
• Metals are generally hard and malleable, meaning they can be bent and shaped without breaking easily.

Physical Properties of Solids

The bonding type in solid materials significantly affects their physical properties.
Krypton, bonded by weak Van der Waals forces, shows distinct characteristics:

• Low melting and boiling points
• Soft and easily broken
• Poor electrical and thermal conductivity

Conversely, copper, with its strong metallic bonds, exhibits opposite properties:

• High melting and boiling points
• Hard and resistant to breaking
• Excellent electrical and thermal conductivity

Understanding these differences helps explain why materials behave the way they do under different conditions.