Question

What kinds of attractive forces exist between particles in (a) molecular crystals, (b) covalent-network crystals, (c) ionic crystals, (d) metallic crystals?

Short answer

In (a) molecular crystals, the attractive forces are London dispersion forces (Van der Waals forces) or, for polar molecules, dipole-dipole forces or hydrogen bonding. In (b) covalent-network crystals, strong covalent bonds form a three-dimensional network between atoms. In (c) ionic crystals, electrostatic forces between oppositely charged ions form ionic bonds. In (d) metallic crystals, metallic bonds arise from the interaction of positively charged metal ions with a delocalized "sea" of electrons.

Step-by-step solution

(a) Molecular Crystals

In molecular crystals, the primary attractive force between the particles is the London dispersion forces (also known as Van der Waals forces) or in case of polar molecules, dipole-dipole forces or hydrogen bonding can also be present. These forces are due to temporary fluctuations in electron distribution around the molecules, creating temporary dipoles that attract other nearby molecules. In case of polar molecules, there are permanent dipoles that interact with the dipoles of nearby molecules.

(b) Covalent-Network Crystals

Covalent-network crystals have a different structure as compared to molecular crystals. In covalent-network crystals, the atoms are connected through covalent bonds forming a three-dimensional network. The attractive forces between the atoms in these crystals are strong covalent bonds, which make these crystals very hard, with high melting and boiling points.

(c) Ionic Crystals

Ionic crystals are composed of positive and negative ions arranged in a lattice structure. The attractive forces between the particles in ionic crystals are electrostatic forces between the oppositely charged ions, also known as ionic bonds. These ionic bonds result in the formation of an ordered and repeating pattern of ions, which gives ionic crystals their characteristic crystal structure.

(d) Metallic Crystals

Metallic crystals have a different arrangement of atoms compared to the other three types of crystals. In metallic crystals, metal atoms are bonded together through metallic bonds, where electrons are delocalized and free to move throughout the lattice. This delocalization of electrons creates a "sea" of electrons around the metal ions, and the attractive forces between the positively charged metal ions and the negatively charged electron cloud hold the particles together in a metallic crystal lattice. So, the attractive forces in metallic crystals are metallic bonds.

Key concepts

Molecular Forces

Molecular forces play a key role in holding together molecular crystals. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonds.
London dispersion forces are weak and arise due to temporary fluctuations in the electron distribution around molecules. These temporary dipoles attract adjacent molecules.
For polar molecules, dipole-dipole forces come into play. These forces are due to the attraction between the positive end of one polar molecule and the negative end of another.

• Hydrogen bonding: A special kind of dipole-dipole interaction occurring when hydrogen is bonded to electronegative atoms such as nitrogen, oxygen, or fluorine. This results in a stronger attraction.

Even though individual molecular forces are weak, collectively, they significantly influence the physical properties of materials like melting and boiling points.

Covalent Bonds

Covalent bonds are the backbone of covalent-network crystals. In these structures, each atom is bonded to its neighbors through strong covalent bonds, forming an extensive three-dimensional network.
These bonds involve the sharing of electron pairs between atoms, which creates a robust and rigid structure.

• Diamond: An example of a covalent-network crystal, where each carbon atom is tetrahedrally bonded to four other carbon atoms, resulting in exceptional hardness.

The strength of these covalent bonds gives covalent-network crystals their characteristic high melting and boiling points, making them very stable.

Ionic Bonds

Ionic bonds are central to the structure of ionic crystals. These bonds form due to the electrostatic attraction between positively charged cations and negatively charged anions.
In an ionic crystal, these ions are arranged in a regular, repeating lattice structure. This ordered pattern minimizes the repulsive forces and maximizes the attractions in the crystal lattice.

• Sodium chloride (NaCl): A typical ionic crystal, where each sodium ion is surrounded by six chloride ions and vice versa, forming a stable and rigid structure.

The ionic bonds make ionic crystals generally hard, brittle, and having high melting and boiling points. However, they tend to be soluble in water and conduct electricity when dissolved or melted.

Metallic Bonds

Metallic bonds are primarily responsible for the formation of metallic crystals. In this bonding type, the bonding electrons are not confined to individual atoms. Instead, they're shared among a lattice of metal cations.
This delocalization of electrons results in a "sea of electrons," which allows them to move freely throughout the crystal lattice.

• Electrical conductivity: This free movement of electrons makes metals excellent conductors of electricity.
• Malleability and ductility: The mobile electrons also allow metal atoms to slide past each other, imparting these properties to metals.
• Luster: The electron sea reflects light, giving metals their shiny appearance.

Metallic bonds, therefore, endow metals with unique physical properties that differentiate them from other crystal types.