Question

Describe, with examples, these types of crystals: (a) ionic crystals, (b) covalent crystals, (c) molecular crystals. (d) metallic crystals.

Short answer

Ionic crystals like Sodium Chloride (NaCl) consist of positive and negative ions bonded strongly. Covalent crystals like Diamond (C) involve atoms connected by covalent bonds, forming a single molecule. Molecular crystals like Ice (H2O) have weak intermolecular forces like Van der Waals forces or Hydrogen bonds. Metallic crystals such as Iron (Fe) include metal atoms with a 'sea' of delocalized electrons around the cationic framework.

Step-by-step solution

Describe and give examples of ionic crystals

Ionic crystals consist of positive and negative ions. They have strong ionic bonds between them. A common characteristic is that they have high melting and boiling points. They also conduct electricity when molten or dissolved. An example of an ionic crystal is Sodium Chloride (NaCl).

Describe and give examples of covalent crystals

Covalent crystals are composed of atoms connected by covalent bonds. The whole crystal is a single molecule. They have extremely high melting and boiling points and are generally hard and brittle. An example of a covalent crystal is a Diamond (C).

Describe and give examples of molecular crystals

Molecular crystals contain molecules at each lattice point. These crystals have weak Van der Waals forces or Hydrogen bonds between the molecules. Their melting and boiling points are usually lower than those of ionic and covalent crystals. They do not conduct electricity. An example of a molecular crystal is Ice (H2O).

Describe and give examples of metallic crystals

Metallic crystals are composed of metal atoms. They have a closely packed regular structure with a 'sea' of delocalized electrons around the cationic framework. They possess properties such as high conductivity and malleability. An example of a metallic crystal is Iron (Fe).

Key concepts

Ionic Crystals

Ionic crystals are fascinating structures made up of positive and negative ions bound together through strong ionic bonds. These bonds result in some rather interesting properties, like extremely high melting and boiling points. This is because a great deal of energy is required to break these bonds. In liquid form or when dissolved in water, ionic crystals are capable of conducting electricity due to the free movement of ions.

One of the most common examples of an ionic crystal is Sodium Chloride (\[ \text{NaCl} \]). Known as table salt, it forms a repeating pattern in a crystal lattice, where sodium ions and chloride ions attract each other. This regular and repeating structure extends in three dimensions, contributing to the crystals’ characteristic hardness and brittleness.

Key features of ionic crystals include:

• High melting and boiling points due to strong ionic bonds
• Conductivity in liquid or dissolved state
• Typically hard and brittle
• Examples: Sodium Chloride, Potassium Bromide (\[\text{KBr}\])

Covalent Crystals

Covalent crystals are a type of crystal where the atoms are linked by covalent bonds forming one gigantic molecule. Unlike ionic crystals, the bonds in covalent crystals are directional, which means they hold the structure in a rigid and predictable form. This tight network of covalent bonds gives these crystals exceptionally high melting and boiling points, as well as making them very hard. However, they are usually quite brittle.

Diamond is the perfect illustration of a covalent crystal. Each carbon atom forms four covalent bonds with other carbon atoms, resulting in a three-dimensional network that extends throughout the entire crystal. This structure makes diamonds incredibly hard and an excellent abrasive.

Key features of covalent crystals include:

• Strong covalent bonds throughout
• High melting and boiling points
• Generally hard and brittle
• Examples: Diamond, Silicon Carbide (\[\text{SiC}\])

Molecular Crystals

Molecular crystals are quite different from ionic and covalent crystals. They are composed of molecules held together by comparatively weak forces—either van der Waals forces or hydrogen bonds. This weak intermolecular bonding results in much lower melting and boiling points for molecular crystals. Due to the absence of free-charged particles, these structures do not conduct electricity.

A common example of a molecular crystal is ice, with water molecules forming a lattice and being held in place by hydrogen bonds. This arrangement explains why ice is less dense than liquid water, allowing it to float.

Key features of molecular crystals include:

• Held together by weak Van der Waals forces or hydrogen bonds
• Lower melting and boiling points
• Non-conductors of electricity
• Examples: Ice, Dry Ice (\[\text{CO}_2\])

Metallic Crystals

Metallic crystals are characterized by a unique type of bonding. They consist of metal atoms that share a "sea" of delocalized electrons which move freely throughout the entire structure. This arrangement results in the remarkable properties of metallic crystals, such as excellent electricity and heat conductivity, as well as malleability and ductility.

An excellent example of a metallic crystal is Iron (\[\text{Fe}\]), which forms into tightly packed structures called lattices. The freely moving electrons enable metals to bend and shape without breaking, hence their usefulness in construction and manufacturing.

Key features of metallic crystals include:

• Delocalized electrons contributing to electrical and thermal conductivity
• Malleable and ductile nature
• Often shiny and reflective
• Examples: Iron, Copper (\[\text{Cu}\])