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Ch:- 3

Giant structures, also known as giant lattices or giant covalent structures, refer to substances where atoms are bonded together by a vast network of covalent bonds in a continuous, repeating arrangement throughout the entire material. This results in materials with distinct physical properties due to the extensive bonding network. These giant structures are characteristic of certain covalent compounds, as well as ionic compounds in a crystalline form.

Types of Giant Structures

There are two primary types of giant structures:

1. Giant Covalent Structures

2. Giant Ionic Structures

1. Giant Covalent Structures

These structures consist of atoms connected by covalent bonds in a large, extended network. Each atom forms strong covalent bonds with multiple neighboring atoms, creating a very stable structure. Some classic examples include:

Diamond

• Structure: In diamond, each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This creates a rigid three-dimensional lattice.

• Properties:

  • Extremely hard: Diamond is the hardest known natural material due to its strong, directional covalent bonds.

  • Very high melting and boiling points: It requires a large amount of energy to break the extensive covalent bonds.

  • Does not conduct electricity: In diamond, all electrons are involved in bonding, and there are no free electrons or ions to carry an electrical charge.

  • Transparent and lustrous: The symmetrical arrangement of carbon atoms reflects light in a distinct way, making diamond transparent and giving it a shiny appearance.

Graphite

• Structure: In graphite, each carbon atom is covalently bonded to three others, forming flat layers of hexagonal rings. These layers are held together by weak van der Waals forces, allowing them to slide over each other.

• Properties:

  • Good conductor of electricity: Graphite has one free electron per carbon atom (due to the three bonds), which can move freely along the layers, making graphite a good conductor.

  • Soft and slippery: The weak forces between layers allow them to slide over each other, which is why graphite is used as a lubricant and in pencils.

  • High melting and boiling points: Despite being soft, graphite has strong covalent bonds within each layer, giving it high thermal stability.

Silicon Dioxide (Silica)

• Structure: Silicon dioxide (SiO₂) has a structure where each silicon atom is bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms, forming a three-dimensional network.

• Properties:

  • Hard and brittle: The strong covalent bonds make it a hard material, but it is brittle because of the rigid lattice.

  • High melting and boiling points: Like other giant covalent structures, a large amount of energy is required to break the bonds.

  • Does not conduct electricity: There are no free electrons or ions available for electrical conduction in silicon dioxide.

Silicon (Si)

• Structure: Silicon forms a giant covalent structure similar to diamond, where each silicon atom is bonded to four others in a tetrahedral arrangement.

• Properties:

  • High melting point: Silicon has strong covalent bonds that require a large amount of energy to break.

  • Semi-conductor: Pure silicon does not conduct electricity well, but its conductivity increases when impurities are added (doping). This property makes it important in the electronics industry for use in semiconductors.

2. Giant Ionic Structures

In giant ionic structures, ions are arranged in a large, repeating three-dimensional lattice held together by strong electrostatic forces of attraction between oppositely charged ions (ionic bonds). These structures are typical for ionic compounds, such as sodium chloride (NaCl) and magnesium oxide (MgO).

Sodium Chloride (NaCl)

• Structure: Sodium chloride has a cubic lattice structure where each sodium ion (Na⁺) is surrounded by six chloride ions (Cl⁻), and each chloride ion is surrounded by six sodium ions.

• Properties:

  • High melting and boiling points: The strong electrostatic attraction between the Na⁺ and Cl⁻ ions requires a lot of energy to overcome.

  • Brittle: When stress is applied, ions of like charge may be forced closer together, leading to repulsion and causing the crystal to break.

  • Conducts electricity when molten or in solution: The ions are free to move when the solid is melted or dissolved, allowing the compound to conduct electricity.

  • Soluble in water: The polarity of water molecules weakens the ionic bonds, allowing the ions to dissociate in solution.

Magnesium Oxide (MgO)

• Structure: Magnesium oxide has a similar structure to sodium chloride but with a higher lattice energy due to the 2+ charge on the magnesium ion and the 2− charge on the oxide ion.

• Properties:

  • Extremely high melting and boiling points: The stronger electrostatic attraction due to the doubly charged ions results in very high thermal stability.

  • Hard and brittle: Like other ionic compounds, magnesium oxide is hard but can fracture if enough stress is applied.

Comparison of Giant Covalent and Giant Ionic Structure:

• Bonding: Giant covalent structures involve atoms connected by covalent bonds, whereas giant ionic structures consist of a lattice of ions bonded by electrostatic forces.

• Electrical Conductivity: Giant covalent structures generally do not conduct electricity (except for graphite), while giant ionic structures can conduct electricity when molten or dissolved in water.

• Hardness and Brittleness: Both types of structures tend to be hard, but ionic compounds are more brittle due to the rigid nature of ionic bonding, whereas covalent compounds like graphite can be soft due to weaker intermolecular forces.

• Melting and Boiling Points: Both types of structures have high melting and boiling points, though ionic structures might be higher when comparing similarly sized compounds due to the strength of electrostatic forces.

Summary

• Giant Covalent Structures (e.g., diamond, graphite, silicon dioxide): atoms bonded by strong covalent bonds in an extended network, leading to high melting points, hardness (or softness in the case of graphite), and electrical conductivity in certain cases.

• Giant Ionic Structures (e.g., sodium chloride, magnesium oxide): ions arranged in a repeating lattice, resulting in high melting and boiling points, brittleness, and electrical conductivity when in molten or dissolved states.