Case Study 1: Types of Solids
Case: A solid is a state of matter in which the particles are closely packed together, which results in a definite shape and volume. There are different types of solids, such as crystalline and amorphous solids. Crystalline solids are further categorized based on their structure and symmetry into four types: ionic solids, covalent solids, molecular solids, and metallic solids.
Ionic solids are formed by electrostatic attractions between oppositely charged ions. These solids have a high melting point and are good conductors of electricity in molten or dissolved form. On the other hand, covalent solids are composed of atoms held together by covalent bonds, resulting in very hard and brittle structures with high melting points. Molecular solids consist of molecules held together by intermolecular forces, while metallic solids are made up of metal atoms arranged in a regular pattern, where electrons are free to move, contributing to their electrical conductivity.
Questions:
Which of the following types of solids consists of positively and negatively charged ions arranged in a regular pattern?
Crystalline solids are characterized by:
Which type of solid has atoms bonded through covalent bonds and is very hard and brittle?
Metallic solids are good conductors of electricity due to the presence of:
Case Study 2: Packing in Solids
Case: The arrangement of particles in a solid determines its properties, such as density, stability, and melting point. In crystalline solids, the particles are packed in a regular and orderly manner, forming a lattice structure. The most efficient packing arrangement is the closest packing, which allows the particles to be as close together as possible. There are two types of closest packing: hexagonal closest packing (hcp) and cubic closest packing (ccp).
In the hcp arrangement, each particle is surrounded by 12 others, forming a hexagonal pattern. In the ccp arrangement, the particles are arranged in a face-centered cubic (fcc) lattice. These arrangements minimize the void space between the particles, which makes the solid denser and more stable.
Questions:
Which packing arrangement in solids results in the closest packing of particles?
What is the characteristic feature of cubic closest packing (ccp)?
The efficiency of packing in hexagonal closest packing (hcp) is:
Which of the following solids has the highest packing efficiency?
Case Study 3: Defects in Solids
Case: Defects in solids occur due to the presence of irregularities in the arrangement of particles in the crystal lattice. These defects can be classified into three main types: point defects, line defects, and plane defects.
Point defects occur when there is a vacancy or an impurity in the crystal lattice. These can be further divided into vacancy defects (missing particles), interstitial defects (extra particles placed in the spaces between particles), and substitutional defects (atoms of different elements replace some of the original atoms). Line defects involve the misalignment of rows of atoms, and plane defects involve the misalignment of entire planes of atoms.
Defects in solids can affect their properties such as electrical conductivity, strength, and optical properties.
Questions:
Which type of defect in solids involves the missing particles in the crystal lattice?
Substitutional defects occur when:
Line defects in crystals result in:
Plane defects in solids refer to:
Case Study 4: Electrical Properties of Solids
Case: The electrical conductivity of solids depends on the availability of free electrons or ions. In metallic solids, free electrons are present due to the delocalization of electrons in the metal's crystal lattice, which allows them to conduct electricity. Ionic solids, however, do not conduct electricity in the solid state because the ions are fixed in place, but they do conduct electricity when molten or dissolved in water, as the ions are free to move.
In contrast, covalent solids and molecular solids are poor conductors of electricity because they lack free-moving electrons or ions. The electrical conductivity in solids is thus influenced by the type of bonding and the ability of the particles to move within the lattice.
Questions:
Which type of solid is a good conductor of electricity due to the presence of free electrons?
Ionic solids conduct electricity in the:
Which of the following solids are poor conductors of electricity due to the lack of free-moving electrons?
The electrical conductivity of metallic solids is primarily due to:
Case Study 5: Packing Efficiency and Unit Cell
Case: The concept of unit cells is crucial in understanding the properties of crystalline solids. A unit cell is the smallest repeating unit in the crystal lattice, which, when repeated, forms the entire crystal. The packing efficiency of a unit cell is defined as the fraction of the volume of the unit cell that is occupied by particles.
In a cubic unit cell, the packing efficiency varies depending on the arrangement of particles. For example, in a simple cubic unit cell, the packing efficiency is about 52%. In a body-centered cubic (bcc) unit cell, the packing efficiency is higher, around 68%, while in a face-centered cubic (fcc) unit cell, the packing efficiency reaches about 74%, the highest of all unit cell types.
Questions:
What is the packing efficiency of a simple cubic unit cell?
Which type of unit cell has the highest packing efficiency?
The packing efficiency of a body-centered cubic (bcc) unit cell is approximately:
A unit cell that is repeated in space to form a crystal structure is called a: