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Questions and Answers
What is the glass transition temperature Tg associated with?
What is the glass transition temperature Tg associated with?
Which statement accurately describes the arrangement of atoms in crystalline solids?
Which statement accurately describes the arrangement of atoms in crystalline solids?
Which characteristic does NOT apply to amorphous solids compared to crystalline solids?
Which characteristic does NOT apply to amorphous solids compared to crystalline solids?
Which of the following best describes the internal energy of a crystalline solid?
Which of the following best describes the internal energy of a crystalline solid?
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What type of forces hold together molecular solids?
What type of forces hold together molecular solids?
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How many total atoms are present in a simple cubic unit cell?
How many total atoms are present in a simple cubic unit cell?
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In a body centered cubic (bcc) structure, how many atoms are found in one unit cell?
In a body centered cubic (bcc) structure, how many atoms are found in one unit cell?
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What is the total number of atoms in a face centered cubic (fcc) unit cell?
What is the total number of atoms in a face centered cubic (fcc) unit cell?
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Which of the following metals commonly crystallizes in the face centered cubic system?
Which of the following metals commonly crystallizes in the face centered cubic system?
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In a simple hexagonal unit cell, what is the value of atoms located at the corners?
In a simple hexagonal unit cell, what is the value of atoms located at the corners?
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How many total atoms are present in a closed packed hexagonal (hcp) structure?
How many total atoms are present in a closed packed hexagonal (hcp) structure?
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What defines the closed packed hexagonal (hcp) arrangement in comparison to the simple hexagonal system?
What defines the closed packed hexagonal (hcp) arrangement in comparison to the simple hexagonal system?
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What does the term 'atomic packing factor' refer to in crystal structures?
What does the term 'atomic packing factor' refer to in crystal structures?
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What primarily causes the formation of dipole bonds in molecules?
What primarily causes the formation of dipole bonds in molecules?
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What defines the equilibrium inter-atomic distance (I.A.D.) in a bonding context?
What defines the equilibrium inter-atomic distance (I.A.D.) in a bonding context?
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Which states of matter have the largest interatomic distance?
Which states of matter have the largest interatomic distance?
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What type of bonding occurs in molecular solids?
What type of bonding occurs in molecular solids?
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What characterizes crystalline solids?
What characterizes crystalline solids?
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Which of the following describes the arrangement of atoms in liquids?
Which of the following describes the arrangement of atoms in liquids?
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What type of forces primarily prevent atomic overlap in solids?
What type of forces primarily prevent atomic overlap in solids?
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What is a common feature of Van der Waals forces?
What is a common feature of Van der Waals forces?
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What primarily determines the physical and chemical properties of any material?
What primarily determines the physical and chemical properties of any material?
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What is the charge of neutrons in an atom?
What is the charge of neutrons in an atom?
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Which type of bond arises from the sharing of electrons between two atoms?
Which type of bond arises from the sharing of electrons between two atoms?
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What must be true about the number of protons and electrons in a neutral atom?
What must be true about the number of protons and electrons in a neutral atom?
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What aspect of atomic weight affects a material the least?
What aspect of atomic weight affects a material the least?
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What is a negatively charged ion formed by an atom receiving extra electrons called?
What is a negatively charged ion formed by an atom receiving extra electrons called?
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Which type of bonding does NOT involve the sharing of electrons?
Which type of bonding does NOT involve the sharing of electrons?
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What is the outermost shell of an atom where valence electrons are located called?
What is the outermost shell of an atom where valence electrons are located called?
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What is the primary distinction between allotropy and isomerism?
What is the primary distinction between allotropy and isomerism?
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Which of the following is NOT an allotropic form of silica (SiO2)?
Which of the following is NOT an allotropic form of silica (SiO2)?
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What characterizes reconstructive transformation in silica allotropes?
What characterizes reconstructive transformation in silica allotropes?
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At what temperature does low cristobalite transform into high cristobalite?
At what temperature does low cristobalite transform into high cristobalite?
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Which characteristic is NOT associated with displacive transformation?
Which characteristic is NOT associated with displacive transformation?
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Why are amorphous solids described as 'without shape'?
Why are amorphous solids described as 'without shape'?
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Which statement is true regarding the density of different allotropes of silica?
Which statement is true regarding the density of different allotropes of silica?
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What is one of the main features of amorphous solids compared to crystalline solids?
What is one of the main features of amorphous solids compared to crystalline solids?
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Study Notes
Crystalline Solids
- The atoms in many solids are regularly arranged in a three-dimensional pattern, called a crystal lattice
- This pattern repeats itself
- Different polymorphic forms may be called allotropic forms
- The term "allotropy" refers to polymorphism in inorganic materials, while "isomerism" refers to the phenomena in organic materials
- Silica (SiO2) is an important example of allotropy in dentistry
- It exists in nature in four different allotropic forms:
- α- Quartz (Hexagonal)
- β- Tridymite (Rhombohedral)
- γ- Crystalobailite (Cubic)
- δ- Fused quarts (Amorphous)
- Each form has different physical properties (e.g. density), but all are chemically SiO2.
- On heating the four forms, 2 types of transformation take place:
- Reconstructive transformation
- Displacive transformation
Reconstructive Transformation
- Involves bond breakage
- Needs more time
- Occurs more slowly
- Example:
- Quartz → Tridymite → Crystobalite → Fused quartz at increasing temperatures
Displacive Transformation
- No bond breakage
- Occurs at a specific temperature
- Transformation is rapid
- Accompanied by expansion
- Example:
- Low quartz → High quartz (α → β) at 573°C
Amorphous Solids
- Molecules are randomly distributed without regularity or repetition
- No specific form or shape
- Example: Glasses
- There is tendency for short-range order arrangement
- Don't have a definite melting temperature
- Gradually soften upon heating and harden upon cooling
- The temperature at which they first form a rigid mass upon cooling is called glass transition temperature (Tg)
- Tend to have higher internal energy than crystalline solids
- Less stable structure
Atomic Solids
- Examples: Diamond
- Held together by strong covalent bonds
Molecular Solids
- Examples: Polymers
- Held together by weak van der Waals forces
- Van der Waals forces control their properties
Unit Cell
-
Simple Cubic
- Contains one metal atom per unit cell
- 8 atoms x 1/8 at each corner = one atom
-
Body Centered Cubic (bcc)
- Contains two atoms per unit cell
- 8 atoms at each corner x 1/8 + one atom in the center = 2 atoms
-
Face Centered Cubic (fcc)
- Contains four atoms per unit cell
- (8 x 1/8) + (6 x 1/2) = 4 atoms
- More common among metals (e.g. Gold (Au), Silver (Ag), Copper (Cu), Platinum (Pt))
Hexagonal Crystals
-
Simple Hexagonal
- a = b ≠ C with γ = 120° and α = β = 90°
- Atoms can be imagined at the corners with one atom at the upper face and another at the lower face
- 1/6 atom at each corner + 1/2 atom at each face = 3 atoms per unit cell
-
Closed Packed Hexagonal (hcp)
- Simple hexagonal system with three unshared atoms at the same plane in the center of the hexagonal system
- 2 (1/6 x 6) + (1/2 x 2) + 3 atoms = 6 atoms per unit cell
- Example: Zinc (Zn)
Structure of Atoms
- Basic unit of the structure of any material
- Consists of:
- Central positive nucleus (composed of protons and neutrons)
- Negatively charged electrons orbiting the nucleus in definite orbits (energy levels or shells)
Valence Electrons
- Electrons in the outermost shell
- Determine most of the physical and chemical reactivity of the element
Atomic Number
- Number of protons in any atom
- Must equal the number of electrons
- Represents the atomic number of the element
### Atomic Weight
- Nearly proportional to the weight of protons and neutrons in the nucleus
- Influences the density and specific heat
- Very little influence on mechanical properties
Types of Bonds
- Inter-atomic bonds (also called primary bonds)
- Inter-molecular bonds (also called secondary bonds)
Inter-atomic Bonds (Primary Bonds)
- Strong bonds that hold atoms together within a molecule
- Occur due to the involvement of valence electrons
- Types of primary bonds:
- Covalent bond
- Ionic bond
- Metallic bond
Covalent Bond
- Atoms share electrons to achieve a stable configuration
- Occurs between two nonmetals
Inter-molecular Bonds (Secondary Bonds)
- Weaker bonds that hold molecules together
- Types of secondary bonds:
- Dipole bonds (Van der Waals forces)
- Hydrogen bonds
Dipole Bonds (Van der Waals forces)
- Result from electric imbalances within a molecule
- Molecules have slight positive and negative charges leading to attractions between them
- Weaker than primary bonds
Hydrogen Bonds
- A type of dipole bond
- Occur between molecules containing hydrogen and a highly electronegative atom (like oxygen or nitrogen)
- Can occur between water molecules
Inter-atomic Distance (I.A.D)
- The distance between atoms
- Results from the balance of:
- Inter-atomic repulsive force (caused by electrostatic fields)
- Inter-atomic attractive force (caused by bonding)
- Equilibrium distance is where these two forces are equal
- Influences bond energy and bonding
States of Matter
- Matter can be classified into:
- Gases - high energy, largest interatomic distance, free movement
- Liquids - less energy, less interatomic distance, short-range order
- Solids - lowest energy, least interatomic distance, long-range order
Why Properties of Materials Depend on Atomic Arrangement
- Types of bonds between atoms and molecules
- Inter-atomic distance
- Manner of arrangement of atoms
- Atomic packing
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Description
This quiz explores the structure and properties of crystalline solids, focusing on the concept of allotropy and its significance in materials like silica. It covers the different allotropic forms of silica and the types of transformations that occur upon heating these forms. Test your understanding of crystal lattices, polymorphism, and reconstructive transformations.
