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Questions and Answers
What is primarily responsible for the efficient conduction of heat through metals?
Which type of alloy involves the substitution of host metal atoms with other metal atoms of similar size?
Which property is characteristic of network solids?
What is a significant characteristic of diamond's structure according to the localized electron model?
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Which of the following statements about graphite is true?
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What are the components of a heterogeneous alloy?
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Which crystal structure does diamond adopt according to molecular orbital theory?
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What type of bonding is primarily responsible for the structure of network solids?
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What type of bonding is primarily responsible for the stability of graphite?
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Which characteristic distinguishes p-type semiconductors from n-type semiconductors?
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What type of hybridization is present in the carbon atoms of graphite?
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Which components in molecular solids contribute to their weak intermolecular forces?
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Which Group IVA element is known as an inorganic semiconductor?
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What type of structural arrangement is depicted in the localized electron model for graphite?
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What condition must be met for a material to be classified as an insulator in terms of band gaps?
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What is the primary role of delocalized electrons in graphite?
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What type of bonding characterizes metallic solids?
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Which characteristic is NOT associated with covalent network solids?
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Which of the following best describes the structure of diamond?
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What structure do graphite layers consist of?
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Which type of solid consists of noble gas elements bonded through London dispersion forces?
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What is the main characteristic of atomic solids?
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Which property is associated with metals due to their closest packing arrangement?
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When identifying an alloy, which classification would NOT apply?
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Study Notes
Mobile Electrons in Metals
- Mobile electrons are responsible for efficient heat conduction in metals.
Metal Alloys
- Alloys are substances that contain a mixture of elements and exhibit metallic properties.
- Substitutional alloys occur when some host metal atoms are replaced by atoms of similar size. (e.g., Brass - Cu is replaced by Zn, Sterling silver - 93% Ag, 7% Cu).
- Interstitial alloys form when small atoms occupy the holes within the closely packed metal structure. (e.g., Steel - C atoms in Fe crystal).
- Heterogeneous alloys have components that are not evenly dispersed.
Network Solids
- Network solids are atomic solids with directional covalent bonds that form giant molecules.
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Properties:
- Brittle
- Inefficient conductors of heat and electricity
- Hard
- Colorless
- Insulators
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Key elements:
- Carbon - Examples include Diamond and Graphite.
- Silicon
Diamond
- The hardest naturally occurring substance.
- Each carbon atom is surrounded by a tetrahedral arrangement of other carbon atoms.
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Localized electron model:
- Stable structure via covalent bonds.
- Formed by sp3 hybridized carbon atomic orbitals.
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Molecular orbital theory:
- Large gaps between filled and empty levels.
- Electron transfer is difficult.
Graphite
- Slippery, black, and conductive of heat and electricity.
- Structure is based on layers of carbon atoms arranged in fused six-membered rings.
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Localized electron model:
- Trigonal planar arrangement.
- 120-degree bond angles.
- sp2 hybridization - three sp2 orbitals on each carbon atom form σ bonds with three other carbon atoms. One unhybridized 2p orbital remains perpendicular to the plane.
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Molecular orbital theory:
- All orbitals combine to form π MOs which contribute to graphite's stability.
- Delocalized electrons enable high electrical conductivity.
Semiconductors
- Gap exists between occupied MOs (valence band) and unoccupied ones (conduction band).
- Electrons must enter the conduction band for electron transfer.
- Group IVA elements have band gaps ranging from 0.08 to 3.05 eV (7 to 300 kJ/mol).
- Band gaps exceeding 3.5 eV result in an insulator.
Semiconductors (contd.)
- Group IVA elements are semiconductors due to their 4 valence electrons.
- Inorganic semiconductors (like GaAs) tend to have an average of 4 valence electrons.
Doping in Semiconductors
- Doping manipulates the conductivity of semiconductors by introducing elements with different numbers of valence electrons.
- n-type semiconductor: Conductivity is increased by doping with atoms that have more valence electrons than the host crystal. n-type have more electrons, allowing negative charge to travel in the conductance band.
- p-type semiconductor: Conductivity is increased by doping with atoms having fewer valence electrons than the host crystal. p-type have fewer electrons, allowing “holes” to travel in the valence band.
Molecular Solids
- Exhibit strong covalent bonding within molecules but weak forces between molecules.
- Forces between molecules depend on the nature of the molecules.
- CO2, I2, P4 and S8: No dipole moment; they possess London dispersion forces.
Diffraction of X-rays
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Bragg's Law: nλ = 2d sin θ
- n: Integer
- λ: Wavelength of X-rays
- d: Distance between atoms
- θ: Angle of incidence and reflection
- Diffraction occurs due to:
- Constructive interference when parallel beam waves are in phase.
- Destructive interference when waves are out of phase.
- The distance traveled by waves depends on the distance between atoms.
- A diffractometer is used for X-ray analysis of crystals.
Example of X-ray Diffraction
- X-rays with a wavelength of 1.54 Å were used to analyze an aluminum crystal.
- A reflection was observed at θ = 19.3 degrees.
- Assuming n = 1, the distance between the atomic planes (d) can be calculated using Bragg's Law.
Classification of Solids
- Ionic solids: Ions occupy the lattice points defining their structure.
- Molecular solids: Discrete covalently bonded molecules exist at each lattice point.
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Atomic solids: Atoms occupy the lattice defining its structure.
- Metallic solids: Delocalized, nondirectional covalent bonding.
- Covalent network solids: Atoms are bonded to each other by strong, directional covalent bonds.
- Group 8A (18) solids: Noble gases, attracted by London dispersion forces.
Structures of Metals
- Closest packing assumes metal atoms are hard, uniform spheres.
- Spheres are packed in layers, with each successive layer occupying dimples formed by the previous layer.
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Description
This quiz covers the key concepts related to mobile electrons in metals, the formation and types of metal alloys, and the characteristics of network solids. Learn about the properties of materials such as diamond and how these properties influence their applications. Test your understanding of these fundamental concepts in materials science.
