Transition Metals and Energy Bands

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Questions and Answers

What is the significance of energy band overlap in metals?

It influences the thermal expansion properties.

It determines the color of the metals.

It allows for the prediction of electrical conductivity. (correct)

It controls the solubility of metals in water.

Which model provides a better explanation for the physical properties of transition metals?

Electron-sea model

Crystal field theory

Molecular-orbital model (correct)

Valence shell electron pair repulsion model

How does the molecular-orbital model describe the bonding in transition metals as valence electrons increase?

Bonding remains constant.

Bonding initially strengthens then weakens. (correct)

Bonding becomes weaker.

Bonding becomes completely non-existing.

What dictates the order in which electrons occupy energy levels in a material?

Aufbau principle Signup and view all the answers

Why are alkali metals expected to have half-filled s-bands?

Because they have a single valence electron. Signup and view all the answers

What occurs when electrons are within a filled band concerning electrical conductivity?

They cannot move to conduct electricity. Signup and view all the answers

Which of these statements about the properties of transition metals is true?

They exhibit strong bonding as bonding orbitals fill. Signup and view all the answers

What is one effect of filling the antibonding orbitals in transition metals?

Makes bonds weaker. Signup and view all the answers

What property of the 3d band explains its narrower energy range compared to the 4s and 4p bands?

The orbitals are smaller and overlap less effectively. Signup and view all the answers

Which of the following statements best describes the contribution of partially filled energy bands to metallic properties?

They allow electrons to be easily promoted to higher energy levels. Signup and view all the answers

According to the molecular-orbital model, how many electrons can the 4p band hold?

6 electrons Signup and view all the answers

What process occurs when electrons in a metal are excited by thermal energy?

They move into higher-energy orbitals and free themselves to conduct electricity. Signup and view all the answers

What characteristic is primarily responsible for the unique conductivity of transition metals?

Their partially filled d orbitals. Signup and view all the answers

Which of the following describes a significant outcome of the Pauli exclusion principle regarding electron capacity in orbitals?

It restricts the number of electrons in a given orbital to two. Signup and view all the answers

What is the primary reason for the large span of energy in the 4s and 4p bands compared to the 3d band?

The overlap with neighboring atoms is more effective in the 4s and 4p bands. Signup and view all the answers

Which model helps to understand the delocalization of electrons in metals, contributing to their conductivity?

Electron-sea model Signup and view all the answers

What occurs as the chain length approaches infinity regarding energy states?

Allowed energy states transform into a continuous band. Signup and view all the answers

Which statement best describes the electronic structure of a bulk solid?

It typically features a series of bands corresponding to different atomic orbitals. Signup and view all the answers

How are the 4s, 4p, and 3d orbitals treated in the context of metals like nickel?

Each orbital contributes to an independent band of molecular orbitals. Signup and view all the answers

What is the implication of having many overlapping bands in metal electronic structures?

They complicate the understanding of conductivity mechanisms. Signup and view all the answers

What does the band structure for transition metals, such as nickel, indicate about their electronic configuration?

Their electron configurations influence the formation of multiple energy bands. Signup and view all the answers

What happens to the energy separation between molecular orbitals when a chain is very long?

The energy separation becomes very small. Signup and view all the answers

What is the significance of continuous bands in the context of solids observed under optical microscopes?

They arise as a result of the crystal's large number of atoms. Signup and view all the answers

Why is the electronic structure of most metals considered complicated?

It requires the consideration of multiple types of atomic orbitals. Signup and view all the answers

Study Notes

Chemistry of Engineering Materials - Metals

Metals are held together by a sea of delocalized valence electrons.
This allows them to conduct electricity and be relatively strong without being brittle.
Metallic solids consist entirely of metal atoms.

Alloys

An alloy is a material containing more than one element with the characteristic properties of a metal.
Alloying metals is vital for modifying the properties of pure metallic elements.
Common Uses often involve alloy compositions.
- Example: Bronze (copper and tin) and brass (copper and zinc)
- Pure gold is too soft for jewelry but alloys of gold are harder.
Nitinol (Nickel-Titanium): A 1:1 nickel-titanium alloy with 'shape memory'. Discovered accidentally during a 1961 meeting where a folded wire was heated by a lighter to regain its original shape.

Four Categories of Alloys

Substitutional Alloys: Solute atoms occupy positions normally occupied by solvent atoms. Components have similar atomic radii and chemical-bonding characteristics, like silver and gold alloys.
Interstitial Alloys: Solute atoms occupy interstitial positions in the holes between solvent atoms (e.g., carbon in steel). Solute atoms have much smaller bonding atomic radii than the solvent atoms and these strengthen the lattice structure, and make them harder but less ductile compared to substituted alloys.
Heterogeneous Alloys: Components are not uniformly distributed e.g., pearlite in iron. Properties depend on the composition and the method of formation (rapid or slow cooling).
Intermetallic Compounds: Compounds rather than mixtures. Have definite properties and composition (e.g., Ni3Al). These are often formed with ordered distribution of atoms and generally have better structural stability and higher melting points than constituent metals. However, they are often more brittle.

Types of Steels

Carbon Steels: Contain trace amounts of elements besides carbon and iron. Account for around 90% of steel production. Sub-divided based on carbon percentage
- Low Carbon Steel (Mild Steel) up to 0.3% carbon, used in cables, nails, and chains.
- Medium Carbon Steels(0.3-0.6% carbon), tougher than mild steels and used for girders and rails.
- High Carbon Steel (0.6-1.5% carbon), used in cutlery, tools, and springs.
Alloy Steels: Steels that have elements other than carbon and iron, providing specific properties.
- Examples: Stainless steel (chromium, nickel) used for its high corrosion resistance; Tool steels (tungsten/molybdenum/cobalt) with high hardness and heat resistance suitable for tools.

Types of Stainless Steel

Contains 10%-20% chromium as an alloying element, and other components like Nickel, silicon, and manganese.
Widely used in construction, medical equipment, piping and food processing equipment because of their high corrosion resistance and weather stability.
Example types of stainless steel: 304 Stainless steel.

Types of Tool Steels

Tool steels are notable for their high heat resistance qualities, making them useful for cutting and drilling equipment or environments that require such qualities.
Contains metals such as tungsten/molybdenum/cobalt leading to durability and high hardness.

Heterogeneous Alloys

Components are not uniformly distributed, examples include pearlite in iron.
Properties depend on composition and formation methods (e.g., rapid or slow cooling.).

Intermetallic Compounds

Compounds, not mixtures, with definite compositions and properties.
Atoms are ordered rather than randomly distributed in the structure.
Advantages: Improved structural stability, higher melting points.
Disadvantages: Often more brittle than substitutional alloys.
- Common types include Ni3Al and Nb3Sn (superconductor)
Examples of compounds include AuAl2

Alloys of Gold

Pure gold is termed 24 karat.
Karat number decreases with decreasing gold percentage.
Common alloys: 14 karat (58% gold), 18 karat (75% gold.)
Alloying with other metals (e.g., silver, copper) alters color and properties such as strength and hardness.

Metallic Solids

Held together by a sea of delocalized valence electrons.
This allows them to conduct electricity and be relatively strong without being brittle.
Consist entirely of metal atoms.

Metallic Properties

Metals have a characteristic luster (shine).
They have high thermal conductivity and electrical conductivity (e.g., silver, copper).
They are often malleable (can be hammered into thin sheets) and ductile (can be drawn into wires).

Models of Metallic Bonding

Electron Sea Model: Valence electrons are delocalized and move freely throughout the solid (relatively simple, good for explaining malleability and ductility)
Molecular Orbital Model (Band Theory): Atomic orbitals combine to form molecular orbitals that extend over the entire molecule. Electrons occupy energy bands, and the behavior of these bands (how they are filled and the energy gaps between them) determines the material's properties.

Semiconductors

Intermediate between metals and insulators.
Have a small energy gap between the valence band and conduction band.
Electrical conductivity increases with temperature.
Examples: Silicon, germanium, certain compound semiconductors.
Doping can enhance conductivity.
Understanding doping allows for control over conductivity.
n-type doping (adding electron donors) increases negative charge carriers
p-type doping (adding electron acceptors) increases positive charge carriers.

Artificial Lighting (LEDs)

LEDs are semiconductor devices.
They convert electrical energy into light.
The color of light emitted by an LED depends on the specific semiconductor material band gap.

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Description

Explore the significance of energy band overlap and the molecular-orbital model in understanding the properties of transition metals. This quiz covers key concepts such as electron occupancy, conductivity, and the effects of bonding in transition metals.