Structure of the Atom

Structure of the Atom

Outline

• Structure of the Atom

• Atomic Bonding

  • Metallic Bond

  • Covalent Bond

  • Ionic Bond

  • Van Der Waals Bonding

  • Mixed Bonding

  • Binding Energy and Interatomic Spacing

References

• D.R. Askeland, and W.J. Wendelin, “The Science and Engineering of Materials,” 7th SI ed., Cengage Learning, 2015.

• W.D. Callister and D.G. Rethwisch, “Materials Science and Engineering: An Introduction,” 8th Edition, John Wiley & Sons, Inc., 2010.

Key Terms

• Atomic Number: The number of protons in an atom.

• Atomic Mass: The mass of one mole of atoms.

• Avogadro Constant: The number of atoms in a mole: 6.022 imes 10^{23}.

• Valence: The number of electrons in an atom that participate in bonding.

• Electronegativity: The relative tendency of an atom to accept an electron and become an anion.

• Anion: A negatively charged ion.

• Cation: A positively charged ion.

Structure of the Atom

• Atoms consist of a nucleus surrounded by electrons.

  • Nucleus Composition: Contains protons and neutrons, accounting for the majority of the atom's mass.

  • Valence Electrons: Determine the chemical properties and bonding characteristics of the atom.

Quantum Physics and Bohr Model

• In quantum physics, the energy of an electron is described by four quantum numbers.

• The Bohr Model represents atoms as solid spheres:

  • Electron Behavior: Orbital electrons revolve around the nucleus, and their energy levels are quantized. Electrons can change energy states by jumping to different shells, requiring absorption or emission of energy.

Electronic Structure - Quantum Numbers

• Electrons exhibit both wavelike and particulate properties, being located in orbitals defined by probability.

• Quantum Numbers:

  • Principal Quantum Number (n): Indicates the energy level or shell (e.g., K, L, M, N, O corresponds to 1, 2, 3, etc.).

  • Subsidiary Quantum Number (l): Indicates the type of orbital (e.g., s, p, d, f) with associated values (0, 1, 2, 3,…, n-1).

  • Magnetic Quantum Number (m_l): Specifies the orientation of an orbital (-l to +l).

  • Spin Quantum Number (m_s): Indicates the spin of the electron (1/2 or -1/2).

Atomic Bonding

Types of Atomic Bonds

1. Metallic Bonds (characterized by the 'sea of electrons').

2. Covalent Bonds (electron sharing).

3. Ionic Bonds (electron transfer).

4. Van der Waals Bonds (weak charge attraction).

   • Primary Bonds: Typically stronger bonds (Metallic, Covalent, Ionic).

   • Secondary Bonds: Weaker bonds (Van der Waals).

Metallic Bonds

• Composed of positively charged atom cores surrounded by a 'sea' of delocalized electrons.

• The valence electrons are shared among multiple metal cores, leading to a strong bond characterized by mutual attraction.

• Properties: High electrical and thermal conductivity, high ductility, and high melting temperatures.

Covalent Bonds

• Formed by the sharing of valence electrons between atoms with similar electronegativities.

• Characteristics:

  • Strong in nature with fixed directional bonding relationships.

  • Limited ductility and poor electrical conductivity.

  • Commonly found in ceramics and polymers.

Ionic Bonds

• Form when one atom donates a valence electron to another:

  • The donating atom (metal) becomes a cation (positive).

  • The receiving atom (non-metal) becomes an anion (negative).

• The resulting attraction between oppositely charged ions forms a strong ionic bond.

• Examples: NaCl, MgO, CaF2.

• Predominantly found in ceramics.

Mixed Bonding

• Materials may exhibit a combination of different bonding types.

  • Example: Intermetallic compounds (e.g., Al2Cu) may show both covalent and metallic characteristics due to differing electronegativities.

  • Ceramics and semiconductors can contain a mixture of covalent and ionic bonds.

Ionic-Covalent Mixed Bonding

• The percentage of ionic character can be calculated using: ext{% ionic character} = (1 - e^{- rac{(XA - XB)^2}{4}}) imes 100

  • Where XA and XB are the Pauling electronegativities of the two elements.

• Example Calculation: For MgO with X{Mg} = 1.2 and XO = 3.5,
  ext{Result} = 73.4 ext{% ionic character}.

Van der Waals Bonds (Secondary Bonding)

• Occur between polarized molecules, where uneven distribution of electrons creates dipoles.

• Weak attractions between these dipoles establish Van der Waals bonds.

• Crucial in dictating strength and ductility of polymers.

Van der Waals Forces in Water

• Permanent dipoles are present, leading to significantly higher boiling temperatures in water compared to non-polar molecules (e.g., O2, H2).

• Example: Water's high boiling point is attributed to its permanent dipoles compared to other gases.

Van der Waals Forces in Polymers

• In polymers like polyvinyl chloride (PVC), chlorine atoms carry a negative charge while hydrogen atoms are positive, leading to weak Van der Waals bonds between polymer chains, imparting stiffness. Upon applying force, these bonds can break, enabling chains to slide.

Material Types and Ductility Rank

• Rank the materials from highest to lowest ductility:
  A. Metals
  B. Ceramics
  C. Polymers

Summary of Bond Energies

• Bond Type | Energy (kcal/mol) | Comments

  • Ionic: 150-370, Nondirectional (ceramics), high Tm, high E, low α

  • Metallic: Variable 25-200, Nondirectional (metals), moderate Tm, moderate E, moderate α

  • Van der Waals: Smallest, Directional inter-chain (polymer), intermolecular, low Tm, low E, high α

  • Covalent: Variable large (e.g., diamond) small (e.g., Bismuth), Directional, high Tm, high E, low α

Properties of an Unknown Substance

• An unknown substance with characteristics:

  1. High melting point (>1000°C)

  2. Small thermal expansion coefficient

  3. Very high stiffness

• Potential bonding might likely include:
  A. Metallic
  B. Ionic
  C. Covalent
  D. Van der Waals

# Binding Energy and Interatomic Distance

• Atoms or ions separated by equilibrium spacing correspond to minimum interatomic energy, where no net force is acting to attract or repel the atoms.

Binding Energies for Bonding Mechanisms

Atomic Bonding and Properties

• Bonding Implications:

  • Atomic bonding significantly influences heat conductivity, elastic modulus, melting temperature, and material strength.

• Elastic Modulus: High stiffness correlates with a bond that requires significant force for small separation distances.

Example Properties of Metals

• Sketch interatomic energy versus distance curves based on these properties.

Carbon Allotropes

• Carbon exists in several allotropes, which are pure forms yet exhibit dramatically different properties:

  • Diamond: Exhibits strong covalent bonds (with 4 other carbon atoms), a stiffness of 1100 GPa, and high thermal conductivity (2000 W/mK). It has no free electrons, resulting in poor electrical conductivity.

  • Graphite: Contains layers of carbon with covalent bonding within layers and Van der Waals bonding between layers, providing good electrical and thermal conductivity due to free-moving electrons.

• Carbon Nanotubes: Considered rolled sheets of graphene and combine properties of both diamond and graphite.

Quiz - Dominant Bonding Type

• Identify the dominant bonding in the following substances:

  • Fe:

  • H2O-H2O:

  • CaCl2:

  • H₂-H₂:

  • SiC:

  • Liquid Al:

  • Si:

Key Chapter Concepts

• Primary bonds:

  • Metallic

  • Covalent

  • Ionic

• Secondary bonds:

  • Van der Waals bonds

• Concepts:

  • Binding energy

  • Stronger bonds correlate to stronger materials

  • Relationships between elastic modulus, melting temperature, strength, and coefficient of thermal expansion related to atomic bonding.