L02 Atomic bonding

Types of Atomic Bonding

• Valence Electrons:

  • Outermost shell electrons involved in bonding between atoms

  • Achieve stable configurations (outermost shell is full) by:

    • Giving electrons when shell is < half-full.

    • Receiving electrons when shell is > half-full.

    • Sharing electrons when shell is ~ half-full.

• Primary Bonds:

  • 3 types that form between pairs of atoms

  • Ionic, Covalent, and Metallic bonds

• Secondary Bonds:

  • Weaker than primary so easier to break.

Strong Atomic Bonds

• Ionic Bonding:

  • ionic compounds or salts

  • Formed between metal and non-metal ions (e.g., NaCl).

  • Involves the exchange of electrons between atoms

    

• Covalent Bonding:

  • Found in molecules and covalent network solids (Non-metals ions: CH4, H2O).

  • sharing of electrons among atoms

    

• Metallic Bonding:

  • Present in metals and alloys.

  • Described as a 'sea' of electrons around positively charged metal ions.

    

Ionic Bonding

• Definition: Formed between cations (positive ions) and anions (negative ions).

• cations = atoms that have lost valence electrons to become positively charged ions e.g. Al³⁺

• anions = atoms that have gained valence electrons to become negatively charged ions e.g. O^2-

• Charge neutrality is maintained:

  • Example: 2Al³⁺ + 3O^2- → Al₂O₃.#

• Ionically bonded materials:

  • bonds = strong and dispersed

  • strength of bond depends on difference in electronegativity between two elements

  • strength of bond increases with charge on ion.

Covalent Bonding

• Definition: Bonds between non-metal atoms, common in ceramics and polymers.

• Electron Sharing:

  • Electrons shared between two bonded atoms often leading to unequal sharing and partial ionic character.

  • this occurs unless 2 elements on either side of bond = identical, this will form a diamond

• bonds in covalently bonded materials:

  • very strong

  • localised

  • mutually repelling

Understanding Metallic Bonding

• Occurrence: mostly metals.

• Properties: Typically have 1-3 (small number) of electrons on the outermost shell

• Model: Valence electrons form a 'sea' of electrons

  

Weak Atomic Bonds

• Types: Van der Waals bonds and Hydrogen bonds.

• Van der Waals force: interaction or repulsion energy between molecules

• Hydrogen bonds: between hydrogen atoms

• Hydrogen bonds and van der Waals force normally found in gas molecules, polymers

• Characteristics:

  • Weaker than covalent bonds; can become inactive at higher temperatures.

Mechanism of Weak Bonds

• Formation: Random fluctuations cause assymetry in electron distribution, creating dipoles. this creates dipoles in neighbouring atoms

• Influence: Positive end of one dipole attracts the negative end of another = Van der Waals bonding.

• interaction occurs when atoms are within 5nm of each other, creating slight polarity & attraction

Hydrogen Bonds

• When hydrogen bonds to another atom, it shares its single valence electron, leaving behind a positive charge on hydrogen atom, which is unscreened as theres no other electrons

• This permanent dipole form bonds with other atoms' negative ends.

Crystalline vs. Amorphous Materials

• Crystalline Materials: Long-range ordered atomic structure.

  

• Types of Materials:

  • Metals and ceramics can be fully crystalline.

  • Polymers = never entirely crystalline; they are amorphous or glassy.

    

3 Principle Crystal Structures for Metals

• Types of Structures:

  • Body-centered cubic (BCC).

    

  • Face-centered cubic (FCC).

    

  • Hexagonal close packed (HCP).

    

Limits of Theoretical Properties

• Observation: Material properties often differ from theoretical predictions due to:

  • Variability in atom-atom interactions.

  • Imperfections in materials.

    

Types of Defects in Metals

• Defects include:

  1. Defects in atomic stacking.

  2. Grains and grain boundaries.

  3. Missing grains (pores).

     

• Dislocation Movement: Affects the physical properties of metals.

  

Characteristics of Ceramics

• ceramics Predominantly covalently or ionically bonded materials. With few exception (e.g. diamond)

• ceramics are compounds made of 2 or more atom types (e.g., Al2O3).

• covalent materials = bonds highly directional and mutually repelling, leading to open structures

• ionically bonded = close packed, with as many anions surrounding a central cation and viceversa to try to maximise electrostatic attraction and minimise repulsion.

  

  
  

Properties of Polymers

• Structure: long chains, irregularly packed. amorphous

• Crystallinity: regions within some polymers where the carbon-carbon chains are packed together in a regular, repeating pattern

  • this depends on the atoms bonded to the backbone.

• polymers Can exist as wholly amorphous or partially crystalline.

Summary of Atomic Bonding