Lecture 6b- Ionic and Metallic Bonding

Ionic Bonding

• Definition of Ionic Bonding:

  • Ionic bonds are formed through the electric force of attraction between oppositely charged ions.

• Types of Ions:

  • Cations: Positively charged ions formed when atoms lose electrons (typically metals).

  • Anions: Negatively charged ions formed when atoms gain electrons (typically nonmetals).

Formation of Ionic Compounds

• Process: An atom that tends to lose electrons (cation) interacts with an atom that tends to gain electrons (anion).

  • Example: Lithium (Li) loses one electron to become Li⁺, while fluorine (F) gains one electron to become F⁻, forming lithium fluoride (LiF).

• Composition: Usually consist of a metal (cation) and a nonmetal (anion) but can also involve polyatomic ions.

Properties of Ionic Compounds

• Distinct Properties: The resulting ionic compounds display different properties from their constituent elements.

  • Example: Sodium (Na) metal is reactive and soft, chlorine (Cl) gas is toxic; together they form sodium chloride (NaCl), which is essential for human diets.

• Charge Balance: The overall charge of ionic compounds must be neutral.

  • Example: Potassium chloride (KCl) results from potassium (K⁺) and chloride (Cl⁻), balancing +1 and -1 charges respectively.

• Complex Formation: In aluminum oxide (Al₂O₃), aluminum (Al³⁺) donates three electrons, and oxygen (O²⁻) accepts two, necessitating a 2:3 ratio to balance charges (2 Al³⁺ and 3 O²⁻).

Predicting Ionic Formulas

• Swap and Drop Method: A trick to predict formulas by swapping ion charges.

  • For calcium fluoride (CaF₂):

    • Calcium (Ca²⁺) and fluorine (F⁻) swap charges resulting in CaF₂.

  • For aluminum oxide (Al₂O₃):

    • Al³⁺ and O²⁻ swap to give Al₂O₃.

  • For magnesium oxide (MgO):

    • Both form a balanced compound of Mg²⁺ and O²⁻, hence MgO.

Characteristics and Melting Points of Ionic Compounds

• Crystal Lattice Structure: Ionic compounds form ordered crystal arrays.

• High Melting Points:

  • Determined by ion charge and size—higher charges and smaller sizes lead to higher melting points.

  • Example: Magnesium oxide (MgO) has a higher melting point than sodium fluoride (NaF) due to greater charge magnitude.

Comparing Ionic Compounds

• Charge Priority: When comparing melting points of ionic compounds, consider the charge first. If charges are equal, consider size.

  • Example: Barium oxide (BaO) has higher melting point than cesium oxide (Cs₂O) due to Ba²⁺’s higher charge.

Metallic Bonding

• Definition of Metallic Bonding:

  • Occurs when metal cations are surrounded by delocalized electrons that can flow through the structure.

• Properties:

  • Metals are good conductors of heat and electricity due to freely flowing electrons.

  • Physical properties include:

    • Opaque

    • Malleable and ductile (can be shaped)

    • Shiny appearance.

Alloys

• Definition: Mixtures containing more than one metal.

  • Examples:

    • White Gold: Alloy of gold and palladium.

    • Brass: Alloy of copper and zinc.

    • Steel: Alloy primarily of iron with carbon.

• Natural Occurrence: Most metals are not found in pure form but rather in ores containing metal compounds.

• Extraction Costs: Separating metals from their ores is often expensive due to complex chemical processes.

  • Example: Extracting iron involves separating from iron oxide.