Lecture on Melting Points and Bonding

Overview of Melting Points

• Last class covered February-March and October-November 2023 papers.
• Melting points of some solids discussed from Table 1.1:
  • Magnesium (Mg): Metal
  • Phosphorus (P): Non-metal
  • Sodium Chloride (NaCl): Ionic compound
  • Sulfur (S): Non-metal

Structure Types: Joint vs Simple

• Metals have Joint Structures.
• Phosphorus (P4) and Sulfur (S8) have Simple Structures.
• Ionic compounds (e.g., NaCl) have Joint Structures.

Melting Points and Bonding Types

• Magnesium: 923 K (Metallic Bonding)
• Sodium Chloride: 1047 K (Ionic Bonding)
• Key Point: Ionic Bonds are stronger than Metallic Bonds, hence higher melting points.
• Phosphorus vs. Sulfur: Both have Simple Structures and Covalent Bonding.
• Differing melting points due to Intermolecular Forces and Electron Count:
  • Phosphorus has 60 electrons (15 per atom, 4 atoms).
  • Sulfur has 128 electrons (16 per atom, 8 atoms).

Electronegativity

• Ability to attract shared electrons.
• Example: Cl and H-Cl bond, Cl is more electronegative.
• Trends:
  • Increases from left to right and bottom to top on Periodic Table.
  • Higher nuclear pull and smaller atomic size lead to higher electronegativity.

Intermolecular Forces

• Ammonia (NH3): Strongest force is Hydrogen Bonding.
• Wrong hydrogen bond representations discussed.
• Correct representation involves lone pairs and partial charges.

Melting Points of Ice and Ammonia

• Ice: 273 K, Ammonia: 195 K
• Ice has stronger hydrogen bonding due to more electronegative oxygen and ability to form two hydrogen bonds per molecule.

Oxidation Numbers in Period 3 Elements

• Example Elements: Na, Mg, Al, Si, P
• Oxidation determined by the number of valence electrons an element can lose.

Reactions with Water

• Excess cold water reacts with Silicon Chloride and Phosphorus Chloride to produce respective acids and byproducts (white fumes/precipitate).
• pH of solutions: acidic (around 2).

Chlorine Reactions

• Reacts with water and hot NaOH: Forms compounds like NaClO3 and releases gas.
• Applications in disinfecting water and bleaching.