Diamond ($sp^{3}$):

• Each C bonded to 4 others in a tetrahedron
• Bond angle: 109.5°
• 3D covalent network
• No free electrons → insulator
• Hardest natural substance
• Density: ~3.5 g/$cm^{3}$

Graphite ($sp^{2}$):

• Each C bonded to 3 others in planar hexagonal layers
• Bond angle within layer: 120°
• p-orbital perpendicular to layer → delocalized π system
• Free π electrons → electrical conductor
• Layers held by weak van der Waals forces → soft, lubricant
• Interlayer distance: ~335 pm (weak forces); C-C within layer: ~142 pm (strong)

Diborane ($B_{2}H_{6}$) Structural Labels:

• 2 B atoms
• 4 terminal H atoms (2 per B), held by normal 2c-2e B-H bonds
• 2 bridging H atoms between B atoms, each forming one 3c-2e "banana bond"
• B-H-B bridge bond angle: ~97°
• B-B distance is NOT a direct bond (no B-B bond exists)
• Molecular geometry: $H_{2}B$(μ-H)_{2}$BH_{2}$ in a "butterfly" arrangement
• SMILES: [BH2]1([H][BH2]1)[H] (bridging bonds shown)

$PCl_{5}$ Structural Labels:

• 3 equatorial Cl: bond length ~202 pm, 120° apart
• 2 axial Cl: bond length ~214 pm, 90° to equatorial plane, 180° to each other
• Hybridization: $sp^{3}$d
• Geometry: Trigonal bipyramidal