Chemical Bonding extends to molecular shapes (VSEPR), polarity, and intermolecular forces — explaining why substances have the physical properties they do.
Ionic: electron transfer, giant lattice, high MP, conduct when molten/dissolved. Covalent: electron sharing, single/double/triple bonds. Dative (coordinate): both electrons from one atom (e.g., NH₄⁺, CO). Metallic: delocalised electrons in sea, good conductors, malleable. Electronegativity: ability to attract bonding electrons. Polar bond: unequal sharing (e.g., H-Cl). Non-polar molecule possible with polar bonds if symmetrical (e.g., CO₂, CCl₄).
VSEPR: electron pairs around central atom repel and arrange to maximize separation. Bond pairs and lone pairs counted. Shapes: 2 pairs → linear (180°), 3 → trigonal planar (120°), 4 → tetrahedral (109.5°), 5 → trigonal bipyramidal (90°/120°), 6 → octahedral (90°). Lone pairs repel more than bond pairs → angles reduced. NH₃: pyramidal (107°, one lone pair). H₂O: bent (104.5°, two lone pairs). SF₆: octahedral. XeF₂: linear (3 lone pairs + 2 bond pairs).
London dispersion (van der Waals): all molecules, strength increases with molecular size/surface area. Permanent dipole-dipole: between polar molecules. Hydrogen bonding: strongest IMF — between H bonded to N, O, or F and lone pair on another N, O, or F. Examples: water (high BP for its mass, ice less dense than liquid, high surface tension), ethanol (higher BP than ethane). Properties: boiling point, solubility, viscosity determined by strength of IMFs, NOT covalent bonds.
In ice, the hydrogen bonds form a regular, open, hexagonal crystalline structure where each water molecule is bonded to four neighbours. This structure contains significant empty space, making ice less dense than liquid water. When ice melts, some hydrogen bonds break, the structure partially collapses, and molecules can pack more closely — so liquid water at 4°C is denser than ice. This is anomalous; most substances are denser as solids. This property is crucial for life: ice floats on lakes, insulating the water below and allowing aquatic life to survive in winter.
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