Chemical bonds / Ikatan kimia
Chemical bonds tests why substances have particular melting points, conductivity and solubility in terms of ionic, covalent and metallic bonding. You need to describe formation and relate structure to properties.
Core
- Ionic: metal plus nonmetal; electron transfer; giant lattice of alternating ions; strong electrostatic forces; high melting point; conducts when molten or aqueous, not when solid.
- Covalent: nonmetal plus nonmetal; shared electron pairs. Simple molecular substances have weak intermolecular forces, low melting points and usually do not conduct. Giant covalent substances such as diamond, graphite and silicon dioxide have very high melting points.
- Metallic: lattice of cations in delocalised electrons; malleable, ductile and conducts in solid and liquid states.
Worked example
Explain why solid NaCl does not conduct but molten NaCl does. In solid NaCl, ions are held in fixed lattice positions and cannot move; there are no mobile charge carriers. When melted, the lattice breaks down and Na⁺ and Cl⁻ ions become mobile, so the melt conducts. Dissolving in water also frees mobile ions. Reasoning: conductivity requires mobile ions or electrons, not simply the presence of ions.
Traps
Do not say covalent compounds never conduct: graphite conducts due to delocalised electrons, and acids in water conduct due to ions. Ionic solids are not molecules. Melting point depends on the strength of forces between particles, not just the bond type. In dot-and-cross diagrams, show all valence electrons and include charges and brackets for ions.