Question

explain the order of melting points for the solids you tested using what you know about the molecular structures of ionic, molecular, and covalent network solids.

Answer 1

Short answer: covalent network solids have the highest melting points, ionic solids are intermediate, and molecular (covalent molecular) solids have the lowest — because of the different kinds and strengths of forces that must be overcome on melting.

Why, in more detail:

- Covalent network solids (e.g., diamond, quartz/SiO2, SiC)
- Structure: atoms linked by a continuous 3‑D network of strong covalent bonds throughout the crystal.
- Melting requires breaking many strong covalent bonds, so very large amounts of energy are needed → very high melting points (thousands of °C for diamond, SiO2, etc.).

- Ionic solids (e.g., MgO, NaCl, KBr)
- Structure: a lattice of ions held together by electrostatic (Coulomb) attractions between oppositely charged ions.
- Melting requires overcoming lattice energy so ions can move freely; this takes a lot of energy but generally less than breaking a covalent network.
- Relative ionic melting points depend on lattice energy: proportional to product of ion charges and inversely related to ion size (approx. Q1·Q2/(r1+r2)). So MgO (2+ and 2−, small radii) melts much higher than NaCl (1+/1−), which is higher than KBr (larger ions, lower lattice energy).

- Molecular (covalent molecular) solids (e.g., ice, iodine, naphthalene)
- Structure: discrete molecules held together by intermolecular forces (London dispersion, dipole–dipole, hydrogen bonding).
- Melting only requires overcoming these relatively weak intermolecular forces — the molecules themselves stay intact — so melting points are low.
- Variation within this class: hydrogen bonding (ice) raises mp above many molecular solids; larger, more polarizable molecules have stronger dispersion forces and higher mp (iodine > naphthalene, etc.).

Illustrative ordering (typical values)
- Covalent network: diamond ~3550 °C, SiO2 (quartz) ~1710 °C
- Ionic: MgO ~2850 °C, NaCl ~801 °C, KBr ~734 °C
- Molecular: ice 0 °C, naphthalene ~80 °C, iodine ~114 °C

Conclusion: the order you observed follows the bond/interaction strength that must be overcome on melting — strongest (network covalent) → strong electrostatic lattice (ionic) → weak intermolecular forces (molecular). Exceptions and differences within classes come from charge, ion size, hydrogen bonding, and polarizability.