For each of the following pairs of substances, predict which...

For each of the following pairs of substances, predict which will have the higher melting point and indicate why: (a) $\mathrm{Ar}, \mathrm{Xe} ;$ (b) $\mathrm{SiO}_{2}, \mathrm{CO}_{2} ;$ (c) $\mathrm{KBr}, \mathrm{Br}_{2}:$ (d) $\mathrm{C}_{6} \mathrm{Cl}_{6}, \mathrm{C}_{6} \mathrm{H}_{6}$

Key Concepts

Molecular Structure and Polarizability

The molecular structure, including molecular weight, shape, and the presence of substituents, affects how easily an electron cloud can be distorted (polarizability). Higher polarizability enhances London dispersion forces between molecules, which can lead to higher melting points. Additionally, variations in molecular symmetry and substituent types can introduce dipole-dipole interactions, further influencing melting behavior.

Network Covalent Bonding

Network covalent bonding occurs when atoms are bonded by covalent bonds in a continuous network extending throughout the material. This type of bonding results in very high melting points, as seen in substances where every atom is strongly covalently bonded to several others, leaving the structure highly resistant to thermal disruption.

London Dispersion Forces

London dispersion forces are weak intermolecular attractions that arise from temporary fluctuations in electron distribution. Their strength increases with the number of electrons and the polarizability of the atom or molecule, which explains why larger atoms (or molecules) generally exhibit higher melting points within non-polar substances.

Ionic Bonding

Ionic bonding involves the electrostatic attraction between positively and negatively charged ions. Compounds with ionic bonds typically have high melting points because the forces between the oppositely charged ions in a lattice are much stronger than the forces holding together most molecular substances.

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