= Solution
\b[CO and CO$_2$ are discrete <molecules>, whereas the other oxides gain stability from extended bonding or ionic lattices.] <carbon>'s compact 2p orbitals overlap effectively with <oxygen> 2p orbitals, allowing strong multiple bonds that satisfy the valence requirements within small <molecules>. Intermolecular forces between CO or CO$_2$ <molecules> are much weaker than those internal bonds, so low condensation temperatures and gaseous states at room conditions are unsurprising. CO$_2$ is nonpolar overall but still has <London dispersion forces> and interactions between <electric quadrupoles>; “nonpolar” does not mean no attraction.
For larger Group 14 atoms, overlap with <oxygen> 2p orbitals is less effective for strong localized multiple bonding. SiO$_2$ instead forms an extended network of SiO$_4$ <tetrahedra> with each <oxygen> bridging two <silicon> atoms. Breaking or melting the network involves much more than separating intact SiO$_2$ <molecules>. The Si–O <electronegativity> difference is $3.61-1.92=1.69$, so these are strongly polar <covalent bonds> rather than evidence for a molecular solid.
For Pb–O the difference is $3.61-1.85=1.76$, and ionic Pb$^{2+}$/O$^{2-}$ or Pb$^{4+}$/O$^{2-}$ descriptions provide useful starting points, with appreciable covalent contributions. PbO has layered structures with asymmetric <oxygen> coordination consistent with a stereochemically active 6s$^2$ pair. A common PbO$_2$ structure is rutile-like, with PbO$_6$ octahedra linked through <oxygen>. Extended lattice interactions account for their solid states.https://doi.org/10.1103/PhysRevLett.107.246402[Geometry and Electronic Structure: rutile PbO2, six-coordinate Pb and three-coordinate O]{ref} The similar Si–O and Pb–O <electronegativity> differences show why a single difference threshold cannot determine the entire bonding type, <coordination number> or structure.
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