= Solution
\b[<carbon> uses strong 2s/2p-derived covalent bonding, whereas retention or involvement of the low-energy Pb 6s pair distinguishes the two <lead> oxides.] In CO, <carbon> has an approximately triple-bonded description with one <carbon> <lone pair>; the latter has substantial 2s character, while the sigma and two pi bonding combinations involve the valence shell, particularly <carbon> 2p orbitals for the pi bonds. In CO$_2$, <carbon> is formally in <oxidation state> +4 and its four <valence electrons> participate in two strongly covalent C=O double-bond frameworks. A localized account uses sp hybrids for sigma bonding and two <p orbitals> for the pi systems. <oxidation> states are bookkeeping devices here: neither <molecule> contains a bare C$^{2+}$ or C$^{4+}$ <ion>.
In PbO, the Pb(II) description removes or employs mainly the two 6p <electrons>, retaining a relatively low-energy 6s$^2$ pair. In PbO$_2$, the Pb(IV) description additionally involves those two 6s <electrons>. The stabilization and poor availability of that pair underlie the <inert-pair effect> and the relative preference for Pb(II) chemistry. “Inert” does not mean that the 6s <electrons> have zero influence on bonding or shape: mixing with <oxygen> states and empty Pb states can make the Pb(II) <lone pair> stereochemically active. The more ionic formal electron counts in the <lead> oxides and the covalent molecular-orbital description of the <carbon> oxides are complementary models, not literal assignments of individual <electrons> to immutable <atomic orbitals>.<The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS by Aron Walsh and Graeme W. Watson>[Abstract and electronic-density analysis: oxygen-dependent lone-pair activity in PbO]{ref}
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