oxides of carbon and carbonates
Two breaths of inorganic carbon shape our world: the carbon dioxide we exhale and that warms the planet, and the carbon monoxide that silently kills from a faulty heater. Add the carbonates locked in limestone, seashells, and chalk, and you have the inorganic side of carbon, the part that has nothing to do with organic molecules yet rules the global carbon cycle.
Carbon monoxide, CO, has a carbon-oxygen triple bond and is isoelectronic with N2 (both have 14 electrons), which makes it remarkably stable and inert as a free molecule, yet a superb ligand: its carbon lone pair donates to metals while empty pi orbitals accept back-donation, the basis of metal carbonyl chemistry. CO is toxic precisely because it binds hemoglobin's iron far more tightly than O2 does. Carbon dioxide, CO2, is a linear nonpolar molecule, O=C=O, an acidic oxide that dissolves in water to form a little carbonic acid, H2CO3, which loses protons to give bicarbonate HCO3- and carbonate CO3^2-. The carbonate ion is trigonal planar with three equivalent C-O bonds, a textbook case of resonance/delocalization (one extra pi electron pair spread over three oxygens, bond order about 1.33).
These species matter at every scale. Carbonates make up vast sedimentary rocks (limestone, marble) and the shells of marine life; heating limestone (CaCO3 to CaO + CO2) is the ancient lime kiln chemistry behind cement and mortar. The CO2 / bicarbonate / carbonate equilibrium buffers blood and the oceans and is central to ocean acidification and the carbon cycle. And the CO of metal carbonyls launched a whole branch of organometallic chemistry. They neatly show that 'inorganic' carbon, far from being a footnote, is among the most consequential chemistry there is.
The carbonate ion CO3^2- cannot be drawn with one short double bond and two long single bonds, because all three C-O bonds are measured to be identical. The true picture is a delocalized pi system: one pair spread over three oxygens, giving an average bond order near 1.33.
Equal C-O bonds force a resonance/delocalized picture rather than fixed single and double bonds.
Carbonic acid is real but only a tiny fraction of dissolved CO2 is actually H2CO3; most stays as hydrated CO2. And CO's inertness as a free gas is no contradiction with its toxicity, which comes from how tightly it binds metal centers like hemoglobin's iron, not from any general reactivity.