lesson
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Over 90% of all known chemical compounds share a single element at their center: carbon. What makes this one element able to construct everything from DNA and fuels to diamonds and medicines?
The answer begins with the exact number of electrons carbon shares with its neighbors.
The Rule of Four
A neutral carbon atom has six electrons: two in its inner shell and four valence electrons (outer-shell electrons that participate in chemical bonding). To achieve a stable octet of eight outer electrons, carbon shares four electron pairs, meaning carbon always forms four covalent bonds.
📊A clean, responsive 2D chemistry graphic comparing a carbon atom's electron shell configuration with its bonding capacity. Left card: A carbon nucleus labeled '6p, 6n' surrounded by two concentric shell circles; inner shell has 2 blue dots (electrons), outer shell has 4 orange dots labeled '4 valence electrons'. Right card: A central 'C' letter with 4 outward-pointing arms with open sockets labeled '4 bond sites (tetravalent)'. Color palette: dark navy background (#0f172a), white cards (#1e293b), cyan accents (#38bdf8), orange electrons (#fb923c). Card dimensions fit 350px width cleanly.
In 1858, chemists August Kekulé and Archibald Scott Couper independently proposed that carbon is tetravalent (having a bonding capacity of four), which solved the mystery of how organic chains form.
Does carbon always have to attach to four separate atoms to satisfy this rule?
Single, Double, and Triple Bonds
Carbon always makes four total bonds, but it can share one, two, or three pairs of electrons with another single atom to form single, double, or triple covalent bonds.
📊A 2x2 grid showing four valid carbon bonding patterns with clear line formulas. Top-left: Methane (CH4), central C with 4 single lines to 4 H atoms labeled '4 single bonds'. Top-right: Ethene (C2H4), H2C=CH2 showing a double bond line between carbons, each C holding 2 single bonds to H, labeled '1 double + 2 singles = 4'. Bottom-left: Ethyne (C2H2), HC≡CH with triple bond lines labeled '1 triple + 1 single = 4'. Bottom-right: Carbon dioxide (CO2), O=C=O with two double bonds labeled '2 double bonds = 4'. Each carbon has a small badge showing 'Total = 4'. Clean white background with dark text #1e2945 and blue highlights #2563eb.
Now let's use this rule to deduce a complete structure from an unfamiliar chemical formula.
Deducing Structural Formulas
A structural formula shows the arrangement of atoms in a molecule and the covalent bonds connecting them, drawn as solid lines representing shared electron pairs.