Every carbon atom in an organic molecule forms exactly four bonds — that's non-negotiable, it's how carbon's four valence electrons pair up. In a straight-chain alkane, each interior carbon uses two of its four bonds to connect to its two neighboring carbons, leaving two bonds free for hydrogen. Each of the two end carbons, though, only connects to one neighboring carbon, leaving three bonds free for hydrogen.

Add that up for a chain of n carbons: the two end carbons contribute 3 hydrogens each (6 total), and the (n − 2) interior carbons contribute 2 hydrogens each. Work through the algebra and it collapses into the compact formula every alkane obeys: CₙH₂ₙ₊₂. Methane (n=1) is the one exception worth noting separately, since with a single carbon and no neighbors at all it simply grabs the full four hydrogens — CH₄ — which still satisfies the formula (2×1+2 = 4) even though the "end carbon logic" above doesn't quite apply with only one carbon present.

Once the pattern clicks, you can generate the molecular formula for any straight-chain alkane without drawing a single bond. Want decane (10 carbons)? Plug in n=10: C₁₀H₂₂. Want a 20-carbon alkane found in candle wax? n=20 gives C₂₀H₄₂. This becomes second nature fast, and it's the baseline you'll compare every other hydrocarbon against — any time you calculate degrees of unsaturation later in this unit, you're really asking "how far short of CₙH₂ₙ₊₂ does this molecule fall, and what does that missing hydrogen pair tell me?"

One important caveat: CₙH₂ₙ₊₂ describes the molecular formula, not the connectivity. Butane (C₄H₁₀) and isobutane (also C₄H₁₀) share the exact same formula but have different skeletons — one is a straight chain, the other is branched. The formula tells you the total atom count is right; it says nothing about which structural isomer you're looking at.