One Formula Is Not a Name
By now you can read a skeletal structure and pick out a carbon skeleton with a reactive site sitting on it. Here is the unsettling consequence: a molecular formula like C4H10 does not pin down a single molecule. It is more like a shopping list — four carbons, ten hydrogens — and a shopping list does not tell you what you will cook. The same atoms can be assembled in more than one order, and each order is a different, real substance.
Two molecules that share a formula but differ in their connectivity — which atom is bonded to which — are called [[constitutional-isomer|constitutional isomers]] (the older name is structural isomers). C4H10 is the smallest hydrocarbon where this happens. You can string the four carbons in a straight line to get butane, or branch them so that a central carbon carries three others, giving isobutane (its proper name is 2-methylpropane). Same formula, different wiring, different molecules.
Picture both in shorthand: butane is just C-C-C-C, four carbons in a row, while isobutane is a three-carbon chain C-C-C with a fourth carbon hanging off the middle one as a branch. The hydrogens fill in automatically to give every carbon its four bonds, and both add up to C4H10 — yet they are different substances you could bottle and label separately.
Connectivity, Not Formula, Decides Behaviour
These are not bookkeeping tricks; the isomers behave differently because their shapes differ. Straight-chain butane is a slender rod that lies snugly against its neighbours, so its weak attractive forces add up over a long contact area and it boils at about 0 degrees C. Branched isobutane is a stubby ball that touches its neighbours over less surface, so the same kind of forces hold less firmly and it boils about 12 degrees lower, near -12 degrees C. Identical formula, identical mass, measurably different boiling points — because branching changes how molecules pack and pull on one another.
Change which atom is bonded to which and you can change much more than the boiling point. Consider C2H6O. Wire the oxygen onto the end of an ethyl group and you get ethanol, CH3CH2OH — a drinkable liquid with an O-H bond and a functional group called the hydroxyl. Tuck the same oxygen between two methyl groups instead and you get dimethyl ether, CH3OCH3 — a gas at room temperature with no O-H at all. Same formula C2H6O; one is the alcohol you can pour, the other a gas you cannot. The functional group, the chemistry, the physical state — all flow from connectivity.
Three Flavours of Constitutional Isomer
It helps to sort constitutional isomers into three kinds, because each shows up again and again. Chain (or skeleton) isomers differ in how the carbon backbone branches — butane versus isobutane is the classic case. Positional isomers keep the same skeleton but move a functional group to a different carbon — 1-propanol has the O-H on the end carbon, 2-propanol on the middle one. Functional-group isomers are the most dramatic: the atoms regroup into an entirely different group, like ethanol the alcohol versus dimethyl ether, or an aldehyde versus a ketone sharing one formula.
These categories are a convenience, not a law of nature — two isomers can differ in chain and position at once, and you do not need to label every pair. The point is to train your eye to ask three questions of any formula: can I rearrange the skeleton, can I slide the functional group along it, and can I rebuild the atoms into a different group altogether? Each yes is another constitutional isomer waiting to be drawn.
The Count Explodes
Add one carbon at a time and the number of possible constitutional isomers grows astonishingly fast. The alkanes make the cleanest demonstration. C4H10 has 2 isomers and C5H12 has 3 — still countable on your fingers. But C6H14 jumps to 5, C7H16 to 9, C8H18 to 18, and by C10H22 there are 75. Push on to C20H42 and the count is 366,319; somewhere past C40 it exceeds the number of stars you can see, and there is no simple closed formula for it.
alkane formula constitutional isomers butane C4H10 2 pentane C5H12 3 hexane C6H14 5 heptane C7H16 9 octane C8H18 18 decane C10H22 75 eicosane C20H42 366,319
This explosion is the deep reason carbon-based chemistry is so vast. With only a handful of elements, connectivity alone generates an essentially unlimited library of distinct molecules — which is exactly the resource life and industry draw on. It is also why naming matters so much: only a systematic name can single out which one of the 366,319 C20H42 molecules you mean, and learning that naming system is the very next step on this ladder.
How a Formula Hints at Structure
A formula cannot name a molecule, but it is not silent either — it whispers a clue about how much structure to expect. Compare what a saturated chain would carry to what the formula actually has, and every missing pair of hydrogens signals one ring or one extra bond. This number is the degree of unsaturation (also called the index of hydrogen deficiency), and it narrows the field of possible isomers before you draw a single line.
- Take a formula such as C4H8. A fully saturated four-carbon chain would be C4H10 (the pattern CnH2n+2 for an acyclic alkane).
- C4H8 is short by two hydrogens (10 minus 8), and two missing hydrogens equal one degree of unsaturation.
- One degree means either one ring or one double bond. So C4H8 could be a butene (a chain with one C=C) or a cyclobutane (a four-membered ring) — and these are constitutional isomers of one another.
So the formula is a starting map, not the territory. It tells you roughly how many rings and bonds to budget for, and the rest is the structural detective work this whole rung is teaching you — work that, much later, spectroscopy will help you finish.
A First Glimpse of the Wider Isomer World
Constitutional isomers are only the first branch of a bigger family. They differ in the very order of bonding — who is connected to whom. But two molecules can have the same formula and the very same connectivity, every atom bonded to the same neighbours, and still be different because their atoms point differently in three-dimensional space. Those are stereoisomers, and they are a separate idea waiting for you on a later rung.
Keep the two firmly apart, because beginners blur them constantly. Constitutional isomers differ in connectivity; stereoisomers share connectivity and differ only in spatial arrangement. A useful litmus test: if you can convert one drawing into the other only by breaking a bond and remaking it elsewhere, they are constitutional isomers. If no bonds need breaking — only a rearrangement in space — they are stereoisomers, or perhaps just the same molecule viewed from a new angle.