Ola

Genuinely a coincidence, and a good one, because the two words come from completely unrelated places.

Meth- traces back through "methyl" to a compound obtained by heating wood, and the name was built from Greek roots meaning roughly wine and wood — wood spirit, what we now call methanol.

Eth- comes from "ether", the volatile liquid, itself from a Greek word for the upper air. The two-carbon fragment got named after the substance chemists were already working with, and "ethyl" was coined for it.

So one name comes from wood and one from air, they were coined decades apart by different people for different reasons, and the fact that removing one letter turns one into the other is pure accident of transliteration.

The giveaway is exactly what you noticed: the pattern stops immediately. Propane comes from a fatty acid name, butane from butter. Only from pentane onwards does the system become systematic, using Greek number roots — pent, hex, hept, oct. The first four names are historical debris that the system inherited and never cleaned up.

The reason behind the limit is worth having, because it makes it stop feeling arbitrary.

A liquid holds together because its molecules attract each other enough to stay in contact while still moving. Heat is molecular motion. Raise the temperature and at some point the molecules are moving fast enough that the attraction between them cannot keep them condensed, no matter how tightly you pack them.

Compressing harder does not help because squeezing does not slow anything down — it just puts fast-moving molecules closer together. You end up with a dense, hot fluid rather than a liquid.

Which is why the critical temperature tracks how strongly molecules attract one another. Water, which hydrogen-bonds strongly, has a critical temperature above 370 Celsius. Helium, which barely attracts anything, is around 5 kelvin. Hydrogen is a small, light, weakly attracting molecule, so it sits near the bottom.

On the teaching half: introductory courses do make it look worse than it is, because they front-load the memorisation before the frameworks that organise it.

A great deal of what looks like arbitrary rules in a first course turns into a small number of principles later — where electrons are, where they want to go, what makes a good leaving group, what stabilises a charge. Once those land, most "exceptions" stop being exceptions and become consequences.

It still will not let you calculate an unknown reaction from scratch. But it moves you from memorising outcomes to reasoning about them, which is a different experience of the subject entirely.

The interesting footnote is that a four-carbon ring does exist as a species — it just is not the molecule you built.

Small carbon clusters have been studied extensively, and a four-atom carbon ring is a known arrangement. It is not a square with alternating or cumulated double bonds; it is a distorted rhombus with a bonding pattern that does not map onto the lines-between-atoms picture at all.

Which is a good general lesson about model kits. The stick model is a bookkeeping device for ordinary organic molecules, and it works remarkably well there. Push it into unusual regions — small clusters, electron-deficient compounds, metals — and the sticks stop corresponding to anything, while the atoms carry on existing quite happily.

The "work done per mole per kelvin" version you were given is not wrong, it is just a specific case presented as a definition, which is why it did not generalise for you.

Here is where it comes from. Heat a mole of ideal gas by one kelvin at constant pressure and it expands. Expanding against a constant pressure does work, and if you compute that work it comes out to exactly R joules.

So the statement is true, and it describes one particular situation — constant pressure, one mole, one kelvin. Your textbook picked it because it appears while deriving the difference between the two heat capacities.

But R shows up in places with no work and no expansion anywhere: entropy expressions, equilibrium constants, reaction rates, the distribution of molecular speeds. Those are the clue that the deeper meaning is the energy-per-kelvin one, and the work interpretation is a consequence rather than the definition.

The practical rule I give students: write the full, unelided form unless the elided one is the standard name of a substance they will meet on a label.

That is roughly monoxide, dioxide and tetroxide, and almost nothing else. Everything outside that short list, spell it out and you will be right.