I know, replying to myself is goofy ...Yes, exactly. The ideal gas molecule has no structure, so it can't rotate or vibrate. All it can do it move in three dimensions. f=3. Atomic gasses (at least the ones that don't make dimers) approximate this case and differ from the ideal primarily only in that they take up space reducing the available volume. More complicated molecules can vibrate and rotate and each independent motion is another degree of freedom where energy can be stored. The gas has more capacity to store energy as a function of temperature.