Monatomic ideal gas

The AP Physics 2 exam conventions settle this in four words. The box printed on the table of information, which governs the exam unless a question states otherwise, reads "Ideal gases are monatomic." The qualifier is this course's default, not a special case.

It matters because of one number. Essential knowledge 9.4.A.1.ii says the internal energy of an ideal monatomic gas is the sum of the kinetic energies of the constituent atoms, and attaches the sheet's

Per atom, the same factor appears in $K_{\text{avg}} = \tfrac{3}{2}k_BT$. The $\tfrac{3}{2}$ counts three independent directions of translation and nothing else.

Diatomic molecules break that count, because they can rotate and vibrate as well, so at a given temperature they store more than $\tfrac{3}{2}nRT$. AP Physics 2 sidesteps this by convention rather than teaching it: no other factor is printed on the sheet and the CED supplies none, so a stem that did specify a diatomic gas would have to hand you the factor itself.

Two consequences worth having ready:

• Combining with $PV = nRT$ gives $U = \tfrac{3}{2}PV$, so a single point on a PV diagram fixes the internal energy.
• 9.4.A.1.i explains why there is no potential-energy term at all: the atoms of an ideal gas do not interact through conservative forces, and their internal structure is not considered.

The equation of state is the ideal gas law; the energy bookkeeping is internal energy.