A monotomic gas of say Argon has an adiabatic expansion index of 5/3 say 1.66. This can be defined as .. (3 modes of spin ENERGY + 2 modes of straight line vibrational POWER) / 3 degrees of spin ENERGY. In thermodynamic gas terms 1.66 = (Work Done + internal energy)/(internal energy). ..................................................(2+3)/(3)