The only form of internal energy for an ideal gas is the translational kinetic energy of the particles.
One of the main themes of the first part of this MCAT course will be to develop our idea of the internal energy of chemical substances using the fundamental tools of Mechanics and basic Electricity & Magnetism. We will return to this theme from many directions. Classical physics is extremely helpful to the conceptual understanding of chemistry, but you must always bear in mind the conceptual departures of quantum mechanics. We will devote a great deal of effort to developing the ability to conceptualize chemical change in terms of the main forms of energy from physics. As substances undergo chemical transformation, what is happening to energy at the particle level?
The two main forms of energy that are involved in chemical transformations are the electrostatic potential energy among the particles and kinetic energy. In fact, for an ideal gas, translational kinetic energy of the particles is the only
form of internal energy. Translational kinetic energy is kinetic energy of a particle whose center of mass is moving through space. In a real substance kinetic energy can also exist as rotational kinetic energy and vibrational kinetic energy in addition to translational kinetic energy.
Furthermore, electrostatic force interactions enable real substances to store energy in the form of electrostatic potential energy. When opposite charges move together in a chemical transformation or like charges move apart, electrostatic potential energy decreases.
When an electron falls to a lower energy level, an atom emits a photon. If you have a large number of atoms where this is occurring, you would have a loss of internal energy, a decrease in the total electrostatic potential energy within the system.