A thermoacoustic generator transforms thermal energy into electrical energy. Thermal energy is transferred to a gas by a heat exchanger. The thermal energy causes the gas particles to vibrate rapidly and generate a sound wave. These vibrating gas particles are then used by an alternator to generate electrical energy. The gas used in this kind of generator must be nonreactive to avoid the danger of fire. Using heavier gases increases the amount of electrical energy generated.Which two elements on the table are most likely to be good choices for use as the gas in a thermoacoustic generator?