Final answer:
The 3rd energy level, or the 3rd harmonic, of the atom corresponds to a standing wave with 2 nodes.
Step-by-step explanation:
The model of the atom that suggests each electron energy level corresponds to a standing wave in a box with two closed ends is known as the quantum particle in a box model. In this model, the energy levels are determined by the number of nodes in the standing wave. A node is a point in the wave where the amplitude is zero. The formula to calculate the number of nodes in the nth energy level is given by:
Number of nodes = n-1
Therefore, the 3rd energy level, or the 3rd harmonic, would have 2 nodes.