Final answer:
When electrons de-excite from the third excited state in a hydrogen atom, three spectral lines in the visible region are observed, corresponding to the transitions from the third excited state (n = 4) to n = 2, as well as from n = 5 and n = 6 to n = 2. The correct answer is option: C. 3
Step-by-step explanation:
The student asked how many spectral lines in the visible region are observed when electrons from a hydrogen atom de-excite from the third excited state.
According to the Bohr model of the hydrogen atom, these types of transitions occur when an electron moves from a higher energy level to a lower one, such as from n > 2 to n = 2, which is known as the Balmer series.
The transitions that result in the emission of visible light are from the n = 3 to n = 2 transition (red light), n = 4 to n = 2 transition (green light), n = 5 to n = 2 transition (blue light), and n = 6 to n = 2 transition (violet light).
Hence, a de-excitation from the third excited state (n = 4) would result in the emission of three spectral lines (because the transitions from n = 5, 6 to n = 2 are also possible).