Question

If $H -$ atom is supplied with $12.1eV$ energy and electron returns to the ground state after excitation the number of spectral lines in Balmer series would be:
A. 1
B. 2
C. 3
D. 4

Answer 1

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identify atoms and molecules.

Complete answer:
In a hydrogen atom, there are various energy levels where the excitation of the electrons takes place. The energy of the ground state of a hydrogen atom is $- 13.6eV$ and the energy of the first excited state is $- 3.4eV$ . The separation energy will be equal to the energy difference between the two energy levels. If the provided energy is greater than the separation energy, the electron jumps to the higher energy level. In the case of a Balmer series, the value of ground state is 2.
Energy difference = $( - 13.6 + 12.1)eV = - 1.5eV$
This means that the electron will jump to the higher energy level. So, the electron jumps to the third energy level.
The number of spectral lines = ${n_3} - {n_2}$
Here, ${n_3} = 3$ and ${n_2} = 2$
Number of spectral lines in the Balmer series = 1

So, the correct answer is Option A.

Note:
When an electric excitation is applied on the hydrogen atomic gas at low pressure, a bluish light is emitted. When a ray of this light is passed through a prism, a spectrum of several isolated sharp lines is obtained. The wavelength of various lines shows that spectrum lines lie in the visible, UV and IR region of the spectrum.