Balmer Series

Atoms and molecules exhibit very interesting behavior when they absorb and release energy. The electrons in an atom traditionally exist at their lowest energy state, called the ground state, and labeled as n equal to one. However, when an atom absorbs energy, the electrons become excited and move to a higher energy level. When the electrons relax down to either a lower energy state or to the ground state, the excess energy is released as emitted light.

You may be familiar with these energy states, as they are depicted in the Bohr Model, which describes an atom as a nucleus with orbiting electrons in shells or orbitals. These shells are the same as the energy levels and are labeled by n. The wavelength of the emitted light depends on the difference between the high and low energy levels. High energy emitted light results from electrons relaxing from a higher energy level, and low energy emitted light results from electrons relaxing from a lower energy level.

The emission spectrum is a measure of emitted radiation across a range of wavelengths. With pure elemental species, the emission behavior appears as lines of specific wavelengths rather than a broad spectrum. Since different atoms have different energy levels, these spectral lines vary from element to element and depend on the transitions those electrons make between energy states when excited. For example, there are six named series of spectral lines for hydrogen, one of which is the Balmer Series.

The Balmer Series of spectral lines occurs when electrons transition from an energy level higher than n = 3 back down to n = 2. The visible light spectrum for the Balmer Series appears as spectral lines at 410, 434, 486, and 656 nm. The h alpha line is the red line at 656 nm and occurs due to the transition from n= 3 to n = 2. The cyan, blue, and violet lines correspond to energy level transitions from n = 4, 5, and 6, respectively, back down to n = 2. Additional spectral lines can be measured outside of the visible range.

Johann Balmer quantified the visible lines using the Balmer formula. Here, lambda is the observed wavelength, c is a constant, n is the lower energy level of two, and m is the higher energy level. The combination of the Balmer equation and the Bohr Model gives us the Rydberg equation, which describes the spectral lines of many different elements. In this equation, lambda is the recorded wavelength, and R_H is the Rydberg constant. The initial levels marked by n-initial represent the higher energy level that the electrons excite to and n final is the lower energy level that the electrons relax back to. For the Balmer series, n_final = 2.

In this lab, you will measure and observe the emission spectrums of hydrogen, helium, and neon and use the Rydberg Equation to determine the location of the spectral lines.