The emission spectrum of a chemical element or a chemical compound is the set of frequencies of the electromagnetic radiation emitted by the electrons of its atoms when they perform a transition from one state to a higher energy towards a lower energy .
For each transition between states, the energy of the emitted photon is equal to the energy difference of the two states according to the equation :
which correlates the energy of the transition with the frequency of the photon of light emitted (h is Planck’s constant) . Since in each element or chemical compound there are many possible transitions, the set of photons of different frequencies emitted by the element or by the molecule is its the spectrum .
The emission spectrum of each element or molecule is unique, for this reason its analysis, known as spectroscopy, can be used to qualitatively and quantitatively analyze a substance.
The emission spectra are distinguished:
- Discrete lines spectra: it correspond to a succession of discrete wavelength
- Band spectra: the lines are close to each other around some wavelength forming separate bands
- Continuous spectra: continuous succession of wavelength
The discrete spectra and band spectra are emitted by gas and vapor at low pressure. The first are due to isolated atoms, the second to diatomic molecules. The spectra are characteristic of the elements that emit them.
In the hydrogen atom, the quantum leaps of an electron between the orbitals are described by the equation Rydberg :
with ni > nf represent the initial and the final energy levels, me and e respectively the mass and the charge of the electron, c is the speed of light, h Planck’s constant and ε the dielectric constant. In the spectrum of hydrogen this equation realizes perfectly the presence of some series of lines, called Lyman, Balmer and Paschen. The Balmer series is the only which fall in the visible, and corresponds to the transition from the energy levels greater than level 2; the most important line of this series is called Hα, its wavelength is approximately 6563 Å, which corresponds to red light, and is given by the electron transition between levels 3 and 2.
Absorption and Emission Spectra
With the grating spectrometer described in one of previous posts we have examined a number of light sources with the aim of highlighting the emission and absorption lines of some chemical elements. In particular, we have used spectral lamps. These are constituted by a glass tube, inside which there is a low pressure gas (hydrogen, nitrogen, carbon dioxide), when a voltage of 3000-5000 V is applied to the two electrodes a discharge of ionized gas is formed that emits light in correspondence to the characteristics emission lines of the gas contained in the tube.
Pdf document with the complete description of the spectroscopy experiments: SpettroscopiaAtomica_ENG