Emission spectrum
From Wikipedia, the free encyclopedia
The emission spectrum of an element or compound is the relative intensity of electromagnetic radiation of each frequency emitted by atoms or molecules of that element or compound when they are excited.
Each atom's atomic emission spectrum is unique and can be used to determine if that element is part of an unknown compound. Similarly, the emission spectra of molecules can be used for chemical analysis.
Contents |
[edit] Origins
When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbits. When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a photon. The wavelength (or, equivalently, frequency) of the photon is determined by the difference in energy between the two states. These emitted photons form the element's emission spectrum.
The fact that only certain colors appear in an element's atomic emission spectrum means that only certain frequencies of light are emitted. Each of these frequencies are related to energy by the formula:
- Ephoton = hf,
where E is the energy of the photon, f is its frequency, and h is Planck's constant. This concludes that only photons having certain energies are emitted by the atom. The principle of the atomic emission spectrum explains the varied colors in neon signs, as well as chemical flame test results mentioned above.
The frequencies of light that an atom can emit are dependent on states the electrons can be in. When excited, an electron moves to a higher energy level/orbital. When the electron falls back to its ground level the light is emitted.
[edit] Radiation from molecules
As well as the electronic transitions discussed above, the energy of a molecule can also change via rotational, vibrational and vibronic (combined vibrational and electronic) transitions. These energy transitions often lead to closely-spaced groups of many different spectral lines, known as spectral bands. Unresolved band spectra may appear as a spectral continuum.
Molecular emission is the mechanism behind the sulfur lamp and the deuterium arc lamp.
[edit] Spectroscopy
Light consists of electromagnetic radiation of different wavelengths. Therefore, when the elements or their compounds are heated either on a flame or by an electric arc they emit energy in form of light. Analysis of this light, with the help of spectroscope gives us a discontinuous spectrum. A spectroscope or a spectrometer is an instrument which is used for separating the components of light, which have different wavelengths. The spectrum appears in a series of lines called the line spectrum. This line spectrum is also called the Atomic Spectrum because it originates in the element. Each element has a different atomic spectrum. The production of line spectra by the atoms of an element indicate that an atom can radiate only a certain amount of energy. This leads to the conclusion that electrons cannot have just any amount of energy but only a certain amount of energy.
The emission spectrum can be used to determine the composition of a material, since it is different for each element of the periodic table. One example is astronomical spectroscopy: identifying the composition of stars by analysing the received light. The emission spectrum characteristics of some elements are plainly visible to the naked eye when these elements are heated. For example, when platinum wire is dipped into a strontium nitrate solution and then inserted into a flame, the strontium atoms emit a red color. Similarly, when copper is inserted into a flame, the flame becomes green. These definite characteristics allow elements to be identified by their atomic emission spectrum. Not all lights emitted by the spectrum are viewable to the naked eye, it also includes ultra violet rays and infra red lighting, an emission is formed when an excited gas is viewed directly though a spectroscope.
[edit] Absorption spectra
When light passes through a cold, dilute gas, atoms or molecules in the gas absorb light at their characteristic frequencies. They eventually re-emit this light, with the re-emitted photon travelling in a random direction. A spectrum of light which has passed through the gas will have dark lines (absence of light from the continuous spectrum) at the characteristic frequencies of the gas. The pattern of dark lines, known as the absorption spectrum, is the inverse of the emission spectrum.
