- How are spectral lines produced
- How are spectral lines both emission and absorption are formed
Bohr's atomic model explains how electrons jumping between orbits in atoms cause emission and absorption line spectra and why each element has its own spectral signature.
Atomic Structure
The basic particles that go into forming atoms are protons, neutrons, and electrons. Protons are positively charged, electrons are negative, and neutrons are electrically neutral. Electrons have about 1/2000 the mass of protons and neutrons.
The first reasonably correct model for the structure of the atom was devised by the Danish physicist Niels Bohr. Bohr's model of the atom is not perfect and has been improved since Bohr's time. However Bohr's model works well enough to help us understand spectral line formation without needless complications.
In the Bohr model the protons and neutrons are in the nucleus, and the electrons orbit the nucleus. This picture is somewhat analogous to the solar system, with the nucleus like the Sun and the electrons like the planets. However the force causing planets to orbit the Sun is gravity, while attractive electrical forces between the positive protons and the negative electrons keep the electrons orbiting the nucleus. The other difference is where electrons can orbit the nucleus.
Energy Levels
Planets can orbit the Sun at any distance. Earth is 150 million kilometers from the Sun because of conditions when the solar system formed, but with different starting conditions, it could be at any distance. Bohr realized that electrons can, on the other hand, only orbit the nucleus in very specific allowed orbits. Electrons in more distant orbits require more energy, so the orbits are called energy levels.
The lowest energy level, which the electron normally occupies, is the ground state. Higher energy levels are called excited states. Electrons can only be in specific energy levels and cannot be between two allowed energy levels. It is however possible for electrons to jump from one energy level to another.
Energy however must be conserved. To jump to a higher level, an electron must gain the energy from someplace. If it jumps to a lower level, it must release the energy. This gaining and releasing energy causes spectral lines.
Absorption Lines
For an electron to jump to a higher energy level, it must gain energy. One way is to absorb a photon (the smallest unit or a particle of light) having the right amount of energy. The energy of a photon of light is related to its wavelength, so jumping to higher levels, electrons absorb the appropriate wavelength photons and produce absorption lines. This mechanism causes light from a hot source, that has a continuous spectrum, passing through a cooler gas to produce an absorption line spectrum.
Emission Lines
If an electron in an excited state jumps down to a lower energy level, it releases that energy as a photon. The wavelength is determined by the energy difference between the two levels.
The electron can get to the higher energy level by absorbing a photon, but that is not the only way. In a hot gas the atoms are more likely to collide. During a collision, some of the heat energy of the gas is transferred to the electrons, so they jump to higher levels. When they jump back down, they emit a photon. So hot transparent gasses have emission line spectra.
Spectral Signatures
Each type of atom has its own unique number of protons and electrons. Hence each element has a unique set of energy levels. The amount of energy corresponding to the various energy levels of hydrogen, for example, will be different from the levels for helium, carbon, oxygen, or any other element. Combining atoms into molecules changes the energy levels, so each type of molecule also has its own unique set of energy levels. The difference in energy levels determines the wavelength of the spectral line produced when an electron jumps to a different level, so each atom or molecule has its own unique set of spectral lines. Each element or compound therefore has its own unique spectral signature that astronomers use to identify chemical compositions of celestial bodies and chemists use to identify compositions of unknown samples.
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