What Can Happen to an Electron When Sunlight Hits It?
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Energy and Electrons    | When an electron is striking past a photon of light, it absorbs the quanta of energy the photon was conveying and moves to a higher free energy state. 1 way of thinking about this higher free energy state is to imagine that the electron is now moving faster, (it has just been "hit" by a rapidly moving photon). But if the velocity of the electron is at present greater, it's wavelength must also have changed, so it can no long stay in the original orbital where the original wavelength was perfect for that orbital-shape. So the electron moves to a dissimilar orbital where over again its own wavelength is in phase with its self. Electrons therefore have to jump around inside the cantlet every bit they either gain or lose free energy. This property of electrons, and the energy they absorb or requite off, can be put to an every 24-hour interval employ. Almost any electronic device you lot buy these days comes with i or more Light Emitting Diodes (usually called "LEDs"). These are tiny bubbles of epoxy or plastic with two wire connectors. When electricity is passed through the diode it glows with a characteristic colour telling yous that the device is working, switched on and ready to exercise it'southward piece of work. Deep in the semiconductor materials of the LED are "impurities", materials such as aluminum, gallium, indium and phosphide. When properly stimulated, electrons in these materials move from a lower level of energy upwards to a higher level of energy and occupy a different orbital. And then, at some point, these higher free energy electrons give upwardly their "extra" energy in the grade of a photon of light, and fall back down to their original energy level. The light that has suddenly been produced rushes away from the electron, atom and the LED to color our world. Typically, the light produced by a LED is just i colour (ruddy or green being stiff favorites). Although they are cheap, easy to brand, don't toll a lot to run, LEDs are not ordinarily used to light a room, because they cannot usually produce the wide range of different colors needed in "white" light. This is because of the quantum nature of the atoms being used in the LED and the quantum energies of the electrons inside them. When an excited electron within a LED gives up energy it must do so in those lumps called quanta. These are fixed packets of free energy that cannot be inverse or used in fractions; they must ever exist transferred in whole amounts. |
 | Thus, an excited electron has no option merely to give off either i quanta or 2 quanta of energy, it cannot give up 1.5 quanta, or two.3 quanta. Also, the electron can only move to very express orbitals within the atom; information technology must end up in an orbital where the wavelength is now uses is "in phase" with itself. These 2 restrictions limit the quality of the quanta of energy being released by the electron, and thus the nature of the photon of light that rushes away from the LED. Since the energy given off is strongly restricted to quanta, and quanta that allow the electron to move to a suitable place inside the atom, the photons of lite are similarly restricted to a tiny range of values of wavelength and frequency (a property we see equally "color"). Many LEDs have electrons that can but give up quanta of energy that, when converted into photons, produce light with a wavelength of virtually 700 nm - which nosotros then come across every bit red low-cal. These electrons are and so restricted in the quanta they tin can emit that they never shine blue light, or greenish light, or xanthous light, only red light. |
Lines in Spectra   | Long, long earlier their were LEDs in our lives, scientists trying to sympathise electrons in atoms noted a like phenomenon when light was either shone on certain materials or given off by certain materials. In 1859 the German physicist Gustav Robert Kirchoff, and his older friend Robert Wilhelm Bunsen came upwardly with a clever idea. They used Bunsen's burner to strongly heat tiny pieces of diverse materials and minerals until they were so hot that they glowed and gave off low-cal. Sodium, for example, when heated to incandescence, produced a strong yellow lite, but no blue, dark-green or ruby. Potassium glowed with a dim sort of violet light, and mercury with a horrible green light but no red or yellow. When Kirchoff passed the emitted light through a prism it separated out into its various wavelengths (the same way a rainbow effect is produced when white light is used), and he got a shock. He could but see a few thin lines of low-cal in very specific places and often spread far apart. Clearly glowing sodium was not producing anywhere near all the different wavelengths of white low-cal, in fact it was only producing a very feature ring of light in the yellow region of the spectrum - just like a LED! Kirchoff and Bunsen advisedly measured the number and position of all the spectral lines they saw given off past a whole range of materials. These were called emission spectra , and when they had nerveless enough of them it was clear that each substance produced a very characteristic line spectrum that was unique. No 2 substances produced exactly the same series of lines, and if two different materials were combined they collectively gave off all the lines produced by both substances. This, idea Kirchoff and Bunsen, would be a expert style of identifying substances in mixtures or in materials that needed to exist analyzed. So they did. In 1859 they found a spectrum of lines that they had never seen earlier, and which did not correspond to any known substance, and so, quite rightly, they deduced that they had found a new element, which they called cesium from the Latin give-and-take meaning "heaven blue". (Guess in what part of the spectrum they found the lines!). |
| Breakthrough Numbers and Levels of Energy | All the research on atomic structure and the hideously difficult-to-understand properties of electrons come together in the topic of "electron energy". An atom such equally lithium has 3 electrons in various orbitals surrounding the atomic eye. These electrons can be bombarded with energy and if they blot enough of the quanta of energy being transferred they jump near and in the most extreme instance, leave the lithium atom completely. This is chosen ionization . The amount of energy needed to remove the offset electron from a lithium is 124 kilocalories/mole, an amount of energy that is not difficult to supply, and so lithium atoms ionize hands. Notwithstanding, it takes near 1740 kilocalories/mole of energy to dislodge the second electron from effectually the lithium ion (it is at present an "ion" because information technology has already lost one electron). It takes a massive 2820 kilocalories/mole to dislodge the third and terminal electron from around the lithium ion. Partly this departure in the corporeality of energy needed to dislodge different electrons away from the lithium atomic center is due to the fact that the center of the lithium atom is carrying the positive charges of three protons. Moving a negatively charged electron away from a positively charged atomic center needs more and more than free energy as the amount of un-neutralized charge increases, thus; Li --> Li+ + e- Li+ --> Li++ + east- Li++ --> Li+++ + due east- However, the amount of energy needed to remove the first electron is a good measure of what information technology takes to stimulate an electron to exit its atom, and how tightly information technology is held in that location in the start identify. Within the atom, as Bohr pointed out, there are different possible positions for electrons to exist found as divers by the chief breakthrough number , normally written as " n ". |
 | Bohr defined the energy of electrons located at these different locations of quantum land by the formula: Eastn = - Eo/n2 In this formula Eo is a whole collection of concrete constants, which for an atom such every bit hydrogen has a value of 313 kilocalories/mole. Using this formula it is possible to calculate how much energy an electron has at each of the other, different, quantum states (n = two, north = 3, n = 4, etc.). This is usually presented in the form of a diagram (see left). For an electron at the ground state (n = 1) to be moved upwardly to the next level (n = ii) information technology must blot a quantum of energy that is the perfect amount to make this motion. If the quantum is as well small the electron could not achieve the next level, then it doesn't attempt. If the quantum is likewise big the electrons would overshoot the next level, so again, information technology does not try. Simply quanta of exactly the right size volition exist absorbed and used. Similarly, if an electron is already at the second level (northward = 2), and there is a infinite for the electron at the lower level (n = 1), it tin release a quantum of energy and drop down to the lower level. Simply the corporeality of energy given off will exist a whole number quantum. If this free energy is given off as low-cal (such as happens with emission spectra) then the photons rushing away from the falling electron volition be of only one size and quality (colour). Hence glowing sodium, or LEDs, only give off very discrete bands of lite with distinct colors or bands within their spectrum. All this implies that if white low-cal (with all the possible wavelengths, colors and possible quanta of energy) is shone on certain materials or substances only certain wavelengths (and their quanta of energy) will be absorbed by the electrons in that substance. Only a narrow band of calorie-free will take just the right quanta to move an electron to the adjacent level, or the level above that, and so on. That wavelength will be taken out of the spectrum of low-cal and leave a dark band of no-light backside. Absorption spectroscopy, therefore, is the equal and opposite of emission spectroscopy. However, in both kinds, information technology is the absorption of quanta to move electrons, or the emission of quanta to movement electrons effectually in the atom that is the reason why only sure wavelengths of light are affected. |
| The Breakthrough Atom - - a Summary | Although Bohr'due south original picture of a quantum atom has been modified in the years since he first proposed the concept, never the less, the principal principles still stand:
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| BIO dot EDU © 2003, Professor John Blamire | |
Source: http://www.brooklyn.cuny.edu/bc/ahp/LAD/C3/C3_elecEnergy.html
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