Orbital (band)

https://www.iitutor.com Different materials vary greatly in their ability to conduct electricity. In Physics syllabus, it describes about energy bands and semiconductors, what is physics and about the hsc physics formula sheet. The conduction strength depends on the resistance of the material's crystal lattice structure. In good insulators, the atoms in the lattice share electrons through strong covalent bonds. This conducts poorly as electrons are held tightly and are not available to conduct electricity through the lattice. Good conductors, like metallic lattices, are formed by ionic bonding consisting of an orderly array of positive metal ions. To maintain stability and as the electro-negativity of the metal ions are low, valence electrons are "delocalised" and are free to move as "sea of electrons" throughout the lattice. Delocalised electrons in the metal lattice move randomly between atoms. Under the influence of an electric field, the random motion of the electrons decreases and the electric field is established with a net motion of electrical current. In 1913, the Danish physicist Niels Bohr realised that the electrons in any atom only have a very limited range of energies (E1, E2, E3, ...). These specific "energy levels" vary from atom to atom. An electron can have an amount of energy corresponding to any one of these energy levels, but it cannot have an amount of energy in between. The reason behind this behaviour remained a mystery for some time. The electrons were believed to orbit the nucleus of the atom, so the different energy levels were called "orbitals". In 1923, de Broglie put forward the idea that, just as light showed particle characteristics (a photon), electrons could exhibit wave properties as "matter waves". He fundamentally observed the patterns formed by standing waves in a string to question what would happen, if the string was bent into a circle. Stable patterns would form when multiples of the wavelength corresponded to the circumference (or length) of the string. Extending this to the circumference of the Bohr orbit in an atom, and using the wavelength of an electron, only certain orbits would be stable—exactly those for which the circumference was equal to a multiple of the wavelength. The cornerstone of de Broglie's idea was that the electron orbiting the atom must have a standing-wave pattern of vibration. If it did not, its orbit will destructively interfere with itself, causing the electron to slip out of position. Hence, the orbital level represents an energy level of the electrons. With a standing wave pattern, only integer numbers of wave¬lengths are permitted between the nucleus and the orbiting electron. Intermediate electron energy levels can't be stable as they would produce destructive interference in the orbiting electron. The electron can absorb energy and move to a higher standing-wave energy level but this energy absorption must be of a specific amount; that is, the difference in energy between the electron's initial and final orbitals. The same electron can move to a lower energy orbital by emitting radiation energy as a photon; this photon contains an amount of energy exactly equal to the difference between the initial and final energy levels of the electron. In summary, De Broglie proposed that we think of electron orbiting as stable, stationary matter waves. The energy of the electrons is quantised. Only certain values, El, E2, E3, E4 ... are allowed; they correspond to these allowed quantum states. Earlier atomic models had circular orbits with only certain allowed radii; correspondingly, in the de Broglie atom, the only orbiting modes allowed are those that fit a whole number of wavelengths around the circumference. One whole wavelength corresponds to the lowest energy level, and increasing numbers of wavelengths correspond to higher energy levels. PD4311 http://youtu.be/KYYWZhsgVPc

• published: 23 Aug 2013
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