Atomic orbital model - Wiki..

The Rutherford Bohr model of the hydrogen atom. From Wikipedia, the free encyclopedia The Atomic Orbital Model is the currently accepted model of the electrons in an atom. It is sometimes called the Wave Mechanics Model. In the atomic orbital model, the atom consists of a nucleus surrounded by orbiting electrons.These electrons exist in atomic orbitals, which are a set of quantum states of the negatively charged electrons trapped in the electrical field generated by the positively charged nucleus. Classically, the orbits can be likened to the planets orbiting the sun. However, the atomic orbital model can only be described by quantum mechanics, in which case the electrons are more accurately described as standing waves surrounding the nucleus. 1 Historical context 2 Current theory 2.1 Wave-like properties 2.2 Particle-like properties 2.3 Describing the electrons 2.4 Predictions of the model 3 See also 4 External links Main article: Atomic theory After the discovery of the photoelectric effect, the connection between the structure of electrons in atoms and the emission and absorption spectra of atoms became an increasingly useful tool in the understanding of electrons in atoms. The most prominent feature of emission and absorption spectra was that these spectra contained discrete lines. The significance of the Bohr model was that it related the lines in emission and absorption spectra to the energy differences between the orbits that electrons could take around an atom. This was achieved by giving the electrons some kind of wave-like properties. In particular, electrons were assumed to have a wavelength (a property that had previously been discovered, but not entirely understood). The Bohr model was therefore not only a significant step towards the understanding of electrons in atoms, but also a significant step towards the development of the wave/particle duality of quantum mechanics. The premise of the model was that electrons had a wavelength, which was a function of its momentum, and therefore an orbiting electron would need to orbit at a multiple of the wavelength. The Bohr model was thus a classical model with an additional constraint provided by the 'wavelength' argument. In our current understanding of physics, this 'wavelength' argument is known to be an element of quantum mechanics, and for that reason the Bohr model is called a Atomic orbital model - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Atomic_orbital_model 第1页 共4页 2010/11/15 10:06 Electron atomic and molecular orbitals. The chart of orbitals (left) is arranged by increasing energy (see Madelung rule). Note that atomic orbits are functions of three variables (two angles, and the distance from the nucleus, r). These images are faithful to the angular component of the orbital, but not entirely representative of the orbital as a whole. semi-classical model. The Bohr model was able to explain the emission and absorption spectra of Hydrogen. The energies of electrons in the n=1, 2, 3, etc. states in the Bohr model match those of current physics. However, this did not explain similarities between different atoms, as expressed by the periodic table, such as why Helium (2 electrons), Neon (10 electrons), and Argon (18 electrons) exhibits similar chemical behaviour. Modern physics explains this by noting that the n=1 state can hold 2 electrons, the n=2 state can hold 8 electrons, and the n=3 state can hold 8 electrons. In the end, this was solved by the discovery of modern quantum mechanics and the Pauli Exclusion Principle. With the development of quantum mechanics, it was found that the orbiting electrons around a nucleus could not be fully described as particles, but needed to be explained by the wave-particle duality. In this sense, the electrons have the following properties: Wave-like properties The electrons do not orbit the nucleus in the sense of a planet orbiting the sun, but instead exist as standing waves. The lowest possible energy an electron can take is therefore analogous to the fundamental frequency of a wave on a string. Higher energy states are then similar to harmonics of the fundamental frequency. The electrons are never in a single point location, although the probability of interacting with the electron at a single point can be found from the wavefunction of the electron. Particle-like properties There is always an integer number of electrons orbiting the nucleus. Electrons jump between orbitals in a particle-like fashion. For example, if a single photon strikes the electrons, only a single electron changes states in response to the photon. The electrons retain particle like-properties such as: each wave state has the same electrical charge as the electron particle. Each wave state has a single discrete spin (spin up or spin down). Describing the electrons Because the electrons around a nucleus exist as a wave-particle duality, they cannot be described by a location and momentum. Instead, they are described by a set of quantum numbers that encompasses both the particle-like nature and the wave-like nature of the electrons. Each set of quantum numbers corresponds to a wavefunction. The quantum numbers Atomic orbital model - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Atomic_orbital_model 第2页 共4页 2010/11/15 10:06 are: The principal quantum number, n, is analogous to the harmonic of the electrons. That is, the n=1 states are analogous to the fundamental frequency of a wave on a string, and the n=2 states are analogous to the first harmonic, etc. The azimuthal quantum number, l, describes the orbital angular momentum of each electron. Note that this has no classical analog. The number l is an integer between 0 and (n - 1). The magnetic quantum number, ml, describes the magnetic moment of an electron in an arbitrary direction. The number ml is an integer between -l and l. The spin quantum number, s, describes the spin of each electron (spin up or spin down). The number s can be +1⁄2 or -1⁄2. These quantum numbers can only be determined by a full quantum mechanical analysis of the atom. There is no way to describe them using classical physical principles. A more technical analysis of these quantum numbers and how they are derived is given in the atomic orbital article. Furthermore, the Pauli Exclusion Principle states that no two electrons can occupy the same quantum state. That is, every electron that is orbiting the same nucleus must have a unique combination of quantum numbers. Predictions of the model Under quantum mechanics, each quantum state has a well-defined energy. When applied to atomic orbitals, this means that each state has a specific energy, and that if an electron is to move between states, the energy difference is also very fixed. Consider two states of the Hydrogen atom: State 1) n=1, l=0, ml=0 and s=+1⁄2 State 2) n=2, l=0, ml=0 and s=+1⁄2 By quantum theory, state 1 has a fixed energy of E1, and state 2 has a fixed energy of E2. Now, what would happen if an electron in state 1 were to move to state 2? For this to happen, the electron would need to gain an energy of exactly E2 - E1. If the electron receives energy that is less than or greater than this value, it cannot jump from state 1 to state 2. Now, suppose we irradiate the atom with a broad-spectrum of light. Photons that reach the atom that have an energy of exactly E2 - E1 will be absorbed by the electron in state 1, and that electron will jump to state 2. However, photons that are greater or lower in energy cannot be absorbed by the electron, because the electron can only jump to one of the orbitals, it cannot jump to a state between orbitals. The result is that only photons of a specific frequency will be absorbed by the atom. This creates a line in the spectrum, known as an absorption line, which corresponds to the energy difference between states 1 and 2. The atomic orbital model thus predicts line spectra, which are observed experimentally. This is one of the main validations of the atomic orbital model. The atomic orbital model is nevertheless an approximation to the full quantum theory, which only recognizes many electron states. The predictions of line spectra are qualitatively useful but are not quantitatively accurate for atoms and ions other than those containing only one electron. Atomic orbital model - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Atomic_orbital_model 第3页 共4页 2010/11/15 10:06 Atomic orbital — Details of how atomic orbitals are characterized, and approximations used to calculate them. Molecular orbital — Details on the structure of electrons in compounds. quantum chemistry — Study of how chemical bonds are formed by orbital electrons to form molecules. Solid state physics and condensed matter physics — Study of electrons in crystalline materials. Bonding in Organic Molecules. Atomic-Orbital Models (http://caltechbook.library.caltech.edu /122/7/BPOCchapter6.pdf) PDF (1.2MB) Retrieved from "http://en.wikipedia.org/wiki/Atomic_orbital_model" Categories: Atomic physics This page was last modified on 14 November 2010 at 02:51. Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply. See Terms of Use for details. Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. Atomic orbital model - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Atomic_orbital_model 第4页 共4页 2010/11/15 10:06