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Topic: Electron counting

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In the News (Mon 19 Aug 19)

  Electron counting
In the inorganic chemistry and organometallic chemistry of transition metals, electron counting is a formalism used for characterizing a compound and for understanding its electronic structure and bonding.
The valence shells of a transition metal are filled when they contain 18 electrons: 2 each in the 5 d orbitals, or 10 total; 2 each in the 3 p orbitals, or 6 total; and finally 2 in the single valence shell s orbital.
The electrons contributed by the metal atom or ion are summed with the electrons contributed by each ligand (ie, those valence electrons of each ligand participating in some way in a bonding interaction with the metal, and not otherwise occupied only in intraligand bonding or in lone-pairs not interacting with the metal center).
www.ebroadcast.com.au /lookup/encyclopedia/el/Electron_counting.html   (403 words)

 The Organometallic HyperTextBook: Electron Counting
Each derives from a simple count of the number of electrons that may be accommodated by the available valence orbitals (one s and three p for organic chemists; organometallic chemists get five bonus d-orbitals in which to place their electrons).
There are two distinct methods that are used to count electrons, the neutral or covalent method and the effective atomic number or ionic method.
Therefore, the methyl group is a one electron donor, not a two electron donor as it is under the ionic formalism.
www.ilpi.com /organomet/electroncount.html   (1139 words)

 Electron counting - Wikipedia, the free encyclopedia
Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding.
Polyhedral skeletal electron pair theory for cluster compounds, including transition metals and main group elements such as boron including Wade's rules for polyhedral cluster compounds, including transition metals and main group elements and mixtures thereof.
Similar for H: neutral counting: H contributes 1 electron, the C contributes 1 electron (the other 3 electrons of C are for the other 3 hydrogens in the molecule): 1 + 1(1) = 2 valence electrons.
en.wikipedia.org /wiki/Electron_counting   (1094 words)

 Evans Analytical Group LLC   (Site not responding. Last check: 2007-10-20)
These include an ion counting electron multiplier, a Faraday cup, and two ion image detectors.
The ion counting electron multipliers are the most sensitive detectors.
Electron multipliers can also be made from continuous dynode materials rather than discrete dynodes.
www.eaglabs.com /en-US/references/tutorial/simsinst/detector.html   (774 words)

 High count rate electron probe microanalysis Journal of Research of the National Institute of Standards and Technology ...
This paper discusses the collection and counting of the emitted x rays, which are diffracted into the gas flow or sealed proportional x-ray detectors.
Counting longer means the analysis time increases and may become excessive to get to the desired uncertainty.
Counting at higher rates has its limitations, which are a function of the detector physics and the detecting electronics.
www.findarticles.com /p/articles/mi_m0IKZ/is_6_107/ai_99376674   (791 words)

 Quantum Numbers   (Site not responding. Last check: 2007-10-20)
One of the key insights in quantum chemistry is that, because an electron is not a classical particle located at a definite point in space, even a single electron can "surround" the nucleus of an atom, filling a volume roughly as big as the whole atom.
Instead of thinking of electron shells neatly nested inside one another, it is better to visualize electrons in interpenetrating orbitals, like a lot of ripples on a pond.
Each individual electron cloud extends down to "touch" the nucleus, and all the electrons in an atom come under the direct influence of the nucleus, although some are influenced more strongly than others.
www.howe.k12.ok.us /~jimaskew/cquantum.htm   (955 words)

One of the projects that we are pursuing in Qlab is electron counting.
The goal is to demonstrate this phenomena and to study the possibility of precision measurment of the frequency.
Our proposed implementation of the electron counting experiment intends to use the Josephson Bifurcation Amplifier (JBA) developped in our group as a detector.
www.eng.yale.edu /qlab/proj_desc/elect_count_proj.htm   (359 words)

 Glossary : SLAC Virtual Visitor Center
An electron in a atom is bound because of its electrical attraction to the nucleus, which makes the mass of the atom slightly less than the sum of the masses of the electron plus the rest of the atom without that electron.
Electrons carry electrical current and successful manipulation of electrons allows electronic devices, such as the one you are using, to function.
The picture and text on the video terminal in front of you is caused by electrons being accelerated and focused onto the inside of the screen, where a phosphor absorbs the electrons and light is produced.
www2.slac.stanford.edu /vvc/glossary.html   (5645 words)

 Single Electron Tunneling
Verify or refute the belief in the molecular electronics field that nonlinearities in the transport are due to charge motion in the molecules.
The standard, shown in the schematic, consists of the electron counter, a capacitor, and a single-electron electrometer to monitor the process (not shown).
Electron Counting Capacitance Standard Demonstrated - At present, the most advanced application for SET devices within the project is a new capacitance standard based on the fundamental definition of capacitance.
emtech.boulder.nist.gov /div817b/whatwedo/set/set.htm   (2731 words)

 The double-slit experiment (September 2002) - Physics World - PhysicsWeb   (Site not responding. Last check: 2007-10-20)
The electron biprism was to become widely used in the development of electron holography and also in other experiments, including the first measurement of the Aharonov-Bohm effect by Bob Chambers at Bristol University in the UK in 1960.
To film the build-up process, the electron source, the electron biprism and the rest of the experiment therefore had to be extremely stable: if the interference pattern had drifted by a fraction of fringe spacing over the exposure time, the whole fringe pattern would have disappeared.
Counting losses and noise in conventional TV cameras mean that it is difficult to know if each flash of the screen really corresponds to an individual electron.
physicsweb.org /article/world/15/9/1/1   (3316 words)

 International Union of Pure and Applied Chemistry
Electron deficient compounds - Molecules or ions that contain too few electrons to allow their 	bonding to be described exclusively in terms of two-center, two-electron, i.e.covalent, bonds.
Electronic chemical potential - The quantity that measures the escaping tendency of electrons from a species in its ground state.
Electronic configuration - The allocation of electrons within an atom or a molecule to a set of correspondingly atomic or molecular orbitals complying with the Pauli exclusion principle.
www.iupac.org /reports/1999/7110minkin/e.html   (1852 words)

 Single-electron ammeter based on bidirectional counting of single-electrons
An electron entering or escaping from one of the quantum dots influences the current flowing through the point contact, and thus the change in the electron occupation in a quantum dot can be monitored.
When an electron travels through the two quantum dots, the current through the point contact shows three different values depending on the location of the electron as shown in Fig.
It should be especially useful for studies on nanoelectronics, which seek to examine electron transport through nanostructures, single molecules, and biological cells, and often require measurement of extremely small currents.
www.physorg.com /news69934210.html   (789 words)

 18-Electron rule - Wikipedia, the free encyclopedia
The valence shells of a transition metal can accommodate 18 electrons: 2 in each of the five d orbitals (10 in total); 2 in each of the 3 p orbitals (6 in total); and 2 in the s orbital (see Electron counting).
Because putting electrons into them is a favourable process, and as each orbital can take two electrons, the greatest stability is achieved when there are a total of 18 electrons in these orbitals - this includes both the electrons that come from the metal, and those donated to it from the ligands.
In fact, 18 electrons is often a recipe for non-reactivity in either a stoichiometric or catalytic sense.
en.wikipedia.org /wiki/18-Electron_rule   (1243 words)

 Lewis structure - Wikipedia, the free encyclopedia
The total number of electrons represented in a Lewis structure is equal to the sum of the numbers of valence electrons on each individual atom.
The 18-electron rule is operative on atoms from period 4, which have to achieve 18 electrons to fill their orbitals and achieve a noble gas configuration.
Electrons in covalent bonds are split equally between the atoms involved in the bond.
en.wikipedia.org /wiki/Electron_dot_notation   (1949 words)

 Chpt 11.html
We are only counting the number of s electrons that are donated to the metal.
The virtue in using this method is that the d electron count is precisely how many electron are present in what we have labelled the important valence region.
Therefore, if the polyene with all carbon atoms connected to the metal would give an electron count greater than this, then the polyene instead distorts, bends away from the metal to a lower coordination number.
www.chem.uh.edu /Courses/Albright/Chem6312/ch11/chpt11.html   (874 words)

 Proportional Counters
The signals induced on these electrodes by the motions of electrons and ions in the counting gas mixture contain information on the energies, arrival times, and interaction positions of the photons transmitted by the window.
Once a gas atom is ionized by photons or charged particles, the free electron is attracted to the positive anode wire at the center of the cell.
In X-ray astronomy, where count rates are typically low and system simplicity is a good thing, the analog readout, in which it is the centroid of the induced charge distribution which is encoded, is more common.
imagine.gsfc.nasa.gov /docs/science/how_l2/proportional.html   (849 words)

 Radiation Detection Physics Lab - Current Research - Single electron counting   (Site not responding. Last check: 2007-10-20)
The single electron counting approach provides a "zoom-in" view of the ionization process induced by radiation in the detector medium - a process which is stochastic in its nature.
By counting individual ionization charges rather than integrating and multiplying them, we get a better insight to the primary ionization process, un-obscured by additional statistical fluctuations or by integration.
The individual ionization charges, deposited in the gaseous medium or emitted from the solid surface, are individually counted, providing the full statistical information on the primary interaction process (Publications).
www.weizmann.ac.il /home/detlab/counting.html   (216 words)

 Free electrons   (Site not responding. Last check: 2007-10-20)
One not-so-bad approximation is to divide the problem into several parts: the nuclei, the core electrons, and the valence electrons.
The core electrons to a reasonable approximationa still form filled, relatively inert shells surrounding their nuclei, which can be considered to be a single unit: an ionic core.
If we have N electrons, the ground state will be to fill up the N states with the lowest available energies.
carini.physics.indiana.edu /p615/free-electrons.html   (617 words)

 Quantum Electrical Metrology: National Electrical Standards, Single Electron Tunneling
Atomic force microscope image of an electron counter, the heart of a new capacitance standard based on counting electrons.
To operate the ECCS approximately 100 million electrons are placed, one at a time, on the capacitor.
We have counted up to 3 electrons added to a single dot, and have measured the energy spectrum associated with adding electrons on this dot.
www.eeel.nist.gov /817/whatwedo-817g/set/set.html   (2795 words)

 How do I find the number of protons, electrons and neutrons that are in an atom of...?
For example, removing an electron from an atom of krypton forms a krypton ion, which is usually written as Kr The plus sign means that this is a positively charged ion.
The 35 remaining electrons were outnumbered by the 36 positively charged protons, resulting in a charge of +1.
If you could grab atoms of krypton and count the number of neutrons each one had, you would find that most would have 48, others would have 47, some would have 50, some others would have 46, a few would have 44 and a very few would have 42.
education.jlab.org /qa/pen_number.html   (840 words)

 photon counting
Astronomers, in the early 1950s, were the first to use photomultipliers in the photon counting mode for measurements on stars of very low magnitude.
The power of the photon counting technique stems from the fundamental principle upon which it is based: electromagnetic radiation is quantised and so counting these quanta (photons) provides the best and most direct method of measuring the intensity of the radiation.
Setting up a photon counting system used to be complicated requiring voltage dividers, high voltage power supplies, photomultiplier housings, low noise amplifiers and fast discriminators.
www.photoncounting.com /index.html   (263 words)

 OSU Physics: Calendar of Events
Still, electron dynamics in quantum dots and other nanostructures have usually been inferred from dc measurements rather than observed directly.
Detecting the motion of individual electrons in a quantum dot allows direct access to dynamical information, and allows electron counting experiments analogous to the photon counting experiments of quantum optics.
By extending these techniques to more complex nanostructures, we can also investigate such issues as the quantum measurement problem, detection of individual electronic spins, and development of a current standard based on electron counting.
www.physics.ohio-state.edu /calendar/event.php3?id=768   (166 words)

 ONE ELECTRON, TWO ELECTRONS ... Method for Counting Individual Electrons Leads to New Electrical Standard from NIST
Electrons are the negative-charged particles that orbit the positive-charged nucleus in an atom.
The first is an electron pump, based on ultra-small electrical devices called tunnel junctions (thin metal-insulator-metal sandwiches through which electrons can flow).
The capacitor is designed so that very few of the electrons placed upon it "leak out," dramatically improving the accuracy of the capacitance measurement.
www.nist.gov /public_affairs/releases/n99-16.htm   (628 words)

 The Structure of Semiconductor Surfaces and Interfaces: Projects
It is widely accepted that the surfaces of these polar semiconductors should reconstruct such that all the dangling bonds on the electropositive surface atoms (III) are unoccupied and all those on the electronegative atoms (V) are doubly occupied, with the resulting surface band gap similar to that of the bulk.
This is in contrast to the traditional view of III-V(001) reconstructions, where the valence electrons in excess of the bonding states should exactly fill the V dbs and leave all III dbs empty.
Impact: The results suggest that the electron counting model may not be nearly as universal as it is currently regarded.
stm2.nrl.navy.mil /~lwhitman/Projects.html   (2068 words)

The number of nonbonded electrons at the metal plus the number of electrons in the metal-ligand bonds should be 18.
A molecular polarizability effect occurring by an intramolecular electron displacement (sometimes called the "conjugative mechanism" and, previously, the "tautomeric mechanism") characterized by the substitution of one electron pair for another within the same atomic octet of electrons.
If P(x,y,z) dx dy dz is the probability of finding an electron in the volume element dx dy dz at the point of a molecular entity with coordinates x,y,z, then P(x,y,z) is the electron density at this point.
www.chem.qmul.ac.uk /iupac/gtpoc/E.html   (2409 words)

 GV Instruments
Typically in helium analysis the count rate for 3He will be few thousand ions per second at most.
In the case of Tritium analysis the 3He count rate may be a few ions per second.
However, if extremely small samples/large ratios are required the electron multiplier ion counting system is recommended due to its optimum signal to noise ratio.
www.gvinstruments.co.uk /Noble_Gas_MS.htm   (952 words)

 crdits.info - Electron counting (via CobWeb/3.1 planetlab1.tamu.edu)   (Site not responding. Last check: 2007-10-20)
Electron counting iѕ a formaliѕm for for claѕѕifying compoundѕ and uѕed explaining or predicting electronic ѕtructure and Ƅond ing.
The numƄerѕ of electronѕ "donated" Ƅy ѕome ligandѕ dependѕ on the geometry of the metal-ligand enѕemƄle.
The neutral counting approach iѕ uѕually conѕidered eaѕier Ƅut the "ionic counting" approach rewardѕ the uѕer with a knowledge of oxidation ѕtateѕ, which can Ƅe valuaƄle.
crdits.info.cob-web.org:8888 /7689   (1069 words)

 Influence of the vertical emittance on the calculability of the Synchrotron Ultraviolet Radiation Facility Journal of ...   (Site not responding. Last check: 2007-10-20)
A method to include the influence of the vertical electron beam emittance onto the calculability of synchrotron radiation is introduced.
At the Synchrotron Ultraviolet Radiation Facility SURF III (6) the electron energy and orbital radius are determined by the magnetic flux density B and the radio-frequency [v.sub.RF].
The electron beam current is determined using a well established electron counting procedure (8,9).
www.findarticles.com /p/articles/mi_m0IKZ/is_5_107/ai_95792245?lstpn=article_results&lstpc=search&lstpr=external&lstprs=other&lstwid=1&lstwn=search_results&lstwp=body_middle   (692 words)

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